MUSCLE 



555 



chief duty of a writer on elementary physiology in regard to the 

 whole question. 



The muscle-fibre, the unit out of which the anatomical muscle is 

 built up, is surrounded by a structureless membrane, the sarcolemma. 

 The length and breadth of a fibre vary greatly in different situations. 

 The maximum length is about 4 cm. ; the breadth may be as much 

 as 70 ,a and as little as 10 /A. When we come to analyze the muscle- 

 fibre and to determine out of what units it is built up, the difficulty 

 begins. The fibre shows alternate dim and clear transverse stripes, 

 and can actually be split up into discs by certain reagents. It also 



FIG. 163. LIVING MUSCLE OF 

 WATER -BEETLE (highly magnified). 



(SCHAFER.) 



s, sarcolemma; a, dim stripe; , bright 

 stripe ; c, row of dots in bright stripe, 

 which represent expansions of the inter- 

 stitial substance (sarcoplasm). 



FIG. 164. PORTION OF 

 LEG MUSCLE OF INSECT, 

 treated with dilute acetic 

 acid. (ScHlFER.) 



S, sarcolemma; D, dot-like 

 enlargement of sarcoplasm ; 

 K, Kra use's membrane. The 

 sarcous elements have been 

 swollen and dissolved by the 

 acid. 



shows a longitudinal striation, and can be separated into fibrils. 

 Some have supposed that the discs are the real structural units 

 which, piled end to end, make up the fibre. The fibrils they con- 

 sider artificial. This view is erroneous. It seems certain that the 

 fibres are built up from fibrils ranged side by side, and that the discs 

 are artificial. The contents of the muscle-fibre appear to consist of 

 two functionally different substances, a contractile substance, and an 

 interstitial, perhaps nutritive, non-contractile material of more fluid 

 nature. The contractile substance is arranged as longitudinal fibrils 

 embedded in interfibrillar matter (sarcoplasm). In a muscle im- 

 pregnated with chloride of gold, the interfibrillar matter appears as a 

 network. 



Schafer has described the contractile elements of the muscle-fibre 

 (Figs. 163, 164) as fine columns (sarcostyles), divided into segments 

 (sarcomeres) by thin transverse discs (Krause's membranes), occupy- 

 ing the position of the middle of each light stripe. Krause's mem- 

 brane is probably only an optical appearance. If an actual partition 



