THE ANTIBRACHIAL MUSCLES. 597 



of the interosseous membrane. It usually possesses also an origin by means of a 

 slender slip from the coronoid process of the ulna or the medial epicondyle of the 

 humerus. The muscle-fibres pass into a strong tendon at the middle of the forearm, 

 and this passes downward beneath the lateral part of the annular ligament and 

 extends along the volar surface of the thumb to be inserted into the base of its ter- 

 minal phalanx. 



Nerve-Supply. — By the anterior interosseous nerve from the eighth cervical 

 and first thoracic nerves. 



Action. — To fllex the terminal phalanx of the thumb ; continuing its action, it 

 will also flex the proximal phalanx and assist in the flexion of the hand. 



Relations. — In the forearm it is covered by the flexor sublimis digitorum, the 

 flexor carpi radialis, and the brachio-radialis, and has resting upon it the radial ves- 

 sels. Deeply it is in relation with the pronator quadratus and the wrist-joint. In 

 the hand its tendon is covered by the opponens poUicis and the flexor brevis poUicis, 

 and it rests upon the adductor pollicis. 



Variations. — The head from the coronoid process or medial epicondyle of the humerus is 

 sometimes absent and the muscle is frequently connected with the flexor profundus digitorum or 

 even fused with it. 



Occasionally there arises from the lower part of the anterior and external surfaces of the 

 radius a muscle which has been termed \h& flexor carpi radialis brevis. Its insertion varies some- 

 what, being sometimes on one of the carpal bones, at other times on either the second, third, or 

 fourth metacarpals, and at others, again, into the transverse carpal ligament. Although asso- 

 ciated by name with the flexor carpi radialis, it is more probably a derivative of the deeper layer 

 of the flexor musculature and is supplied by the volar interosseous branch of the median nerve. 



The majority of the variations of the flexor longus pollicis and flexor profundus digitorum 

 find an explanation in the historical development of the muscles. In the lowest group of the 

 mammalia, the monotremata, the two muscles are fused with each other and also with the 

 flexor sublimis to form a common long flexor, from the tendon of which the tendons of the flexor 

 sublimis arise. In slightly higher forms this common flexor can be seen to be composed of five 

 portions, which, from their points of origin and relations, may be termed the condylo-ulnaris, 

 condylo-radialis, centralis, ulnaris, and radialis, and as the scale is ascended one finds at first a 

 part of tj;ie condylo-ulnaris and later the whole of that portion separating from the common mass 

 and joining the tendons of the sublimis. In still higher forms the centralis and condylo-radialis 

 portions follow the example of the condylo-ulnaris, the flexor sublimis digitorum in man being 

 composed of these portions of the common mass. 



The ulnaris and radialis portions remain, as a rule, united and, after the separation of the 

 superficial portions is completed, constitute the flexor profundus. In man and a few other forms 

 the radialis separates from the ulnaris to form the flexor longus pollicis. 



The connections which occur between the sublimis, profundus, and flexor longus pollicis 

 are consequently to be regarded as relics of the historical development of the muscles, as the 

 incomplete separation of a common flexor mass. 



In the lower terrestrial vertebrata the superficial and deeper layers, corresponding practi- 

 cally to the sublimis and profundus (plus the flexor longus pollicis), are distinct, their fusion in 

 the monotremes being a secondary condition, which forms the starting-point for the differentia- 

 tion of the mammalian arrangement of the muscles. In these lower forms both layers insert 

 into the palmar aponeurosis, the extension of the deeper layer to the digits being due to the 

 separation of the layer of the aponeurosis to which the deeper muscle-layer is attached and its 

 differentiation into tendons. 



It may be added that in the lower vertebrates the palmaris longus is not represented as a 

 separate muscle, and it is to be regarded as a portion of the superficial sheet which has retained 

 its original relations to the palmar aponeurosis, its occasional absence being ascribed to its 

 sharing the history of the flexor sublimis and being incorporated in that muscle. 



{cc) The Deep Layer. 

 I. Pronator quadratus. 



I. Pronator Quadratus (Fig. 588). 



Attachments. — The pronator quadratus is a flat quadrangular sheet extending 

 across between the lower portions of the radius and ulna. It arises from the volar 

 surface of the ulna and passes laterally and slightly distally to be inserted into the 

 lateral and anterior surfaces of the lower end of the radius. 



Nerve-Supply. — By the anterior interosseous branch of the median nerve from 

 the seventh and eighth cervical and the first thoracic nerves. 



Action. — To pronate the forearm. 



