HISTOLOGY 



Many undoubtedly smooth fibers have their fibrils lying in a waved 

 position that simulates striation. This is the more easily seen because the 

 angles of such waves hold quantities of the stain in a crude physical way, 

 especially iron haematoxylin. In this connection we shall mention the 

 unique case of the muscle cell, described by Schaper from the salaman- 

 der's mesentery, which showed alternate light and dark bands. (As 

 Schaper's description deals with the in- 

 dividual myo-fibrils, however, we must 

 accept it as a remarkable exception.) 



In the Crustacea and Insecta the dif- 

 ferentiation of cardiac muscle is more 

 complete than in any lower forms. In 

 the insect it is precisely the same as the 

 other body muscles, except the unimpor- 

 tant difference of having slightly smaller 

 fibers and slightly narrower striations. 

 This is pictured in Figure 141. 



In the lobster we find a case that is 

 much like that of the vertebrates, an 

 apparent syncytium, in which striated 

 myo-fibrils are developed (Fig. 92). Its 

 bodies cf unknown meaning. One embryology would probably show a multi- 



of these lies in close contact with a , ... , . . , 



nucleus ; / marks two of the fibril cellular origin, but as this is not known, 

 we shall study the adult form. 



Any section shows a great wealth of 

 fibers running in all possible directions. 



This apparent lack of aim in placing cardiac fibers in any particular 

 direction has been mentioned previously and has its mechanical advan- 

 tages. The fibers are not individual so far as boundaries can be 

 detected, but form strands of one large syncytium. 



The sarcoplasm is very abundant in proportion to the other contents 

 present and contains three prominent objects, nuclei, myo-fibrils, and 

 characteristic bodies of doubtful function. 



The nuclei are fine and large, as are most of those in the lobster's 

 tissues. Like other lobster nuclei, they have a delicate but rigid nuclear 

 membrane, finely distributed chromatin, and a small but very clearly 

 defined nucleolus. In a transection of almost any single connecting 

 strand there are to be seen from one to three nuclei, more rarely none 

 or more than three. Figure 92 shows a typical transection. 



The myo-fibrils are especially worth study in such a section as the 

 above because they are comparatively few in number for such a well- 

 differentiated muscle cell and serve to do away with the idea, so preva- 

 lent among beginners who study only human histology, that a muscle fiber 



t. n. 



FIG. 92. Transection of a single 

 muscle fiber in heart of the lobster. 

 Shows two nuclei, about twelve 

 fibril bundles and four of the x 



bundles; conn.t.n., connective-tis- 

 sue nucleus or sarcolemma nu- 

 cleus. 



