Understanding the Axial Skeleton

This lecture will cover the skeletal system, specifically the axial skeleton portion of the skeleton. And this lecture is a mixture of both lecture material and lab material. So we'll kind of cover that as we go through. You'll also get a highlight of those bones that students find to be most problematic, at least those that I see students find most problematic. So hopefully this will help in a variety of ways. Okay, so. The adult skeleton typically has 206 bones. However, that is not absolute because people vary within single digits less or more in terms of the number of bones. There's a reason why that is because certain bones don't fuse with maturity in some individuals. Some individuals have bones that fuse more so than we would imagine, and so therefore we get some variability. this and that's totally normal. At birth there are way more bones than 206 and with time and maturity these bones begin to fuse into the singular bones that we identify in the adult skeleton. Now the axial skeleton we define as being those bones that compose an axis, a central axis of the body and the portions that we're talking about here are specifically the skull the vertebral column, and then finally the thoracic cage. Notice that none of these three portions of the skeleton have anything to do with the appendages, so the arms or the legs, and that's important to keep in mind because the axial skeleton does not have anything to do with the arms and legs. It provides an axis for which those appendages may connect. Now this is an overview of the actual axial skeleton. We see here again it is just the skull, vertebral column, and thoracic cage. We see the principal and primary bones of the thoracic cage and the vertebral column. The skull, it just highlights a few, but there are a lot more bones than what are shown here. All right, let's start with the skull. This is by far, I would say, the most difficult aspect of the axial skeleton. The cranial bones form a large, rounded, hollow cranium. Obviously, we all understand and know that. This encloses the brain. Also, we understand that as well. The facial bones form the face, so they're there to provide an appearance for us. And in addition to that, they protect the entrances to the digestive system, to the respiratory system, as well as an attachment site for facial muscles. And I cannot understate the importance of this. Our facial features and our ability to make... Expressions with our face are such a huge aspect of communication that we often take for granted. And so while this seems kind of like a passive thing, that it's not all that important, it's actually hugely important, as important as their protection for influences of the digestive and respiratory system, for example, if not more so. Here's a view of the skull. Obviously, the body is not a paint-by-number system. This has been colored specifically so that you can see the demarcations between each bone. And so we're going to go through all of these bones. Yes, unfortunately, we are going to go through all of these bones. And we will go through a select set of features on each of those bones when appropriate and when they are indeed most difficult. Because, again, we tend to highlight here those aspects of the lab material that students find most troubling. Here is, so I should point out this is an anterior view, which again I think we can understand and what I've imagined. This is an inferior view. So assume that the vertebral column itself had been removed, right, so we're looking at an inferior view minus the mandible. This bone is a wonderfully fun bone. Students really love this. This is the vomer bone. The vomer bone is a small thin plow-like bone that's found in the interior portion of the nasal cavity and helps make up a portion of the nasal septum. So we see that right here. Now you don't need to know any of the particular features of the vomer bone, but you do need to know what the vomer bone looks like. So again, it's placement here anteriorly, it's placement here inferiorly, and a tad bit posteriorly. Now that's kind of a weird bone to lead into this, but we've got the skull with the calvaria cut off. So this is the skull cap otherwise known as the calvaria. and we've got a plane that goes through there right and so we've opened that up so now we can see this is a superior view of the internal cranial cavity and from here we see a very different perspective on the skull than what we have seen when we look at the skull intact and some of the most difficult features of this are learning where bones begin and end so keep that in mind where you're working in lab as well as some of these nuanced features of the sphenoid. The sphenoid and arguably the ethmoid are the two most difficult bones here on the inside of the skull. So we'll go through them as we get there, as well as all the openings of the skull. So all these holes that are highlighted here, these are also in a very difficult set of structures for students to master. All right, so let's look at the the cheekbone, right, and we're going to look at the zygomatic arch, right. So this whole structure here across here is the zygomatic arch and it's composed in part by the zygomatic bone here. This is highlighted in green. We can see it here on a lateral view, here in an inferior view, here in an anterior view. And the zygomatic bone contains on it what we call the temporal process. And so this green portion here, enlarged, is this portion here, is going to connect to the temporal bone, specifically here, this portion of the temporal bone. And because it's articulating with the temporal bone, we call it the temporal process. It is the process by which the temporal bone touches, and that's why we call it that. And so therefore, this process is named the temporal process of the zygomatic bone. And that's really important because this structure here is called the zygomatic process of the temporal bone. And it's called the zygomatic process because it's the portion of that bone that reaches out and touches the zygomatic bone. And these are structures that students routinely get confused. They will often think the temporal process is on a temporal bone and the zygomatic process is on the zygomatic bone. But that's not the case at all. The bone is... The process itself is not named after the bone for which it's a part of, but rather the bone that it is reaching out to and connecting to. So we see that here a little bit more up close. Now, please bear in mind, this figure is wrong. This is the publisher's mistake, as it turns out, which is, I find humorous considering how much money you pay for the book. This is actually the zygomatic process. process of the temporal bone. So that's this structure right here, zygomatic process of the temporal bone. And this structure here is the temporal process