vni.] THE MUSCLES, 



337 



man and his order, but even in such forms as the Do- and 

 the Bat. 



Interossei. These muscles are very generally present, 

 existing even in the cold-blooded Vertebrates, and their 

 essential nature is that of flexores breves. Even in man half 

 of each pair of interossei acts as an extensor, being inserted 

 into the dorsum of the third phalanx ; while half of each pair 

 acts as a flexor, being inserted into the palmar side of the 

 first phalanx. Indeed, it is by a modification of what are 

 essentially interossei that some of the small special muscles 

 of the pollex and little finger are constructed, as the abductor 

 pollicis and digiti minimi. 



The distinction between " palmar " and "dorsal " interossei 

 is really unimportant, referring only to the mode of their 

 origin from the metacarpals. 



23. In the ABDOMINAL REGION, the body-wall of man is 

 composed of muscular layers such as normally exist in other 

 Vertebrates above Fishes. The muscles ma'y, however, be 

 severally more developed or less developed, or more complex 

 or less complex, thao they are in him. 



On entering the class of Fishes we lose the superimposed 

 lamellse of differently directed muscular fibres of which the far 

 greater portion of the abdominal muscles in the higher forms 

 consist. In their place we have exclusively antero-posteriorly 

 directed fibres, and it is as if the recrus muscle had increased 

 vastly and entirely at the expense of the completely aborted 

 oblique muscles. That this is so, seems to be demonstrated 

 by those Batrachians which begin life with so many and close 

 resemblances to Fishes, which subsequently they lose. Thus 

 in the great persistent larva, the Axolotl, we find no truly 

 oblique abdominal muscles, but only as it were a hyper- 

 trophied rectus ; while in the rarely attained adult condition 

 (in which it closely resembles the genus Amblystoma] we 

 meet with the normal muscular abdominal structure of super- 

 imposed oblique lamellae. 



Sometimes the number of these lamellae is greater than in 

 man : thus the obliqmis externus may consist of three dis- 

 tinct layers, as e.g. in the Iguana. 



The obliquus internus of man is of moderate development. 

 It may be less developed than in him, as e.g. in the common 

 Chameleon. On the other hand it may be much larger, 

 relatively, as in Iguana, where it lines the whole of the 

 thorax ; or it may be, as in Menopoma and Menobranchus, 

 the largest muscle of the body, being continued on forwards 



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