100 



PHYSIOLOGY 



are called muscle cells (Fig. 60). They are usually 

 long and slender and they are arranged in groups in 



such a way that when 

 or the muscle as a whole, 

 each cell grows shorter 

 grows correspondingly 

 the effort of the indi- 

 Through the activity of 

 able to change its form 

 place. 



Theory of Muscular 

 theories have been ad- 

 muscle cell contracts but 

 With the aid of a com- 

 tain definite changes 

 place in the structure of 

 tion. These may be 



'SJ3 



they contract, the group, 

 contracts (Fig. 61). As 

 and thicker, the muscle 

 shorter and thicker and 

 vidual cell is magnified, 

 these cells an animal is 

 and move from place to 



Contraction. — Various 

 vanced to explain how a 

 they are not satisfactory, 

 pound microscope, cer- 

 may be seen to take 

 the cells during contrac- 

 accounted for by the 



a 





■MR 



Fig. 60.— Muscle cells. A, cross section from intestine of a dog; £, isolated cell 

 from intestine of a rabbit; C, part of a single fiber of voluntary muscle from 

 the leg of a rabbit ; p, protoplasm ; w, nucleus. (From Sedgwick and Wilson, 

 after Ranvier.) 



theory that when protoplasm disintegrates, as it always 

 does during activity, the resulting heat causes the pas- 