of the zygomatic bone. Alright, let's look at the mandible. The mandible is a unique bone. Again, it's our lower jaw. We can see that view here, articulated with the skull here, highlighted in purple. from a lateral view. Now, there are several structures on here that are important to remember. We've got the coronoid process here, which does not form an articulating portion of the actual temporal mandibular joint, which is the joint that's formed between the skull and the actual mandible. But nonetheless, the coronoid process is a structure that you will need to know for lab, so keep that in mind. You've got the mandibular condyle here, which is a condyle, by the way, something that you should know, is... a rounded structure that is used for a bone to rock on or move on another bone. The ramus here is that portion of the bone that juts upward to articulate with the actual skull. The angle of the mandible is seen here, and then finally we have the body of the mandible here. The holes that you see here and there's actually another one paired over on the other side too. You can't see it. These foramen are called the mental foramen. You want to think of mental as referring to the chin, interestingly enough in this case. Let's talk about cavities of the skull. Cranial cavity by far is the largest cavity in the skull. It encloses, cushions, and supports the brain. The smaller cavities that we see in the skull are going to be the orbits, they're going to be the oral cavity, they're going to be the nasal cavity, and finally the paranasal sinuses. So there's several other small cavities that exist within the skull. The largest is by far the easiest to remember and the easiest to associate with the skull. But these other four are not by any means unimportant. Here's a coronal section of the skull where we can actually see a lot of these cavities. Now again we can identify the cranial cavity is by far the largest, but we've got all these other cavities as well. We've got the two orbits seen here and here. We've got the paranasal sinuses, of which there are many. We've got frontal sinuses here and here. The ethmoid sinuses, you see them highlighted in red here and here. We've got the nasal cavity, which in and of itself is not a sinus, but it's connected to the sinuses. We've got the maxillary sinus here and here. The only sinus that we're missing here is called the sphenoidal sinus, and that's because this plane will not give you a view of that. But nonetheless, we'll show you that in a little bit here. Finally, the other cavity we've got is the oral cavity here. The sinuses themselves lighten the skull. They're there to reduce the weight of the skull overall so that it's manageable. These are lined with an epithelium that produces mucus and helps to humidify in warm inhaled air. Because of this, it helps provide resonance to the voice, and that's actually very important. I think we've all been sick enough that we can remember a time when our voice sounded very nasally, and that's because with a sinus infection, that airflow gets cut off, and all of a sudden our voice sounds greatly different. It lacks that resonance that we would often find with the sinuses. By far one of the creepiest pictures in this book. These are the scattered view of the different sinuses. And the only ones you haven't seen so far yet are these even farther back, these posterior most ones called the sphenoidal. These are highlighted in orange. I'm going to show you another view of those here right here. So I've shown you the frontal sinuses, the maxillary sinuses, the ethmoidal sinuses. The last sinus that we haven't talked about is the sphenoidal here, this big sinus here in the very posterior most aspect to the rest of the sinuses. This is the ethmoid bone, and the ethmoid bone is by far one of the most challenging bones for students to master. And if I can impart a little bit of advice here right up front, you're going to be working with a disarticulated resin cast ethmoid bone, which means that it's not an actual bone, but rather a molded cast of the bone. The reason that you're not working with the real ethmoid bone, disarticulated at least, is because it is so fragile that it will infect. break on you and so you will not get a good specimen to work with. But the resin cast is really important that you and one of the keys to tackling this properly is making sure that you can actually orient this bone properly. So making sure that you can tell superior from inferior, anterior from posterior view, and lateral view is finally. That's key and crucial because without being able to do that properly Naming off the structures on the ethmoid bone is going to be particularly difficult. Now from a superior view like this You should be able to see this structure that juts upward and this is important to understand because we're looking this view here is Actually this view here You're seeing only this tiny portion of the ethmoid bone here and the crista Gali looks like a tiny little mohawk that sticks up into the cranial cavity and what's it? What's key and crucial to know here is that you can't actually see the perpendicular plate anywhere near as well as it's labeled here. Perpendicular plate is much more easily seen and in fact very readily seen on the anterior view of the actual ethmoid. So I'm going to go ahead and skip forward so that we can actually see this. This is an anterior view. And again, we can see this plate that sticks downward. Very long, very impressive. It's called the perpendicular plate. This is going to help form the nasal septum. And this is the crista galli. Now you can see that that structure actually sticks upward. This is going to sit next to the brain. This is going to be a portion of the nasal septum that helps form the two different passageways of the nose. So that's key and crucial to understand here. Now, sutures of the skull. A suture is an immovable... fibrous joint and this is something that you will learn in the articulations. chapter, right? So these are immovable joints. You don't want these things to move. Remember, these things are the joints that link together are skull bones. We really don't want those skull bones to move, right? They're fibrous joints. They are held together by a dense regular connect tissue that firmly seals these bones together, right? And they allow the cranium to grow and expand throughout childhood. Very important aspect of things there. We can see the various different sutures that you need to know in this class in these two views. This is a superior view. This is a posterior view. And on the superior view, we can see the coronal suture here. The coronal suture lies on a coronal or frontal plane and therefore is defined as or named the coronal suture. And that separates or more to the point actually connects the frontal bone. to the two parietal bones