THE MUSCULAR SYSTEM 



389 



light band, form a sarcomere; a series of sarcomeres form a 

 sarcostyle. The sarcous element is filled with longitudinal 

 canals or pores, which communicate with the bright end of the 

 sarcostyle ; when the muscle contracts, the bright or clear part 

 of the contractile substance passes into the pores, and for the 

 time being is almost lost to view, while the sarcomere shortens 

 and widens ; when the muscle relaxes, the clear substance 

 emerges from the pores, and the sarcomere lengthens and 

 narrows. It is generally believed that the fibril constitutes the 

 contractile substance of the fibre, the sarcoplasm being of a 

 nutritive nature. When the'tands are examined by polarised 



light, the dim band is found 

 to be doubly refractive 

 (anisotropous), while the 

 light band is singly refrac- 

 tive (isotropous). 

 In some muscles, like those 





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Fig. 1 10. — Living Muscle of India- 

 Beetle (Highly Magnified) (Schafer). 



s, Sarcolemma ; a, dim band ; b, light band ; 

 c, row of dots : bright stripe, which 

 appears to be the enlarged ends of 

 rod-shaped particles, d, but in reality 

 represents expansions of the interstitial 

 substance (sarcoplasm). 



Fig. hi. — Portion of Leg Muscle 

 of Insect treated with Dilute 

 Acetic Acid (Schafer). 



S, Sarcolemma ; D, dot-like en- 

 largement of sarcoplasm ; K, 

 Krause's membrane. The sarcous 

 elements have been swollen and 

 dissolved by the acid. 



of the heart and diaphragm, the sarcoplasm exists in relatively 

 large amounts ; such muscles are deeper in colour than ordinary 

 skeletal muscle, they contract more slowly, and may be said 

 never to tire. Striped muscle may therefore be dark as in the 

 case of the heart, full tinted as in the ordinary skeletal muscles of 

 the larger herbivora, or pale bloodless-looking structures, such as 

 are found in the rabbit and some fishes. In the rabbit both pale 

 and red fibres may be mixed up in the one muscle. These pale 

 striated muscles exhibit structural differences from ordinary 

 striated muscle. 



The nerve supply to muscle is both motor and sensory : through 

 the sensory nerves the brain is made acquainted with the position 



