we're going to use this glove today to draw the muscles of the thumb hopefully to clarify the anatomy precisely of how the muscles surround the thumb we're going to start with the extrinsic saan the dorsal aspect and first we'll start with the extensor pollicis longus it arises from the posterior surface of the ulna in the middle third moves distally through the third dorsal compartment of the dorsal retinaculum at the wrist and if you extend your thumb you can see it we will follow the EPL out to its insertion distal to the interphalangeal joint now in addition to the EPL being a single tendon we also want to draw the dorsal apparatus which is a group of fibers that starts at the metacarpal phalangeal joint and is somewhat triangular in shape similar to the dorsal apparatus of the fingers and continues with the EPL the EPL becomes essentially part of that so we'll come back to the store Xalapa radisson demonstrate how the small intrinsic muscles insert into it the extensor pollicis brevis is the next extrinsic muscle we'll consider it arises from the posterior surface of the radius instead of the ulna and it proceeds distally through the first dorsal compartment ending with its insertion distal to the metacarpal phalangeal joint having a little difficulty with my ink here let's see if we can get something on there that's much better okay so the EPB sensor pollicis brevis even though it's an extrinsic normally would be described as inserting here we know that it can be variable and it can go more distally the abductor pollicis longus also starts on the posterior surface of the ulna as well as the interosseous membrane as well as the radius so the APL has a much larger origin moves distally through the same first compartment as the EPB and is normally described as inserting just distal to the CMC joint however we know that this has a lot of variability as well and often has multiple insertions and may extend more distally than what's drawn here now let's turn to the palmar surface and to the flexor pollicis longus the flexor pollicis longus arises obviously from the anterior surface it arises from the interosseous membrane the medial border of the ulna as well as the medial epicondyle so the FPL has a rather large forearm origin it precedes distally through the carpal tunnel along with the flexors to the fingers turns and begins to enter the pulley system of the thumb at the level of the metacarpal phalangeal joint and inserts just distal to the interphalangeal joint so the FPL now we've clearly drawn the next muscle we're going to consider is an intrinsic muscle so I'm going to change colors to a red marker hopefully to keep this visually a bit clearer for you the opponent's pollicis is a muscle that arises from the flexor retinaculum and the tubercle of the trapezium and it is a somewhat triangular shaped muscle that inserts along the entire surface of the first metacarpal I'm going to put lines across it so that we can clearly identify it as a muscle and as we know that's what pulls that first metacarpal over into rotation it does not go distal to the MP joint the next muscle we're drawing is the largest muscle in the hand and that is the adductor pollicis it has two heads the oblique head arises from the capitate and from the base of both the second and third metacarpals and it too is a somewhat triangular muscle that then insert through the web space and into the dorsal apparatus now we're drawing on the surface and we are unable to draw this accurately because this muscle is deep inside the web space but you can appreciate that the muscle goes through the web space and inserts into the dorsal apparatus as well as an insertion into the base of the proximal phalanx the transverse head of the adductor pollicis goes from the third metacarpal across and it goes specifically usually into the dorsal apparatus so both of these would insert distal to the metacarpal phalangeal joint therefore the abductor can flex the MP and extend the IP as well as add up I'm also going to draw these muscles so that we can identify them the two bellies of the adductor pollicis okay we now have those two intrinsic muscles and now we're going to look at the muscles that over lie the opponent's pollicis so I'm going to switch to the color blue we're going to look at the flexor pollicis brevis it has two heads one is the d-pad which actually arises from the capitate as well as the flexor retinaculum excuse me the trapezoid as well as the capitate not the flexor retinaculum will make that blue and the adjacent muscle belly of the flexor pollicis brevis which is the superficial belly which arises from the flexor retinaculum and the tubercle of the trapezium these two go to insert into the dorsal apparatus so just like the adductor the flexor pollicis brevis can flex the MP joint and extend the IP joint because of the insertion into the dorsal apparatus now the last thumb muscle is the abductor pollicis brevis often call the most important thumb muscle that lies on the radial aspect of the first metacarpal superficial to the opponent's pollicis adjacent to the flexor pollicis brevis and it - along with the flexor pollicis brevis inserts into the dorsal apparatus as well as the proximal phalanx it arises it has a rather broad or origin because it arises from the flexor retinaculum trapezium and scaphoid so it has a firm origin so that it can generate some power now we've just drawn the eight thumb muscles but the muscle we have not drawn is actually a muscle to the index finger which can be considered an influence on the thumb especially the thumb CMC joint and that muscle is the first dorsal interosseous as we know it has two bellies it has an owner belly which arises from the first metacarpal and that owner belly inserts into the radial tubercle excuse me usually that inserts into the dorsal apparatus I think the owner belly the radial belly however it arises from this first metacarpal and this radial belly which also goes up and inserts into the tubercle and perhaps also the dorsal apparatus that is attached to the base of the metacarpal and therefore has some influence on stabilizing the first metacarpal so we've drawn all of the muscles schematically that can influence thumb motion I hope this has helped you to clarify where and how the muscles originate and where they attach and therefore how they influence thumb motion