we see here. The sagittal suture here lies along a sagittal plane and it lies between the two parietal bones here and here. Again because it lies along a sagittal plane it is named after the plane that it lies along. I'm going to go ahead and move down here to the inferior most portion of this figure. So we can see the posterior view. This structure here is not related at all to any plane that it is, but rather it's called the lambdoid or lambdoidal suture. One of the most misspelled words on the first practical is lambdoid. Now, the key to this is understanding something right up front. It is spelled literally lamb, L-A-M-B, doid, D-O-I-D. So there's both a... B and a D in the middle of that word. And this is where I get a lot of spelling mistakes. Students typically guess this correctly, but they can't spell it correctly. They either forget the B or they forget the D in the middle. And so I'm left with either lambdoid or lambvoid, but never lambdoid. Right. And that's something that I want you to remember. If you've got to say it. Like that, to remind yourself that it's lamb-doid, then do it so that you spell it correctly. Fontanelles are soft spots. These are, we've all, or most of us, I guess, would have probably had experience with these. We see these on babies. We don't see these, obviously, on adults, thank goodness. But these are soft spots in the skull, right? And these are regions in which there is still a fibrous membrane remnant. where the cranial bones have not yet totally ossified. So they're literally the soft spots in the infant skull. They close up by about 15 months in age. The whole idea behind a fontanelle is that it allows the actual cranial bones to overlap so that when the child is passing through the birth canal, it eases the baby's passage for both mother and child. And this is very important. You do not want a great deal of resistance there any more than there already is. And you don't want this to be even more difficult. Babies often have, obviously, a not yet fully ossified skeleton because it makes it easier, makes it more flexible, including the cranium here can change shape because of the fact that we need to ease the passage of this, make birth as easy as possible on both mother and child. The newborns frequently have a cone-shaped head due to this temporary deformation that occurs when they're passing through the birth canal. We can see these fontanelles here, the anterior fontanelle, the sphenoidal fontanelle, the mastoid fontanelle, and then finally the posterior fontanelle here, posteriorly. Alright, here's an important term, or important concept. This is something that is more lecture related than lab, and this is just simply being able to list the bones that form the orbital complex, right? It's seven bones here. Some of them are far more participa- far more largely participating in the orbit than others, but still all seven of them help to form the orbit. The frontal bone, the lacrimal bone, the zygomatic, the maxilla, the palatine, the ethmoid, and the sphenoid bone all play a role in forming the orbit. So we can see them here, right? Now the frontal bone is a fairly large portion of it. as is the sphenoid and the zygomatic. The maxillary bone also plays a very large role. And then we get some very tiny roles. that are played here. We get the Palatine bone here highlighted in this kind of lime green, not a huge role, but still is there. We've got the Ethmoid bone here, not a huge role, but yet still there. And then finally this little blue kind of island there in the middle, this is the Lacrymal bone. So there are three bones for which play a relatively small role, and then four bones that play a much a much larger, much more significant role in the formation of the actual orbit. This is a view of the Palatine bone. This contributes very little to the actual orbit, but it's still there. And the point I want to make is that you need to be able to identify the Palatine bone based upon view, as well as a structure on the Palatine bone known as the horizontal plate. Now the Palatine bone looks like it's shaped like an L, albeit a very ornate L, but nonetheless still like an L. And the point I'm trying to make here is that this plate here that helps make up a portion of the hard palate within the actual oral cavity. This is the horizontal plate and you can see that horizontal plate here and you can see it from a very different view here in this mid-sagittal section. The auditory ossicles are the tiny ear bones that allow us to be able to actually perceive the sense of hearing, right? So these are the tiny little bones. housed within the petrous region of each temporal bone and they rattle against each other to bring vibrations from sound waves into the inner ear so that we can actually form a sensation from them. These three bones, the malleus, otherwise often called the hammer, you must call it, you must be able to spell it as the malleus, the incus, otherwise affectionately termed the anvil, and then finally the stapes, which again more affectionately called the stirrup. You must be able to name them and spell them by the Malleus, Incus, and Stapes. The hyoid bone, not associated with the ear at all. Hyoid bone is a slender, curved bone located in the inferior portion of the skull, but it's not a portion of the skull at all. It actually doesn't articulate with the skull at all. As a matter of fact, it doesn't articulate with any bone on the skeleton. It is the only floating bone in the body with no articulations to other bones. And it's a particularly important bone because it serves as sites for attachment of muscles for the tongue and the larynx, as well as ligaments. We also know that the hyoid bone is more important for us in terms of a forensic perspective, right? So it often is one of many pieces of information that can be used to determine whether somebody committed suicide versus somebody was actually strangled and then posed to look like they committed suicide. So. Hanging if somebody hangs themselves most of the time the highway bone will not break because the the actual Orientation of the rope simply does not support that right it Should not break the hyoid because it shouldn't be up against the hyoid However, if somebody is strangled most of the time that force is coming from directly in front of or directly behind That person and it puts a great deal of pressure on the hyoid and the hyoid often snaps and so this bone has particularly important forensic significance and so one of the reasons why it's important as well this is the hyoid bone the only thing you have to know about the hyoid bone is to be able to look at it and identify the hyoid bone the vertebral column again this this material here is greatly lecture material until we actually start looking at the individual bones. But the point I want to make here is that