592 HUMAN ANATOMY. 



flexors qr extensors of the digits, this extension being due in some cases to the 

 differentiation of the fascia of the hand into tendons continuous with those of the an- 

 tibrachial muscles, in other cases to end-union of antibrachial and hand muscles. It 

 will be convenient, however, to regard the long flexors and extensors of the digits, 

 formed in this way, as antibrachial muscles. 



Comparatively studied, an arrangement of the antibrachial muscles in distinct 

 layers is clearly perceivable, three layers being found in the pre-axial and two in the 

 post-axial muscles. In both cases the superficial layer takes its origin from the 

 humerus, while the remaining layers are attached above to the bones of the forearm. 

 Secondary adaptations have in some cases interfered with the distinctness of the 

 layers, but the primary conditions will be taken as the basis for the classification of 

 the muscles. 



The antibrachial fascia completely invests the muscles of the forearm and is 

 the downward continuation of the brachial fascia. It is especially strong upon the 

 dorsal surface of the forearm, where it is attached to the olecranon process and the 

 entire length of the posterior border of the ulna, and anteriorly it is strengthened in 

 its upper part by the fibres of the semilunar fascia of the biceps. At the wrist it is 

 attached to the bones of the forearm and carpus, and becomes thickened by trans- 

 verse fibres to form the dorsal and volar carpal ligaments. 



The anterior arundar ligament (ligamentum carpi volare) lies on the anterior sur- 

 face of the wrist, covering the flexor muscles in that region (Fig. 577). Laterally 

 and medially it is connected with the dorsal ligament. This, the posterior annular 

 ligament (ligamentum carpi dorsale), is a stronger transverse band on the posterior 

 surface of the wrist, and is attached laterally to the outer surface and to the styloid 

 process of the radius, and passes inward and slightly downward to the styloid process 

 of the ulna and to the pisiform and cuneiform bones, making attachments to the ridges 

 on the posterior surface of the radius and ulna and thus converting the six intervening 

 grooves into canals which lodge the tendons of the long extensor muscles (Fig. 579). 

 Beginning at the radial side, the first canal transmits the tendons of the extensor ossis 

 metacarpi pollicis and the extensor brevis poUicis ; the second, the tendons of the two 

 extensores carpi radiales ; the third, that of the extensor longus pollicis ; the fourth, 

 those of the extensor communis digitorum and the extensor indicis ; the fifth, that of 

 the extensor minimi digiti ; and the sixth, that of the extensor carpi ulnaris. Each^ 

 of the canals is lined by an independent synovial membrane. 



(«) THE PRE-AXIAL MUSCLES. 

 {aa) The Superficial Layer. 



1. Pronator radii teres. 3. Palmaris longus. 



2. Flexor carpi radialis. 4. Flexor carpi ulnaris. 



5. Flexor sublimis digitorum. 



I. Pronator Radii Teres (Fig. 576). 



Attachments. — This muscle (m. pronator teres), thick and band-like, arises by 

 two heads {a) from the inner condyle of the humerus, the adjacent intermuscular 

 septa, and the deep fascia, and {b^ from the medial border of the coronoid process of 

 the ulna. It passes downward and laterally and is inserted into about the middle of 

 the outer surface of the radius. The median nerve passes downward between the 

 two heads. 



Nerve- Supply. — By the median nerve from the sixth cervical nerve. 



Action. — To pronate and flex the forearm. 



Variations. — The pronator teres is formed by a combination of portions from the super^ 

 ficial and deep layers of the forearm musculature, the condylar head representing the superficial 

 portion and the coronoid head the deep one. In the lower mammals the pronator quadratus 

 frequently extends well up towards the elbow-joint, and the coronoid head of the pronator 

 teres represents the uppermost portion of this muscle, its lower portion persisting as the pro- 

 nator quadratus. Not infrequently the coronoid portion of the muscle is completely separate 

 from the condylar head, or it may be rudimentary or represented only by a connective-tissue 



