The pharynx is a passage common to both the respiratory and the digestive systems. It is a musculomembranous junction of the respiratory and the digestive tubes between the oral and nasal cavities rostrally and the esophagus and larynx caudally. It consists of a nasal, oral, and a laryngeal part. Here is a sagittal section of the head to show the three major parts of the pharynx.
For orientation purposes, Let us first identify some of the landmarks that we can easily recognize. Here is the hard palate and here is the soft palate. Here is the tongue. Let us again see the soft palate as this structure serves as the boundary between the parts of the pharynx.
The nasopharynx is located dorsal to the soft palate and it is the respiratory channel of the pharynx. The oropharynx is ventral to the soft palate. This is for the digestive channel of the pharynx.
And finally, the laryngopharynx is a part where the air from the nasopharynx crosses to reach the larynx and the food and water from the oropharynx cross into the esophagus. Thus, it is considered as both respiratory and digestive channel. Here is a schematic diagram showing the parts of the pharynx.
Here is the nasopharynx, the oropharynx, and the laryngopharynx. The piriform recess is a continuation on the floor of the oropharynx on either side of the larynx. This is the site in dogs where the bones may become large and close the laryngeal opening of the airway.
It is also important to locate the five openings in the pharynx. The pharyngeal opening or the isthmus of phoses is the opening from the oral cavity to the oropharynx. The caudal nares or quanae is the osseous opening between the caudal nasal cavity and the nasopharynx.
The pharyngeal opening of the auditory tubes are the slits in the lateral walls of the nasopharynx leading to the middle ear. The laryngeal opening or the aditus laryngeus is the opening to the larynx surrounded by rostral laryngeal cartilages. The esophageal opening or the aditus esophagi is the opening at the caudal end of the laryngopharynx into the esophagus. The esophagus is a connecting tube between the laryngopharynx and the stomach. In medium-sized dogs, it is approximately 30 cm long and 2 cm in diameter when it is collapsed.
The esophagus is divided into three regions based on their location. Cervical, thoracic, and abdominal region. The cricopharyngeal muscle acts as a cranial sphincter while the cardiac sphincter of the stomach acts as the caudal sphincter.
In the neck, the esophagus lies dorsal to the trachea then shifts to the left of the trachea as you can see on this image. Inside the thorax, it returns to a position dorsal to the trachea, passing over the tracheal bifurcation. The esophagus enters the abdominal cavity via the esophageal hiatus of the diaphragm.
Here is a schematic illustration of the diaphragm. There are three openings present, caval foramen for the entrance of the vena cava, aortic hiatus for the entrance of the aorta, and the esophageal hiatus for the esophagus. Here is the esophageal hiatus. After the esophagus, the food will enter the stomach.
The stomach is the largest dilation of the gastrointestinal tract caudal to the esophagus. It is a musculoglandular organ intersposed between the esophagus and the small intestine. It varies greatly in size.
It stores and partly mixes the food, and its intrinsic glands intermittently add enzymes, mucus, and hydrochloric acid for the digestion of food. Now let us locate the different parts of the stomach. The greater curvature is a long convex surface of the stomach extending from the cardia to the pylorus.
The greater amentum is attached to this structure. Also present at the greater curvature is the spleen. We will discuss further the anatomy of this organ when we discuss the lymphatic system.
The lesser curvature, on the other hand, is a short, concave surface of the stomach extending from the cardia to the pylorus. it is the attachment site of the lesser omentum the cardia or the cardiac ostium is the opening of the oesophagus into the stomach the cardiac sphincter is present and served as the functional sphincter at the entrance the funnel-shaped cardia is rather wide which may be related to the ease of the dog in vomiting the fundus is the blind expanded part of the stomach at the left side above the level of the cardia while the body is the largest part of the stomach where the actual mixing of the food is happening the pylorus is the distal opening of the stomach phyloric sphincter is the functional sphincter at the distal end gastric folds or rugae are internal folds of the stomach mucosa to increase the surface area the parietal surface of the stomach lies against the diaphragm and liver while the visceral surface is in contact with the adjacent abdominal organ the small intestine extends from the pylorus of the stomach to the iliocolic orifice leading into the large intestine it is the longest portion of the alimentary canal having an average length in the living animal of three point five times the length of the body it is divided into duodenum jejunum and ilium In this slide, you can see mostly the jejunum with its mesentery called the mesojejunum. Let us discuss each segment.
