176 CHAS. W. GREENE 



tissue. The muscle as a whole is marked off from the great lateral 

 muscle by a pretty definite fibrous partition. This separation is 

 not loose enough to be easy of dissection, but when the connective 

 tissue is softened, as in the process of cooking, the muscle very 

 readily separates from the deep portion of the lateral muscle. 



I have examined the histological structure of this dark muscle, 

 however, chiefly with special reference to the normal loading of fat. 

 The muscle possesses the usual gross arrangement of sarcolemma, 

 sarcoplasm, and fibrillae. The fibrillae are band shaped and 

 appear in cross section as doubly refractive lines not unlike the 

 appearance of bacilli arranged side by side. These fibrillae vary 

 in size, but are in the neighborhood of from 1 to 1.2m in cross 

 section in their long diameter. At the surface they are radially 

 arranged with reference to the axis of the fiber, forming a rather 

 definite layer around the circumference of the fiber. But they 

 are irregularly placed throughout the central portion of the fiber, 

 as shown in figs. 1 and 2. 



The most striking characteristic of the normal dark muscle is 

 the relatively large amount of sarcoplasm. The sarcoplasm forms 

 thin layers between the doubly refractive lines shown in cross 

 section of the fibrillae, and fills up the spaces where groups of 

 fibrillae are brought in contact, i.e., the angles of Cohnheim's 

 areas. In paraffin sections of adult muscle, both from the young- 

 salmon and from mature adults, there is always present in the 

 sarcoplasm a number of clear globules or vacuoles. The number 

 and size of these vacuoles is so great as to obscure the usual rela- 

 tions of the sarcoplasm, fig. 1. The vacuoles are largely in the 

 angles of Cohnheim's areas, but may be present in spaces between 

 the fibrillae in the individual rows. These vacuoles represent 

 spaces from which fat has been extracted during the imbedding 

 process. On the whole the muscle fibers are characterized by 

 resemblance to the more generalized type of striated fiber. Espe- 

 cially in the younger fibers do we find an excessive amount of 

 sarcoplasm both around and between the fibrillae. Other types 

 of striated muscle of the salmon have smaller fibrillae and rela- 

 tively less sarcoplasm. 



