598 HUMAN ANATOMY. 



Variations. — The pronator quadratus usually occupies about the lower fourth of the fore- 

 arm, but it may be considerably reduced or, on the contrary, may extend as high as the middle 

 of the forearm or even higher. It represents the lower portion of a muscle-sheet which extends 

 in some of the lower mammals almost the entire length of the forearm, the upper portion of 

 this sheet being represented, as already pointed out, by the coronoid head of the pronator teres. 



{d) THE POST-AXIAL MUSCLES. 



The post-axial muscles of the forearm may be regarded as consisting of two 

 layers, the more superficial of which arises from the external condyle of the humerus, 

 while the deeper one is attached to the bones of the forearm. As was the case with 

 the pre-axial muscles, constituents of both layers have extended into the hand to act 

 as extensors of the digits. 



{aa) The Superficial Layer. 



1. Brachio-radialis. 4. Extensor communis digitorum. 



2. Extensor carpi radialis longior. 5. Extensor minimi digiti. 



3. Extensor carpi radialis brevior. 6. Extensor carpi ulnaris. 



I. Brachio-Radialis (Fig. 576). 



Attachments. — The brachio-radialis, sometimes termed the supinator loiigus, 

 arises from the external condylar ridge of the humerus and from the lateral inter- 

 muscular septum. Its fibres form a strong muscle which, at about the middle of the 

 forearm, passes into a tendon which is inserted into the base of the styloid process of 

 the radius. 



Nerve-Supply. — By the musculo-spiral nerve from the fifth and sixth cervical 

 nerves. 



Action. — To flex the forearm. If the arm be in a position of complete prona- 

 tion, it will produce a slight amount of supination. 



Relations. — In its upper part it is in contact medially with the brachialis anti- 

 cus, a portion of whose lateral border it covers, and with the radial nerve. Below it 

 rests upon the upper portion of the extensor carpi radialis longior, the supinator, the 

 pronator teres, the flexor sublimis digitorum, and the radial artery and nerve. It is 

 crossed near its insertion by the tendons of the abductor longus pollicis and extensor 

 brevis pollicis. 



Variations. — The brachio-radialis is sometimes wanting. It may be inserted a consider- 

 able distance above the base of the styloid process of the radius, a condition characteristic of 

 the lower mammals, or it may pass as far down as the carpal bones or even^^the base of the 

 third metacarpal. 



2. Extensor Carpi Radialis Longior (Figs. 576, 579). 



Attachments. — The longer of the radial carpal extensors (m. extensor carpi 

 radialis longus) lies immediately posterior to the brachio-radialis. It arises from the 

 lower third of the external supracondylar ridge of the humerus, the external inter- 

 muscular septum, and the extensor tendon common to it and the neighboring super- 

 ficial muscles. About the middle of the foiearm it is continued into a tendon which 

 passes beneath the posterior annular ligameiu in the second compartment, along with 

 the extensor carpi radialis brevior, and is iiiserted into the base of the second meta- 

 carpal. 



Nerve-Supply. — By the deep division of the musculo-spiral nerve from the 

 sixth and seventh cervical nerves. 



Action. —To extend and slightly abduct the hand. 



Variations. — The extensor carpi radialis longior is occasionally fused with the extensor 

 carpi radialis brevior. It may send tendinous slips to the first and third metacarpals and to the 

 trapezium. 



3. Extensor Carpi Radialis Brevior (Fig. 579). 



Attachments. — The shorter radial carpal extensor (m. extensor carpi radialis 

 brevisj is fused with the neighboring superficial extensors where it arises from the 



