74 



THE ORGANIZED ANIMAL 



Fig. 4-6. Striated muscle is identified by the striking 

 cross-striations that show under the microscope as 

 seen in this photograph. The nuclei appear as black 

 elongated elipses distributed along the edge of the 

 fibers. 



laxes slowly, a behavior which is quite 

 satisfactory for the kind of job it has to do. 

 The skeletal muscles are usually attached 

 to bones and they constitute the large mus- 

 cles of the body. It is this muscle-bone com- 

 bination that is responsible for the move- 

 ment of the body as a whole. These muscle 

 cells are peculiar in that they are not 

 marked off by definite cell membranes, and 

 a single skeletal muscle fiber is composed 

 of many cells whose nuclei lie at regular 

 intervals along the periphery of the fiber 

 just under the surrounding membrane 

 (sarcolemma). The fiber is called a syncy- 

 tium, a name applied to any mass of proto- 

 plasm which contains many nuclei without 

 discrete cell membranes. Another marked 

 difference between this muscle and the 

 preceding is that there are evenly spaced 

 dark and light transverse bands extending 

 throughout the fiber. These striations iden- 

 tify the tissue as striated muscle ( Fig. 4-6 ) . 



The skeletal fibers contract suddenly with 

 considerable force, an essential feature in 

 moving the body. They can contract rapidly 

 again and again with only momentary rest 

 periods. 



Cardiac or heart muscle is characteristic 

 of vertebrates and is not found in the heart 

 of any of the lower forms. It differs from 

 skeletal muscle in that all of the fibers 



Sbeatb 

 axis cylinder 



x-secTion of axon 



Fig. 4-7. Nerve tissue as found in the spinal cord and 



ganglia. 



