DEVELOPMENT OF THE MUSCLES 375 



by a dark dotted line. This line is termed Dobie's line or Krause's membrane 

 (Fig. 376, k), because it was believed by Krause to be 'an actual membrane, con- 

 tinuous with the sarcolemma, and dividing the light band into two compartments. 

 In addition to the membrane of Krause, fine clear lines may be made out, with a 

 sufficiently high power, crossing the center of the dark band; these are known as 

 the lines of Hensen (Fig. 376, H). 



Schafer has worked out the minute anatomy of muscular fiber, particularly in 

 the wing muscles of insects, which are peculiarly adapted for this purpose on 

 account of the large amount of interstitial sarcoplasm which separates the sarco- 

 styles. In the following description that given by Schafer will be closely followed. 



A sarcostyle may be said to be made up of successive portions, each of which 

 is termed a sarcomere. The sarcomere is situated between two membranes of Krause 

 and consists of (1) a central dark part, which forms a portion of the dark band 

 of the whole fiber, and is named a sarcous element. This sarcous element really 

 consists of two parts, superimposed one on the top of the other, and when the fiber 

 is stretched these two parts become separated from each other at the line of Hensen 

 (Fig. 376, A). (2) On either side of this central dark portion is a clear layer, most 

 visible when the fiber is extended; this is situated between the dark center and the 

 membrane of Krause, and when the sarcomeres are joined together to form the 

 sarcostyle, constitutes the light band of the striated muscular fiber. 



When the sarcostyle is extended, the clear intervals are well-marked and plainly 

 to be seen; \vhen, on the other hand, the sarcostyle is contracted, that is to say, 

 when the muscle is in a state of contraction, these clear portions are very small 

 or they may have disappeared altogether (Fig. 376, 5). When the sarcostyle is 

 stretched to its full extent, not only is the clear portion well-marked, but the dark 

 portion the sarcous element is separated into its two constituents along the 

 line of Hensen. The sarcous element does not lie free in the sarcomere, for when 

 the sarcostyle is stretched, so as to render the clear portion visible, very fine 

 lines, which are probably septa, may be seen running through it from the sarcous 

 element to the membrane of Krause. 



Schafer explains these phenomena in the following way: He considers that each 

 sarcous element is made up of a number of longitudinal channels, which open 

 into the clear part toward the membrane of Krause but are closed at the line of 

 Hensen. When the muscular fiber is contracted the clear part of the muscular 

 substance is driven into these channels or tubes, and is therefore hidden from 

 sight, but at the same time it swells up the sarcous element and widens and shortens 

 the sarcomere. When, on the other hand, the fiber is extended, this clear sub- 

 stance is driven out of the tubes and collects between the sarcous element and 

 the membrane of Krause, and gives the appearance of the light part between 

 these two structures; by this means it elongates and narrows the sarcomere. 



If this view be true, it is a matter of great interest, and, as Schafer has shown, 

 harmonizes the contraction of muscle with the ameboid action of protoplasm. 

 In an ameboid cell, there is a framework of spongioplasm, which stains with 

 hematoxyiin and similar reagents, enclosing in its meshes a clear substance, hyalo- 

 plasm, which will not stain with these reagents. Under stimulation the hyaloplasm 

 passes into the pores of the spongioplasm; without stimulation it tends to pass 

 out as in the formation of pseudopodia. In muscle there is the same thing, viz., 

 a framework of spongioplasm staining with hematoxyiin the substance of the 

 sarcous element and this encloses a clear hyaloplasm, the clear substance of 

 the sarcomere, which resists staining with this reagent. During contraction of the 

 muscle i. e., stimulation this clear substance passes into the pores of the spongio- 

 plasm; while during extension of the muscle i. e., when there is no stimulation 

 it tends to pass out of the spongioplasm. 



In this way the contraction is brought about: under stimulation the proto- 