it's actually 26 bones in composition. You've got 24 individual vertebrae. You've got 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 sacral. These are all fused together. And then finally, 3 to 5 coccyx bones, or 3 to 5, sorry, vertebrae in the coccyx. The vertebral column itself has several functions. It provides vertical support for the body, as well as a protection for the delicate spinal cord. This is actually crucial. That's the reason why it's there. Another big aspect of the actual vertebral column, it supports the weight of the head and it allows for the transfer of weight through the axial skeleton to the appendicular So it serves as a transfer of weight, which is again extremely important. This is a view of the vertebral column. Now let's go through the individual bones, right, so we get a better view of these. There are some common structures that we see amongst all the actual bones here. All of them have a body, which is this big portion here, upon which the rest of the other vertebrae actually sit upon. It's called the body or the centrum. Now, they've all got a spinous process here, and they've all got transverse processes here and here. And the portion of the bone that links the spinous process to the transverse process is this bridge of bone here that we call the lamina. We see a lamina here, and then a lamina here. And then the spinous process and the transverse processes are bound to the body. through yet another bridge of bone here. And this bridge of bone here that is encircling this whole area is called the pedicle. Now there are two very special vertebrae, the atlas and the axis. Atlas is C1, axis is C2. These are the only vertebral bones that you will have to be able to number. The rest of them you can just simply identify by cervical or thoracic. But in this case you must be able to identify them specifically. And that's because they don't look like any of the others. They're very unique. The atlas is very flattened in its orientation. It looks much more like a ring than anything else. And that's intentional because this large process, this portion of the bone here, is meant to actually surround this vertical lying structure of C2 or the axis. And this bone that sticks upward here is called the dens. D-E-N-S. Now, it actually has another name. It's a little bit more complicated. Almost no students ever use it. And that's called the edontoid process. Most students call it dens because dens is far, far easier to spell. Either way is fine, though. The dens sticks upward and allows the formation of a pivot joint to form here. So this allows... your head to actually shake. So if you can shake no to say no. In part because of this pivot joint that's formed here between C1 and C2. This is the sacral bone. The structures here that you need to know are at the very top of the bone where the last lumbar vertebra rests upon. This portion here is called the sacral promontory. This is an anterior view. Now, if we go to a posterior view here, we can see the median sacral crest here of these ridges of bumps that form the back of these. These are actually the original spinous processes from the vertebra themselves. And the other structure here that you need to see, and it's actually better seen here on their actual real bones. That is this surface here. This is called the auricular surface. There's one over here and one over here. Now, when the sacrum articulates with the hip bone, otherwise referred to as the ascoxa, right? That's a bone that you want to keep in mind, the ascoxa. When the ascoxa and the sacrum come together, that's called the sacroiliac joint. But when the ascoxa is not present here, we're looking at that service that helps... form that joint on the sacrum is referred to as the auricular surface. So please keep that in mind. It's not a joint unless two bones are actually connected together. The thoracic cage consists of thoracic vertebrae posteriorly, the ribs laterally, and the sternum anteriorly. So it's a chain, a cage formed by these three sets of structures. And it's a correct... protective cage around things that are particularly vital to us, the heart, the lungs, the trachea, the esophagus. It provides us a flurry of attachment points for muscles, pardon me, as well as having a relatively complicated sternum. That sternum itself is actually split up into three separate portions, the xiphoid process. the body and finally the manubrium. So we're gonna we're gonna highlight those first here. The manubrium is this superior most portion of the sternum we see it here. The body is here. And then finally, you have the xiphoid process down here, that little point that comes off the inferior end of that sternum. Here are the ribs, ribs 1 through 11 all the way down. Ribs 1 through 7 are actually what's called the true ribs. That means that these ribs have their own costal cartilage that connect them directly to the actual sternum. The false ribs, 8 through 12, do not. They share, sorry, 12 ribs there, they share a connection to another costal cartilage to allow them to connect to the actual sternum. Or they don't at all connect. These ribs 11 and 12 are actually called floating ribs, specifically because of the fact that they share no attachment site whatsoever with the sternum. So they don't connect at all to the sternum. All right, for ribs, both males and females have 12 pairs of ribs. The true ribs are 1 through 7. False ribs are technically 8 through 10. And then finally you have the floating ribs, which are 11 and 12. So I've explained all that up front here. So this slide is much more of an actual summary or overview. And here is a view of the rib. The rib itself is actually not complicated. So it's not a complicated structure. bone to understand. However, there are a couple things that I do want to make clear because there are a couple of big mistakes that students make. The shaft of a bone is the the shaft that you can grab onto and hold on to. Think of a shaft there, right? So if I were to take a pipe cleaner and wrap it around the entire bone here and say name this structure be specific, in lab I'm asking for the shaft of the bone. Now Keep that in mind because there is actually a shallow groove that runs along this curved portion of the actual bone here. And this groove is called the costal groove. And we will often paint this structure so that you can see this indentation on the bone. And for whatever reason, it's often misidentified as the shaft. Remember, if I wrap an entire pipe cleaner around it, that is the shaft. However, If there's just a painted indentation along that bone, that's actually the costal groove and not the shaft. Now, this is how it articulates at the head of the actual rib and the tubercle of the rib that actually form these articulations between the vertebra. And that allows the ribs to sit in place properly.

