THE STRUCTURE OF VOLUNTARY MUSCLE 



181 



largely conditioned by the varying relations, spatial and quantitative, of 

 the sarcoplasm to the sacostyles. Thus in the higher vertebrates, two 

 types of voluntary muscular fibre are distinguished, according to the 







io. 37. Transverse sections of the pectoral muscles of a, the falcon, &, the goose, and c, 

 the domestic fowl. It will be noticed that the relative amount of granular or red fibres 

 present varies directly as the bird's power of sustained flight. (After KNOLL.) 



amount of sarcoplasm they contain : one rich in sarcoplasm, more granular 



in cross-section, and generally containing haemoglobin ; and the other poor 



in sarcoplasm, clear in cross-section, and containing no haemoglobin. From 



the fact that the granular fibres are B 



found chiefly in those muscles which 



have to carry out long-continued 



and powerful contractions, it seems 



reasonable to regard the interstitial 



sarcoplasm as the local food-supply 



of the active sarcostyles, although 



some authors have endowed the 



sarcoplasm with a contractile power 



of its own, differing only by its 



extremely prolonged character from 



the quick twitch of the sarcostyles. 



The connection between structure 



and activity of the muscle-fibres is 



well shown by Fig. 37. 



I 





In some animals, such as the rabbit, 

 we find muscles consisting almost entirely 

 of one or other of these varieties ; but in 

 most animals (amongst which we may 

 reckon frog and man) the two varieties 

 occur together in one muscle, so that what 

 we have to say about the properties of 

 voluntary muscle, which rests nearly 

 entirely on experiments with frog's 

 muscle, really has reference to a mixed 

 muscle, i.e. muscle containing both red 

 and white fibres. 



Since the sarcous element represents the 

 contractile unit of the muscle, a know- 

 ledge of its intimate structure should be of great importance for the theory of muscular 

 contraction. Unfortunately however we are here at the limits of the demons fcrably 



, 38. Fibrils of the wing- muscles of a wasp, 

 prepared by Reliefs method. Highly 

 magnified. (E. A. SCHAFER.) 



A, a contracted fibril. B, a stretched 

 fibril, with its sarcous elements separated 

 at the line oE Hensen. c, an uncon- 

 tracted fibril, showing the porous struc- 

 ture of the sarcous elements. 



