THE FROG AND VERTEBRATES IN GENERAL 171 



membrane, the sarcolemma, are the ultimate units of muscular 

 structure, and every muscle contains a large number of such units. 

 Distributed through each fiber are several elongated nuclei. It is 

 believed that each fiber represents a greatly modified single cell 

 in which the original nucleus has divided many times without a 

 division of the cell body. The cytoplasm, or sarcoplasm, of the 

 striated fibers exhibits both a transverse and a longitudinal stria- 

 tion, the former being much the more prominent. The longitu- 

 dinal striations are due to the presence of tiny, filamentous strands, 

 the sarcostyles, which extend longitudinally through each cell. 

 These are supposed to be the contractile elements in the fiber. 

 With a sufficiently high magnification, it is apparent that the 

 transverse striation is due to the presence of alternate light and 

 dark bands which run across the muscle. The muscle fibers are 

 thus divided into segments, or sarcomeres, which constitute the 

 dark areas, and these are separated by light bands. When a 

 muscle is contracted, the bands are more closely approximated 

 than when the muscle is relaxed. (W. 32, E.) 



The cardiac muscle tissue, which forms the walls of the heart, 

 represents a distinct type which, however, shows certain morpho- 

 logical relations to the other two types. The cardiac tissue cells, 

 while retaining their identity as do the cells in the unstriated 

 muscle tissue, are attached to each other by strands of cytoplasm. 

 On the other hand they have a definite transverse striation and 

 in this respect resemble the striated muscle tissue. 



Contraction of Muscle Tissue. The question as to what 

 causes muscle tissue to contract when the proper stimulus is re- 

 ceived from the nervous system is one which is still unsolved 

 although many theories have been proposed. In considering this 

 problem, it is interesting to know that muscle tissue can contract 

 when removed from the body of an animal. Contractility, there- 

 fore, is not dependent upon any connection with the body. As an 

 example of this, the study of a 'muscle-nerve preparation' is of 

 value. Such a preparation is made by removing a muscle from an 

 anesthetized animal, say the large gastrocnemius muscle from the 

 hind leg of a Frog. The dissection should be done in such a way 

 as to leave the sciatic nerve, which innervates this region, attached 

 to the muscle. The dissected muscle with the attached nerve can 

 be kept wet with a normal salt solution. The use of such a solu- 

 tion prevents the tissues from drying, and when so treated they 



