186 STRUCTURE OF THE VERTEBRATES 



paratus, the muscles attached to them shifted their position and 

 function. Those of the posterior five arches remained as ele- 

 vators and depressors of the gill arches. 



In the urodele amphibians there is a further degeneration of 

 the arches, with correlated changes of function in the muscula- 

 ture. The anterior muscles become attached to the throat car- 

 tilages and the cranium. In the mammals the final evolutionary 

 stages are found. The visceral muscles control the function of 

 the jaw, the hyoid, and the larynx. Most of these muscles are 

 very small, although those of the jaw are heavy, and those 

 which pass posteriorly from the larynx to the sternum are fairly 

 long and slim. If it were not for their embryological origin, and 

 innervation from cranial nerves, they could be considered with 

 the ventral axial group. 



E. Appendicular Muscles 



The origin of the appendicular muscles is intimately con- 

 nected with the origin of the limbs. The entire appendage arises 

 embryologically as two rows of metameric buds, one epaxial 

 and one hypaxial, which grow distally from the mesodermal 

 somites. (See page 158.) This diagrammatic metamerism in 

 origin is found only in the fish. In more specialized vertebrates 

 there is some fusion of the separate cell masses, until in the 

 mammals each appendage arises as a mass of cells which comes 

 from several segments and grows distally to form the limb. 

 The complete homology between the limbs of different verte- 

 brates is shown by the number of segments included and the 

 spinal nerves which grow out into the appendage. 



The appendicular muscles begin in all vertebrates as dorsal 

 and ventral groups, and no matter how much torsion or spe- 

 cialization of the limb takes place, this relationship is main- 

 tained. After the full development of the limb the muscles fall 

 into two divisions: (1) the extrinsic muscles which have their 

 origin on the girdles, or other bones, and pass out to the limb 

 where they insert; and (2) the intrinsic muscles which lie en- 

 tirely on the appendage. The latter, even including the muscles 

 of the digits, retain the primitive dorsal and ventral relation- 

 ships. 