So hopefully this will help in a variety of ways. Okay, so. The adult skeleton typically has 206 bones.

However, that is not absolute because people vary within single digits less or more in terms of the number of bones. There's a reason why that is because certain bones don't fuse with maturity in some individuals. Some individuals have bones that fuse more so than we would imagine, and so therefore we get some variability.

this and that's totally normal. At birth there are way more bones than 206 and with time and maturity these bones begin to fuse into the singular bones that we identify in the adult skeleton. Now the axial skeleton we define as being those bones that compose an axis, a central axis of the body and the portions that we're talking about here are specifically the skull the vertebral column, and then finally the thoracic cage. Notice that none of these three portions of the skeleton have anything to do with the appendages, so the arms or the legs, and that's important to keep in mind because the axial skeleton does not have anything to do with the arms and legs. It provides an axis for which those appendages may connect.

Now this is an overview of the actual axial skeleton. We see here again it is just the skull, vertebral column, and thoracic cage. We see the principal and primary bones of the thoracic cage and the vertebral column.

The skull, it just highlights a few, but there are a lot more bones than what are shown here. All right, let's start with the skull. This is by far, I would say, the most difficult aspect of the axial skeleton. The cranial bones form a large, rounded, hollow cranium. Obviously, we all understand and know that.

This encloses the brain. Also, we understand that as well. The facial bones form the face, so they're there to provide an appearance for us. And in addition to that, they protect the entrances to the digestive system, to the respiratory system, as well as an attachment site for facial muscles. And I cannot understate the importance of this.

Our facial features and our ability to make... Expressions with our face are such a huge aspect of communication that we often take for granted. And so while this seems kind of like a passive thing, that it's not all that important, it's actually hugely important, as important as their protection for influences of the digestive and respiratory system, for example, if not more so.

Here's a view of the skull. Obviously, the body is not a paint-by-number system. This has been colored specifically so that you can see the demarcations between each bone. And so we're going to go through all of these bones. Yes, unfortunately, we are going to go through all of these bones.

And we will go through a select set of features on each of those bones when appropriate and when they are indeed most difficult. Because, again, we tend to highlight here those aspects of the lab material that students find most troubling. Here is, so I should point out this is an anterior view, which again I think we can understand and what I've imagined. This is an inferior view.

So assume that the vertebral column itself had been removed, right, so we're looking at an inferior view minus the mandible. This bone is a wonderfully fun bone. Students really love this. This is the vomer bone.

The vomer bone is a small thin plow-like bone that's found in the interior portion of the nasal cavity and helps make up a portion of the nasal septum. So we see that right here. Now you don't need to know any of the particular features of the vomer bone, but you do need to know what the vomer bone looks like.

So again, it's placement here anteriorly, it's placement here inferiorly, and a tad bit posteriorly. Now that's kind of a weird bone to lead into this, but we've got the skull with the calvaria cut off. So this is the skull cap otherwise known as the calvaria.

and we've got a plane that goes through there right and so we've opened that up so now we can see this is a superior view of the internal cranial cavity and from here we see a very different perspective on the skull than what we have seen when we look at the skull intact and some of the most difficult features of this are learning where bones begin and end so keep that in mind where you're working in lab as well as some of these nuanced features of the sphenoid. The sphenoid and arguably the ethmoid are the two most difficult bones here on the inside of the skull. So we'll go through them as we get there, as well as all the openings of the skull. So all these holes that are highlighted here, these are also in a very difficult set of structures for students to master.

All right, so let's look at the the cheekbone, right, and we're going to look at the zygomatic arch, right. So this whole structure here across here is the zygomatic arch and it's composed in part by the zygomatic bone here. This is highlighted in green. We can see it here on a lateral view, here in an inferior view, here in an anterior view. And the zygomatic bone contains on it what we call the temporal process.

And so this green portion here, enlarged, is this portion here, is going to connect to the temporal bone, specifically here, this portion of the temporal bone. And because it's articulating with the temporal bone, we call it the temporal process. It is the process by which the temporal bone touches, and that's why we call it that.

And so therefore, this process is named the temporal process of the zygomatic bone. And that's really important because this structure here is called the zygomatic process of the temporal bone. And it's called the zygomatic process because it's the portion of that bone that reaches out and touches the zygomatic bone. And these are structures that students routinely get confused. They will often think the temporal process is on a temporal bone and the zygomatic process is on the zygomatic bone.

But that's not the case at all. The bone is... The process itself is not named after the bone for which it's a part of, but rather the bone that it is reaching out to and connecting to. So we see that here a little bit more up close. Now, please bear in mind, this figure is wrong.

This is the publisher's mistake, as it turns out, which is, I find humorous considering how much money you pay for the book. This is actually the zygomatic process. process of the temporal bone. So that's this structure right here, zygomatic process of the temporal bone.

And this structure here is the temporal process of the zygomatic bone. Alright, let's look at the mandible. The mandible is a unique bone.

Again, it's our lower jaw. We can see that view here, articulated with the skull here, highlighted in purple. from a lateral view.

Now, there are several structures on here that are important to remember. We've got the coronoid process here, which does not form an articulating portion of the actual temporal mandibular joint, which is the joint that's formed between the skull and the actual mandible. But nonetheless, the coronoid process is a structure that you will need to know for lab, so keep that in mind.

