MUSCULAR TISSUE. 141 



of the water-beetle (Hydropkttus piceus) can be made to separate 

 into transverse discs (Rollet, 85). One of these discs would cor- 

 respond to the segment Q, and it is very probable that this is 

 the portion which has long been known under the name of Bow- 

 man's disc. Other reagents, as weak chromic acid, cause a separation 

 of the muscle-substance into longitudinal fibrils. In this case the discs 

 Q are split up longitudinally into a number of very small columns 

 which were at one time regarded as the primary elements of the 

 fiber and termed by Bowman sarcous elements. 



In adult skeletal and skin muscle-fibers of mammalia the posi- 

 tions of the nuclei vary. There are muscles in which the nuclei are 

 imbedded in the sarcoplasm between the muscle-columns (so-called 

 red muscles, as the semitendinosus of the rabbit) ; in 'other muscles 

 they lie immediately beneath the sarcolemma (white muscles, as the 

 semimembranosus of the rabbit ; Ranvier, 89). In the striated mus- 

 cle-fibers of the lower vertebrates and of mammalian embryos the 

 nuclei lie between the fibrillae, or muscle-columns. The red muscle- 

 fibers are rich in sarcoplasm, and the fibrils are grouped in well- 

 marked and large muscle-columns surrounded by sarcoplasm which 

 often contains granules of various sizes, the interstitial granules of 

 Kolliker, often especially abundant at the poles of the nuclei. The 

 white muscle-fibers have a relatively small quantity of sarcoplasm. 

 In cross-sections of the light fibers the fibrils show as fine points, not 

 distinctly grouped, and surrounded by the homogeneous sarcoplasm. 

 Both varieties occur in almost every human muscle, and the relative 

 number of each varies greatly in the different muscles (Schaffer, 93, 

 II, Fig. 100). 



Muscles with transversely striated fibers are, with the exception 

 of those of the heart, subject to the will of the individual, and 

 are characterized by a rapid contraction in which the anisotropic 

 substance increases in size at the cost of the isotropic discs ; the 

 former appears to play the chief role. Besides morphologic dif- 

 ferences, the red and white muscle-fibers appear to possess differ- 

 ences of a physiologic character, in that the contraction in the red 



Fig. 101. Branched, striated muscle-fiber from the tongue of -a dog. 



variety is slower than that in the white (Ranvier, 80). Only the stri- 

 ated muscles of the esophagus, the external cremaster, and a few 

 others, as well as the somewhat differently constructed muscles of 

 the heart, are involuntary. 



