1 8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



teristic tissue is the muscular. The characteristic prop- 

 erty of this tissue is contractility under stimulus of any 

 kind, but, normally, under the stimulus of nerve force. 



Structure. — To the naked eye muscle tissue consists 

 of bundles or fascicles of fibers lying parallel to one an- 

 other ; but under the microscope each fascicle is seen to 

 be composed of a multitude of cylindrical fibers trans- 

 versely striated, and under favorable 

 conditions these again are separable C 



into still finer fibrillse. Each fiber is 



Fig. 9. — A, muscular fibers of voluntary muscle ; B, one fiber broken to 

 show its investing sheath ; C, cells of involuntary muscle. 



invested with a thin membrane of connective tissue (Fig. 

 9). When a muscle contracts the fibers are observed to 

 shorten and thicken' {Y\g. 9, A, B, C). 



Varieties. — There are two kinds of muscle, voluntary 

 and involuntary. The fibers of the one are transversely 

 striated or minutely wrinkled; the fibers of the other 

 are not thus wrinkled (Fig. 9, C). 



Of all tissues, muscle is that one in which the original 

 cell structure is most obscured by modification for func- 

 tion. In perfectly formed striated muscle there is no ap- 