The duodenum is the first of the three divisions of the small intestine. It is closely attached to the right side of the dorsal body wall by a short mesentery called the mesoduodenum. It has a descending, transverse, and an ascending portion. Here is an enlarged...
portion of the descending duodenum. After the stomach, it descends forming the cranial part of the duodenum before becoming the transverse portion. At the descending duodenum, there are two openings. The major duodenal papilla is for the pancreatic duct while the minor duodenal papilla is for the accessory pancreatic duct. This is important for the release of the secretion of enzymes and digestive fluids like bile from the liver to the duodenum for digestion.
We should also be familiar with the different flexures present at the duodenum. Cranial flexure separates the descending duodenum from the pyloric end of the stomach. The caudal flexure separates the descending duodenum from the ascending duodenum, while the duodenal-jejunal flexure separates the ascending duodenum from the jejunum.
The jejunum is the longest and the most movable part of the small intestine, while the ileum is the short terminal part of the small intestine. In this schematic drawing of the dog gastrointestinal tract, here is the jejunum and here is the ileum. The jejunum is attached to the body wall by the mesojejunum, while the ileum is attached via the mesoileum. The mesojejunum and the mesoileum are often collectively referred to as the great mesentery. In situ, the jejunum occupies almost the entire of the abdominal cavity.
If we remove the greater omentum, most of the jejunum will be exposed. The distinction between the ilium and the jejunum is defined by the proximal extent of the iliosecal folds. Take note that the ilium is more muscular and firmer than the jejunum.
It It terminates at the cecocolic junction of the large intestine forming the ileal orifice. The large intestine is composed of the cecum, the ascending colon, the transverse colon, the descending colon, and the rectum and anal canal. The large intestine is short and specialized.
The large intestine of dog is much simpler than that of other domestic animals. Neither haustra nor teinia exist, nor are flexures or spirals are present. In general, it is a simple tube, only slightly larger in diameter than the small intestine. Its most important function is the dehydration of the fecal contents.
The cecum is commonly described as the first part of the large intestine, but this is not true in dogs as the ilium communicates with the ascending colon. The cecum exists as a diverticulum of the ascending colon. The cecum of dog is spiral S shape as shown here. It's a blind pouch with a tapering apex, a body, and a base. It is located at the right side of the abdominal cavity of the dog.
It has a communication with the ascending colon via the psychocolic orifice. For orientation purposes, here is the psychum placed between the junction of the ilium and the ascending colon. And here is the psychocolic orifice which served as the communication site. between the cecum and the colon.
The colon is divided into ascending, transverse, and descending segments and their connecting flexures. This naming is adapted to human anatomy but does not strictly represent an ascending, transverse, and descending position in an animal. The colon lies in the dorsal part of the abdominal cavity and it is shaped like a shepherd's crook or question mark.
The colon is held by the mesocolon. It is noted that most of the variation in the morphology of the colon is present at the ascending colon. In dog, it is simple and short, located on the right side of the median plane, it is not succulated. Like the dodenum, the colon has flexures.
The right colic flexure separates the ascending and the transverse colon. while the left colic flexure separates the transverse and the descending column. The rectum is the distal portion of the large intestine, beginning at the pelvic inlet and ending at the anal canal.
It is held close to the body wall by connective tissue and muscles. The anal canal is the short terminal part of the large intestine. This consists of the internal anal sphincter, or a smooth muscle, and an external anal sphincter composed of a striated muscle.
Anal sac are paired pouches located in between the internal and external anal sphincter. The anal sac opens at the anal canal at a position comparable to the 4 and 8 o'clock. The anus is the external opening of the intestine.
And that concludes our discussion on the first part of Module 5. which is the anatomy of the digestive tract. Take note that we discuss from segment to segment based on the direction of the food upon ingestion to digestion until absorption and later defecation. You may now proceed to part 2 of this module which is discussing the anatomy of the digestive accessory organs like salivary glands, liver, and the pancreas.