MUSCULAR TISSUE. 137 



Each striated muscle-fiber consists of a delicate membrane, the 

 sarcolemma, a muscle protoplasm, in which are recognized very fine 

 fibrils and a semifluid interfibrillar substance (the sarcoplasm) and 

 the muscle nuclei. The sarcolemma is a very delicate, transparent, 

 and apparently structureless membrane, which resists strong acetic 

 acid, even after boiling for a long time. If we examine in an indif- 

 ferent fluid fresh muscle-fibers, the contents of which have been 

 broken without rupturing the sarcolemma, we may see this sheath 

 as a fine glistening line. (Fig. 95.) 



The fibrils of the muscle-protoplasm constitute the contractile 

 part of the muscle-fiber. They are exceedingly fine and extend the 

 entire length of the muscle-fiber. These fibrils are, however, not of 

 the same composition throughout, but are made up of segments 

 which show different physical properties and stain differently. The 

 structure of the fibrils may be expressed in the form of a diagram 

 (Fig. 96) giving the more recently expressed views of the structure 

 of these fibrils. The fibrils present alternating darker and lighter 

 segments, which taken together give the striation which is so char- 



Fig. 95. Striated muscle-fiber of frog, showing sarcolemma. 



acteristic of striated muscle. The darker segments are slightly 

 longer, are doubly refracting, anisotropic, and in general stain more 

 deeply than do the lighter segments, which are slightly shorter and 

 are singly refracting, isotropic. The darker segments, known as the 

 transverse discs, or Briicker's lines, are indicated in the diagram by 

 the letter Q ; the lighter segments, known as the intermediate discs 

 of Krause, are indicated by the letter j. In the intermediate discs 

 of Krause there is found a dark line, which is doubly refractive, 

 which is known as Krause's membrane (z) (Grundmembran), and 

 which, according to certain observers (M. Heidenhain, J. B. Mac- 

 Callum), is continuous through the fibril bundles, as will be stated 

 more fully later. This membrane divides disc j into two equal 

 parts. The transverse disc (Q) is likewise divided into equal parts 

 by a narrow, isotropic band, known as the median disc of Hensen, 

 and designated by the letter H. In the median discs of Hensen 

 H there is found a thin membrane, known as the median mem- 

 brane of M. Heidenhain, and designated as M, which, like the mem- 

 brane of Krause, is continuous through the fibril bundles, uniting 



