92 



NORMAL HISTOLOGY. 

 FIG. 78. 



FIG. 79. 



Striated muscular tissue. 



Fig. 78. Diagrams of the structure of the contractile substance. (Rollet.) Q, sarcous elements, 

 appearing dark in A, light in B; Zand J, sarcoplasm. The sarcoplasm also lies between 

 the sarcous elements in Q, appearing as light bands in A and as dark lines in B. A is the 

 appearance of the fibre when the focal plane is deep ; B, the appearance when the focal 

 plane is superficial (see Fig. 76). The dots Z in A and J in B are optical expressions of 

 differences in the refraction of the sarcoplasm and sarcous elements, and do not repre- 

 sent actual structures. A complete explanation of the way in which a microscopical im- 

 age may contain apparent objects which have no actual existence cannot be entered into 

 here. It is due to the fact that regularly alternating structures of different powers of re- 

 fraction affect rays of light very much as they are affected by a fine grating, producing 

 diffraction spectra. These spectra may interfere with each other, occasioning an alter- 

 nation of light and dark bands or areas above the specimen. When the focal plane is 

 changed the light areas become dark and the dark areas light, but sometimes with an 

 alteration in their outline and relative sizes, as exemplified in the cuts. 



Fig. 79. Cross-section of a muscle-fibre. (Rollet.) The fine reticulum, collected into larger 

 masses at a few points in the midst of the contractile substance, is composed of sarco- 

 plasm. The clear areas within this reticulum are the cross-sections of the sarcous ele- 

 ments. These cross-sections are sometimes called "Cohnheim's areas." Immediately 

 beneath the sarcolemma are cross-sections of two nuclei. 



nucleated, cylindrical cells. The body of these cells is almost ex- 

 clusively composed of a very complex, contractile substance which pre- 