You've got the mandibular condyle here, which is a condyle, by the way, something that you should know, is... a rounded structure that is used for a bone to rock on or move on another bone. The ramus here is that portion of the bone that juts upward to articulate with the actual skull.

The angle of the mandible is seen here, and then finally we have the body of the mandible here. The holes that you see here and there's actually another one paired over on the other side too. You can't see it.

These foramen are called the mental foramen. You want to think of mental as referring to the chin, interestingly enough in this case. Let's talk about cavities of the skull. Cranial cavity by far is the largest cavity in the skull.

It encloses, cushions, and supports the brain. The smaller cavities that we see in the skull are going to be the orbits, they're going to be the oral cavity, they're going to be the nasal cavity, and finally the paranasal sinuses. So there's several other small cavities that exist within the skull.

The largest is by far the easiest to remember and the easiest to associate with the skull. But these other four are not by any means unimportant. Here's a coronal section of the skull where we can actually see a lot of these cavities.

Now again we can identify the cranial cavity is by far the largest, but we've got all these other cavities as well. We've got the two orbits seen here and here. We've got the paranasal sinuses, of which there are many. We've got frontal sinuses here and here.

The ethmoid sinuses, you see them highlighted in red here and here. We've got the nasal cavity, which in and of itself is not a sinus, but it's connected to the sinuses. We've got the maxillary sinus here and here. The only sinus that we're missing here is called the sphenoidal sinus, and that's because this plane will not give you a view of that.

But nonetheless, we'll show you that in a little bit here. Finally, the other cavity we've got is the oral cavity here. The sinuses themselves lighten the skull. They're there to reduce the weight of the skull overall so that it's manageable.

These are lined with an epithelium that produces mucus and helps to humidify in warm inhaled air. Because of this, it helps provide resonance to the voice, and that's actually very important. I think we've all been sick enough that we can remember a time when our voice sounded very nasally, and that's because with a sinus infection, that airflow gets cut off, and all of a sudden our voice sounds greatly different. It lacks that resonance that we would often find with the sinuses. By far one of the creepiest pictures in this book.

These are the scattered view of the different sinuses. And the only ones you haven't seen so far yet are these even farther back, these posterior most ones called the sphenoidal. These are highlighted in orange.

I'm going to show you another view of those here right here. So I've shown you the frontal sinuses, the maxillary sinuses, the ethmoidal sinuses. The last sinus that we haven't talked about is the sphenoidal here, this big sinus here in the very posterior most aspect to the rest of the sinuses.

This is the ethmoid bone, and the ethmoid bone is by far one of the most challenging bones for students to master. And if I can impart a little bit of advice here right up front, you're going to be working with a disarticulated resin cast ethmoid bone, which means that it's not an actual bone, but rather a molded cast of the bone. The reason that you're not working with the real ethmoid bone, disarticulated at least, is because it is so fragile that it will infect. break on you and so you will not get a good specimen to work with.

But the resin cast is really important that you and one of the keys to tackling this properly is making sure that you can actually orient this bone properly. So making sure that you can tell superior from inferior, anterior from posterior view, and lateral view is finally. That's key and crucial because without being able to do that properly Naming off the structures on the ethmoid bone is going to be particularly difficult.

Now from a superior view like this You should be able to see this structure that juts upward and this is important to understand because we're looking this view here is Actually this view here You're seeing only this tiny portion of the ethmoid bone here and the crista Gali looks like a tiny little mohawk that sticks up into the cranial cavity and what's it? What's key and crucial to know here is that you can't actually see the perpendicular plate anywhere near as well as it's labeled here. Perpendicular plate is much more easily seen and in fact very readily seen on the anterior view of the actual ethmoid.

So I'm going to go ahead and skip forward so that we can actually see this. This is an anterior view. And again, we can see this plate that sticks downward. Very long, very impressive.

It's called the perpendicular plate. This is going to help form the nasal septum. And this is the crista galli. Now you can see that that structure actually sticks upward.

This is going to sit next to the brain. This is going to be a portion of the nasal septum that helps form the two different passageways of the nose. So that's key and crucial to understand here.

Now, sutures of the skull. A suture is an immovable... fibrous joint and this is something that you will learn in the articulations. chapter, right?

So these are immovable joints. You don't want these things to move. Remember, these things are the joints that link together are skull bones.

We really don't want those skull bones to move, right? They're fibrous joints. They are held together by a dense regular connect tissue that firmly seals these bones together, right?

And they allow the cranium to grow and expand throughout childhood. Very important aspect of things there. We can see the various different sutures that you need to know in this class in these two views. This is a superior view. This is a posterior view.

And on the superior view, we can see the coronal suture here. The coronal suture lies on a coronal or frontal plane and therefore is defined as or named the coronal suture. And that separates or more to the point actually connects the frontal bone. to the two parietal bones we see here.

The sagittal suture here lies along a sagittal plane and it lies between the two parietal bones here and here. Again because it lies along a sagittal plane it is named after the plane that it lies along. I'm going to go ahead and move down here to the inferior most portion of this figure. So we can see the posterior view. This structure here is not related at all to any plane that it is, but rather it's called the lambdoid or lambdoidal suture.

