The Interstitial Granules of Striated Muscle etc. 309 



fibers of animals like the cat. In some light fibers no liposomes at all 

 can be demonstrated (fig. 3); in others a large nnmber may be shown 



(fig. 1). 



The opacity of a fiber when examined fresh in normal salt solution 

 seems to depend on the number, size, and refractive power of its hpo- 

 somes; but in sections that have been treated with a fixative differ- 

 ences in opacity may be due to other factors. Schaffer apparently 

 believes that a considerable number of the light fibers seen in normal 

 salt solution may be contracted portions of dark fibers. I have not 

 found any evidence to support this view. 



Under normal conditions the proportion of dark and light fibers 

 is fairly constant in any particular muscle of animals of the same 

 species, e. g., the soleus of a well-nourished rat nearly alwa^^s contains 

 a large per cent of dark fibers, and the gastrocnemius only a small 

 per cent. Good accounts of the distribution of the dark and light fibers 

 are given by Knoll and by Schaffer (see review of the literature). My 

 own observations in so far as they extend are essentially in agreement 

 with their results. In the brown rat (Mus decumanus) which has been 

 used to a considerable extent in this research, the gastrocnemius con- 

 sists mainly of light fibers ; the soleus contains a large per cent of dark 

 and intermediate fibers. The diaphragm resembles the soleus. The 

 cardiac fibers contain a large number of small liposomes that seldom 

 stain intensely. These fibers resemble the intermediate fibers of the 

 diaphragm and soleus, tho they are usually classified as dark fibers. 



In the adult ox (fig. 3) there are none of the coarse-droplet fibers 

 such as occur in the dog and cat. The darkest fibers are usually full of 

 small moderately-refractive granules. In many of the pale fibers no 

 interstitial granules can be demonstrated. The granules are fewest and 

 faintest in the panniculus muscle. No striking differences among the 

 other muscles were noted. 



In the frog (R. pipiens was used to the grestest extent) the dark 

 fibers are usually of smaller diameter than the light. The dark fibers 

 contain coarse liposomes. Under certain conditions the light fibers are 

 full of small liposomes (figs. 1 and 4), but under other conditions the 

 liposomes may be so small and faint that they escape ordinary obser- 



