THE MUSCULAR SYSTEM 261 



They may spread out in thin sheets that are ribbon-like, triangular, 

 pinnate, or fan-like. Appendicular muscles are more frequently spindle- 

 shaped and massive. 



Each muscle is well supplied with capillaries and with both motor and 

 sensory nerves. 



As to origin, muscles are sharply divided into two kinds; i. Skeletal 

 (epimeric or myotomic), derived from the dorsal or epimeric portion of the 

 mesoderm; and 2. Visceral (hypomeric) derived from the hypomere. 

 In the trunk region in contrast with the head visceral muscles arise from 

 the splanchnic layer of mesoderm only. 



Skeletal muscles are composed of striped fibers which respond to 

 stimulation by rapid contraction. Most visceral muscles, on the other 

 hand, consist of slow-acting smooth or non-striped fibers. The former 

 are voluntary and under control of the will, the latter are usually involun- 

 tary. Exceptions are found in the heart muscle which is visceral and 

 involuntary, but formed of striped fibers, and in the chewing and facial 

 muscles which are visceral and at the same time striped and voluntary. 



EVOLUTION OF THE MUSCULAR SYSTEM 



The muscles of man and mammals are the last term in the series of 

 transformations of the mechanism of contraction, the evolution of which 

 it is now possible to sketch in fairly firm outlines. 



Contractility appears to be one of the original properties of living cells. 

 Touch an amoeba, and it responds by drawing together into a sphere. 

 There is no single axis, but contraction takes place from all directions 

 towards a center. In some protozoa, however, progressive advance in 

 the function appears in the differentiation of contractile fibrils. A cluster 

 of such fibrils in the stalk of vorticella is so arranged as to contract in one 

 direction only, like a muscle fiber. 



True muscle cells first appear in the animal series in the sponges. 

 The primary independence of muscle and nerve is indicated by the 

 presence of muscle cells in this group which lacks nerves altogether. The 

 epithelio-muscular cells of coelenterates are essentially similar to those 

 of sponges. 



The next step in the evolution of muscles appears in the flatworms, 

 in which muscle cells are aggregated into clusters. The bilateral sym- 

 metry characteristic of the muscles of higher animals also appears in 

 this group. 



Transitional evolutionary stages between flatworms and chordates are, 

 it must be admitted, highly speculative. Even if we accept the assump- 

 tion that annelids resemble the ancestors of vertebrates, there still remains 

 a wide gulf to be filled between flatworms and annelids. In certain 

 characteristics, the muscles of annelids, it is true, strikingly resemble those 