One of the most misspelled words on the first practical is lambdoid. Now, the key to this is understanding something right up front. It is spelled literally lamb, L-A-M-B, doid, D-O-I-D.

So there's both a... B and a D in the middle of that word. And this is where I get a lot of spelling mistakes. Students typically guess this correctly, but they can't spell it correctly. They either forget the B or they forget the D in the middle.

And so I'm left with either lambdoid or lambvoid, but never lambdoid. Right. And that's something that I want you to remember.

If you've got to say it. Like that, to remind yourself that it's lamb-doid, then do it so that you spell it correctly. Fontanelles are soft spots.

These are, we've all, or most of us, I guess, would have probably had experience with these. We see these on babies. We don't see these, obviously, on adults, thank goodness.

But these are soft spots in the skull, right? And these are regions in which there is still a fibrous membrane remnant. where the cranial bones have not yet totally ossified. So they're literally the soft spots in the infant skull.

They close up by about 15 months in age. The whole idea behind a fontanelle is that it allows the actual cranial bones to overlap so that when the child is passing through the birth canal, it eases the baby's passage for both mother and child. And this is very important. You do not want a great deal of resistance there any more than there already is.

And you don't want this to be even more difficult. Babies often have, obviously, a not yet fully ossified skeleton because it makes it easier, makes it more flexible, including the cranium here can change shape because of the fact that we need to ease the passage of this, make birth as easy as possible on both mother and child. The newborns frequently have a cone-shaped head due to this temporary deformation that occurs when they're passing through the birth canal.

We can see these fontanelles here, the anterior fontanelle, the sphenoidal fontanelle, the mastoid fontanelle, and then finally the posterior fontanelle here, posteriorly. Alright, here's an important term, or important concept. This is something that is more lecture related than lab, and this is just simply being able to list the bones that form the orbital complex, right?

It's seven bones here. Some of them are far more participa- far more largely participating in the orbit than others, but still all seven of them help to form the orbit. The frontal bone, the lacrimal bone, the zygomatic, the maxilla, the palatine, the ethmoid, and the sphenoid bone all play a role in forming the orbit. So we can see them here, right?

Now the frontal bone is a fairly large portion of it. as is the sphenoid and the zygomatic. The maxillary bone also plays a very large role. And then we get some very tiny roles. that are played here.

We get the Palatine bone here highlighted in this kind of lime green, not a huge role, but still is there. We've got the Ethmoid bone here, not a huge role, but yet still there. And then finally this little blue kind of island there in the middle, this is the Lacrymal bone.

So there are three bones for which play a relatively small role, and then four bones that play a much a much larger, much more significant role in the formation of the actual orbit. This is a view of the Palatine bone. This contributes very little to the actual orbit, but it's still there. And the point I want to make is that you need to be able to identify the Palatine bone based upon view, as well as a structure on the Palatine bone known as the horizontal plate.

Now the Palatine bone looks like it's shaped like an L, albeit a very ornate L, but nonetheless still like an L. And the point I'm trying to make here is that this plate here that helps make up a portion of the hard palate within the actual oral cavity. This is the horizontal plate and you can see that horizontal plate here and you can see it from a very different view here in this mid-sagittal section.

The auditory ossicles are the tiny ear bones that allow us to be able to actually perceive the sense of hearing, right? So these are the tiny little bones. housed within the petrous region of each temporal bone and they rattle against each other to bring vibrations from sound waves into the inner ear so that we can actually form a sensation from them.

These three bones, the malleus, otherwise often called the hammer, you must call it, you must be able to spell it as the malleus, the incus, otherwise affectionately termed the anvil, and then finally the stapes, which again more affectionately called the stirrup. You must be able to name them and spell them by the Malleus, Incus, and Stapes. The hyoid bone, not associated with the ear at all.

Hyoid bone is a slender, curved bone located in the inferior portion of the skull, but it's not a portion of the skull at all. It actually doesn't articulate with the skull at all. As a matter of fact, it doesn't articulate with any bone on the skeleton.

It is the only floating bone in the body with no articulations to other bones. And it's a particularly important bone because it serves as sites for attachment of muscles for the tongue and the larynx, as well as ligaments. We also know that the hyoid bone is more important for us in terms of a forensic perspective, right?

So it often is one of many pieces of information that can be used to determine whether somebody committed suicide versus somebody was actually strangled and then posed to look like they committed suicide. So. Hanging if somebody hangs themselves most of the time the highway bone will not break because the the actual Orientation of the rope simply does not support that right it Should not break the hyoid because it shouldn't be up against the hyoid However, if somebody is strangled most of the time that force is coming from directly in front of or directly behind That person and it puts a great deal of pressure on the hyoid and the hyoid often snaps and so this bone has particularly important forensic significance and so one of the reasons why it's important as well this is the hyoid bone the only thing you have to know about the hyoid bone is to be able to look at it and identify the hyoid bone the vertebral column again this this material here is greatly lecture material until we actually start looking at the individual bones.

But the point I want to make here is that it's actually 26 bones in composition. You've got 24 individual vertebrae. You've got 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 sacral. These are all fused together.

And then finally, 3 to 5 coccyx bones, or 3 to 5, sorry, vertebrae in the coccyx. The vertebral column itself has several functions. It provides vertical support for the body, as well as a protection for the delicate spinal cord. This is actually crucial.

