THE MUSCLES OF THE HAND. 607 



its greatest breadth over the distal portions of the metacarpals, and is continued 

 onward as four more or less distinct bands, which are inserted into the integument at 

 the bases of the second, third, fourth, and fifth fingers. A little below the lower 

 edge of the aponeurosis transverse bands of fascia (fasciculi transversi) stretch across 

 between the same fingers, lying immediately beneath the skin and being connected 

 to a greater or less extent with one another. These bands constitute the supe7'ficial 

 transverse metacarpal ligamejit beneath the webs of the fingers. 



The anterior annular ligament (ligamentum carpi transversum) (Fig. 578) is a 

 strong band which stretches across from the trapezium and scaphoid bones of the 

 carpus on the radial side to the pisiform and unciform bones on the ulnar side, 

 forming a bridge across the groove on the anterior surface of the carpus which trans- 

 mits the tendons of the long flexors and of the flexor carpi radialis and the median 

 nerve. The canal so formed is divided by a partition into a small radial compart- 

 ment through which the flexor carpi radialis passes, and a large ulnar one which 

 gives passage to the other structures mentioned. The tendons are enclosed within 

 synovial sacs which extend downward to about the middle of the palm and upward 

 to a short distance above the upper edge of the ligament. The sac which surrounds 

 the flexor longus poUicis is usually separate from that which surrounds the remaining 

 tendons of the ulnar compartment ; occasionally the portion surrounding the tendons 

 of the index-finger is also separate. 



Towards either side of the palmar surface of the hand the palmar fascia forms a 

 thin covering for thenar and hypothenar eminences formed by the superficial muscles 

 of the thumb and the little finger respectively. Upon the dorsal surface the fascia is 

 thin, and is continued downward from the lower border of the posterior annular 

 ligament over the extensor tendons to the fingers, where it unites with the aponeu- 

 roses of the tendons. 



(a) THE PRE-AXIAL MUSCLES. 



The pre-axial muscles of the hand are to be regarded, from the comparative 

 stand-point, as being arranged in five layers. Although these layers become con- 

 fused to a certain extent in the human hand, it will, nevertheless, aid in the proper 

 understanding of their relations to group them according to the primary layers from 

 which they are derived. 



{aa) The Muscles of the First Layer. 



1. Palmaris brevis. 4. Flexor brevis pollicis. 



2. Abductor pollicis. 5. Abductor minimi digiti. 



3. Opponens pollicis. 6. Opponens minimi digiti. 



7. Flexor brevis minimi digiti. 



The most superficial layer of the palmar muscles in the lower vertebrates takes 

 its origin from the palmar aponeurosis. The greater portion of the layer, as has 

 already been pointed out, becomes converted in the mammalia into the palmar por- 

 tions of the tendons of the flexor sublimis digitorum, and it is only towards either 

 margin of the hand that it persists as muscles, which show indications of their 

 primary relations in their origin from the palmar aponeurosis or the anterior annular 

 ligament. 



I. Palmaris Brevis (Fig. 576). 



Attachments. — The palmaris brevis is a thin quadrangular sheet which lies 

 immediately beneath the skin of the hypothenar eminence. It arises from the 

 proximal portion of the ulnar border of the palmar aponeurosis and is inserted into 

 the skin of the ulnar border of the hand. 



Nerve-Supply. — By the superficial division of the ulnar nerve from the first 

 thoracic nerve. 



Action. — To wrinkle the skin upon the ulnar border of the hand, deepening 

 the hollow of the hand. 



Variations. — The muscle may be greatly reduced in size and is occasionally wanting. 



