THE MUSCLES. 535 



clear bands and the membranes of Krause, and must necessarily 

 diminish in amount opposite the sarcous elements. 



In the living muscle this change in the position of the sarco- 

 plasm during contraction can be observed; the muscle-columns tend 

 to cause the contracted parts to appear dark, the bulged parts bright, 

 in comparison. 



Appearance of Muscle under Polarized Light. Briicke was the 

 first to point out that the fiber is not composed entirely of a double 

 refracting, or anisotropous, substance. In addition there is a cer- 

 tain amount of singly refracting, or isotropous, material. This 

 investigator points out that there is a difference between the appear- 

 ances presented by living muscle examined in its own plasma and 

 those of dead and hardened muscle examined in glycerin. In living 

 muscle nearly the entire fiber is doubly refracting, the isotropous 

 substance occurring only as fine transverse lines or as rows of rhoin- 

 boidal dots which are united to one another across the anisotropous 

 substance by fine longitudinal lines. Sarcous element is anisotropic ; 

 sarcoplasm is isotropic. 



Nuclei. In muscles that are cross-striped are found a number 

 of clear, oval nuclei. They are sometimes spoken of as muscle-cor- 

 puscles. In mammalian muscle they usually lie upon the inner sur- 

 face of the sarcolemma. In the muscles of the frog and reptiles the 

 nuclei lie in the substance of the fiber surrounded by a small amount 

 of protoplasm. When the nuclei lie immediately beneath the sarco- 

 lemma they are more or less flattened. Each nucleus contains one 

 or two nucleoli. Mitotic figures, denoting division of the nuclei, 

 have been observed. The nuclei are not very readily seen in fresh 

 muscle, due to their being of the same refractive index as the sar- 

 cous substance. Only after they have undergone some spontaneous 

 change or acetic acid has been added to the specimen can they be 

 readily discerned. 



In the rabbit and rays of fishes some of the voluntary muscles 

 present differences from others, both as to appearance and mode of 

 action. Thus, while most of the voluntary muscles are pale and con- 

 tract forcibly when irritated, the soleus and semitendinosus show 

 different characteristics. They are of a deeper color and respond 

 with slow, prolonged contractions when stimulated. Thus, in these 

 animals there are red and white muscles. 



In other animals, this distinction of muscles is not found as 

 regards a whole muscle, but may affect individual fibers. Thus, in 

 the diaphragm many of the fibers have numerous nuclei imbedded 



