THE MUSCULAR SYSTEM 



231 



which is well developed in the anterior limb is small or want- 

 ing in the posterior, and thus the former is a little more 

 typical.* 



The dorsal aspect of the antebrachinm (Fig. 62, a and b) is 

 largely taken up superficially by a single muscular mass, M. 

 dorsalis antcbrachii (da.) which arises from the distal end of 

 the humerus. This separates, spreads' out over the ante- 

 brachium, and divides distally into four slips, three for the in- 

 ter-digital spaces and one for the ulnar side of digit V. Each 

 of these in turn divides into two, which insert by tendons into 

 the bases of the adjoining metacarpals. The muscle is thus an 

 abductor-adductor complex, furnishing the digits with lateral 

 motions, but without any power in extending them. The 

 radial aspect of digit II is alone unsupplied from this system, 

 and this deficiency is made up by the supinator (s), a muscle 

 which underlies the former, arising from the ulnar side of the 

 carpus. It crosses the limb obliquely, and inserts into the 

 internal or free aspect of metacarpal II. Extension of the 

 digits is effected by four short muscles, Mm. extensores breves 

 (x, x), which arise from the distal row of carpalia and become 

 continued into tendons that lie along the dorsum of the sep- 

 arate digits and insert into the bases of the terminal phalanges. 

 Partly along the sides of the dorsalis, and partly covered by it, 

 thus forming a deeper layer, are two long muscles, associated 

 respectively with radius and ulna, Mm. extensor radialis and 

 extensor ulnaris (er. and eu.). These arise from the humerus 

 with the dorsalis and insert, the one along the shaft of the 



* In one point the free limb of Necturus diverges from what is gener- 

 ally believed to be the typical chiropterygium, and that is, it possesses but 

 four digits in each extremity instead of the canonical five which is usually 

 considered primitive. Since the nearest ally of this species, the cave 

 form, Proteus, exhibits a still greater reduction of digits (anterior, 3; 

 posterior, 2), it has been presumed that this is in both cases a secondary 

 reduction. Certain facts, however, lead one to think that the first land 

 vertebrates possessed a smaller number of digits than five, and if this be 

 so, the condition in these two salamanders is primitive, and not a second- 

 ary reduction. According to the reduction theory digit I is assumed to be 

 the one lost, and in accordance with this the four digits present are 

 designated here, both in text and illustrations, as II-V. 