That's the reason why it's there. Another big aspect of the actual vertebral column, it supports the weight of the head and it allows for the transfer of weight through the axial skeleton to the appendicular So it serves as a transfer of weight, which is again extremely important. This is a view of the vertebral column. Now let's go through the individual bones, right, so we get a better view of these. There are some common structures that we see amongst all the actual bones here.

All of them have a body, which is this big portion here, upon which the rest of the other vertebrae actually sit upon. It's called the body or the centrum. Now, they've all got a spinous process here, and they've all got transverse processes here and here. And the portion of the bone that links the spinous process to the transverse process is this bridge of bone here that we call the lamina. We see a lamina here, and then a lamina here.

And then the spinous process and the transverse processes are bound to the body. through yet another bridge of bone here. And this bridge of bone here that is encircling this whole area is called the pedicle. Now there are two very special vertebrae, the atlas and the axis. Atlas is C1, axis is C2.

These are the only vertebral bones that you will have to be able to number. The rest of them you can just simply identify by cervical or thoracic. But in this case you must be able to identify them specifically.

And that's because they don't look like any of the others. They're very unique. The atlas is very flattened in its orientation.

It looks much more like a ring than anything else. And that's intentional because this large process, this portion of the bone here, is meant to actually surround this vertical lying structure of C2 or the axis. And this bone that sticks upward here is called the dens.

D-E-N-S. Now, it actually has another name. It's a little bit more complicated.

Almost no students ever use it. And that's called the edontoid process. Most students call it dens because dens is far, far easier to spell.

Either way is fine, though. The dens sticks upward and allows the formation of a pivot joint to form here. So this allows...

your head to actually shake. So if you can shake no to say no. In part because of this pivot joint that's formed here between C1 and C2.

This is the sacral bone. The structures here that you need to know are at the very top of the bone where the last lumbar vertebra rests upon. This portion here is called the sacral promontory. This is an anterior view.

Now, if we go to a posterior view here, we can see the median sacral crest here of these ridges of bumps that form the back of these. These are actually the original spinous processes from the vertebra themselves. And the other structure here that you need to see, and it's actually better seen here on their actual real bones. That is this surface here.

This is called the auricular surface. There's one over here and one over here. Now, when the sacrum articulates with the hip bone, otherwise referred to as the ascoxa, right?

That's a bone that you want to keep in mind, the ascoxa. When the ascoxa and the sacrum come together, that's called the sacroiliac joint. But when the ascoxa is not present here, we're looking at that service that helps... form that joint on the sacrum is referred to as the auricular surface. So please keep that in mind.

It's not a joint unless two bones are actually connected together. The thoracic cage consists of thoracic vertebrae posteriorly, the ribs laterally, and the sternum anteriorly. So it's a chain, a cage formed by these three sets of structures.

And it's a correct... protective cage around things that are particularly vital to us, the heart, the lungs, the trachea, the esophagus. It provides us a flurry of attachment points for muscles, pardon me, as well as having a relatively complicated sternum. That sternum itself is actually split up into three separate portions, the xiphoid process.

the body and finally the manubrium. So we're gonna we're gonna highlight those first here. The manubrium is this superior most portion of the sternum we see it here. The body is here. And then finally, you have the xiphoid process down here, that little point that comes off the inferior end of that sternum.

Here are the ribs, ribs 1 through 11 all the way down. Ribs 1 through 7 are actually what's called the true ribs. That means that these ribs have their own costal cartilage that connect them directly to the actual sternum.

The false ribs, 8 through 12, do not. They share, sorry, 12 ribs there, they share a connection to another costal cartilage to allow them to connect to the actual sternum. Or they don't at all connect. These ribs 11 and 12 are actually called floating ribs, specifically because of the fact that they share no attachment site whatsoever with the sternum. So they don't connect at all to the sternum.

All right, for ribs, both males and females have 12 pairs of ribs. The true ribs are 1 through 7. False ribs are technically 8 through 10. And then finally you have the floating ribs, which are 11 and 12. So I've explained all that up front here. So this slide is much more of an actual summary or overview. And here is a view of the rib. The rib itself is actually not complicated.

So it's not a complicated structure. bone to understand. However, there are a couple things that I do want to make clear because there are a couple of big mistakes that students make. The shaft of a bone is the the shaft that you can grab onto and hold on to.

Think of a shaft there, right? So if I were to take a pipe cleaner and wrap it around the entire bone here and say name this structure be specific, in lab I'm asking for the shaft of the bone. Now Keep that in mind because there is actually a shallow groove that runs along this curved portion of the actual bone here.

And this groove is called the costal groove. And we will often paint this structure so that you can see this indentation on the bone. And for whatever reason, it's often misidentified as the shaft. Remember, if I wrap an entire pipe cleaner around it, that is the shaft.

However, If there's just a painted indentation along that bone, that's actually the costal groove and not the shaft. Now, this is how it articulates at the head of the actual rib and the tubercle of the rib that actually form these articulations between the vertebra. And that allows the ribs to sit in place properly.

Transcript for:Understanding the Axial Skeleton

Transcript for:
Understanding the Axial Skeleton