THE MUSCULAR SYSTEM 201 



and cut the body nearly at right angles to the longitudinal 

 axis, thus presenting a much more primitive appearance than 

 that found in most fishes, and one that corresponds closely to 

 the embryonic condition. 



Above the urodeles the development of more massive limbs 

 and limb-girdles, the same element that produces such pro- 

 found regional differentiation in the vertebral column, is oper- 

 ative here also in modifying the simple succession of typical 

 myotomes, a condition which was unaffected by the delicate 

 fins and weak girdles sufficient for their primitive aquatic 

 environment. The hip-girdle produces the most direct change 

 through the intrusion of the ilium between two successive 

 myotomes, following the course of a myocomma. By this the 

 axial myotomes become divided into those of the trunk and 

 those of the tail, and the gradual increase in size of the ilium 

 and its extension along the vertebral column, as well as the 

 formation of an immovable sacrum, widens the space between 

 these two groups of muscles, while the differentiation of func- 

 tion between trunk and tail effects profound changes in the 

 muscles themselves. The principal effect of the shoulder-girdle 

 in this regard is an indirect one, for while it presents no in- 

 truding process, as in the case of the ilium, it establishes the 

 anterior point of support and causes the differentiation of the 

 region between it and the skull into a neck, which becomes in 

 some cases extremely mobile. 



In most reptiles and in mammals, however, in spite of these 

 modifications, the succession of myotomes remains distinct, 

 especially dorsally, and through all the regional differentia- 

 tions there may still be seen the segmental character of the 

 musculature; but birds and turtles represent two types of ex- 

 treme specialization in which this continuity becomes broken 

 through local reduction. The cause of this in turtles is the 

 formation of the carapace, in which all free movement of the 

 vertebral elements and ribs is lost through a complete anchy- 

 losis, and in this region the trunk muscles, though laid down 

 in the embryo, are early atrophied and become lost. The only 

 axial muscles retained are those of the neck and tail, together 



