PRACTICAL CONSIDERATIONS : WRIST AND HAND. 613 



Variations. — The first volar interosseus is the most slender of the series and is covered by 

 the oblique portion of the adductor pollicis, with which it may be practically incorporated. 

 Occasionally it is so reduced in size as to appear to be wanting. 



2. Interossei Dorsales (Fig. 590). 



Attachments. — The dorsal interossei are also four in number and lie in the 

 intervals between the metacarpal bones, dorsal to the volar interossei. Each is a 

 bipinnate muscle arising from the adjacent surfaces of the metacarpals which bound 

 the interspace in which the muscle lies. The first and second muscles, counting 

 from the radial side, are inserted into the radial side of the base of the first phalanx 

 and into the membranous expansion of the extensor tendons of the second and 

 third fingers, while the third and fourth are inserted similarly into the ulnar sides 

 of the third and fourth fingers. 



Nerve-Supply. — By the deep division of the ulnar nerve from the eighth 

 cervical and first thoracic nerves. 



Action. — The first and fourth muscles draw the second and fourth fingers 

 away from the third, while the second and third draw the third finger radially or 

 ulnarly, as the case may be. All the muscles flex the first phalanx of the digits to 

 which they are attached. 



Variations. — Occasionally the second dorsal interosseus is inserted into the base of the first 

 phalanx of the index-finger, upon its ulnar side. 



{b) THE POST-AXIAL MUSCLE. 



Normally no post-axial muscles exist in the human hand. Occasionally, however, an ex- 

 tensor brevis digitorivn nianus is more or less perfectly developed. It arises from the dorsum 

 of the carpus, or sometimes from the lower end of the radius and ulna, and passes distally into 

 a varying number of tendons. Most frequently the muscle is small and gives rise to but a single 

 tendon, which joins with the tendon of the extensor digitorum communis of either the second 

 or third digit. Sometimes two tendons occur, passing to the second and third digits, and more 

 rarely three have been observed, passing to the second, third, and fourth fingers. In a single 

 case a fourth tendon was observed which terminated upon the dorsal surface of the fifth 

 metacarpal. 



PRACTICAL CONSIDERATIONS. 



The \Vrist and Hand. — The skin of the wrist and of the back of the hand is 

 thin and freely movable and contains numerous hair-follicles and sebaceous glands. 

 These structures are absent in the palm and on the palmar and lateral surfaces of the 

 fingers, as well as on the dorsal surface of the terminal phalanges. Sudoriparous 

 glands are, on the contrary, relatively more numerous in the palms of the hands 

 than on any other part of the body surface. 



These anatomical conditions and the existence of the subungual and periungual 

 spaces and irregularities render the sterilization of the hands for surgical purposes 

 very difficult. 



The absence of hair-follicles and of sebaceous glands explains the freedom of 

 the palm from the superficial furuncular infections that are so common on the dorsum. 



In the palm the subcutaneous connective tissue, like that in the plantar region 

 and in the scalp between the skin and aponeurosis, is very dense. This similarity 

 has already been alluded to (page 491 ) in relation to the absence of hair-follicles in 

 the two former regions and the frequency of baldness in the latter. 



On the dorsal surface the subcutaneous tissue is loose. As a result, in whitlow, 

 in palmar abscess, in hemorrhagic extravasation, in oedema or cellulitis, the swelling 

 is apt to be much more marked on the dorsum and may be misleading as to the 

 real seat of the trouble. Abscesses immediately beneath the palmar fascia will 

 sometimes point in a metacarpal space on the dorsum. 



The thickness and close adhesion of the skin to the dense fascia beneath, while 

 admirably protecting the vessels and nerves of the palm and enabling it to withstand 

 pressure and friction, greatly increase the pain in cutaneous or subcutaneous infections. 

 On account of this same adhesion, superficial wounds of the palm do not gape, and 

 heal readily if non-infected and kept at rest. 



