THE STRUCTURE OF VOLUNTARY MUSCLE 201 



dinally striated, the striation being apparently due to the develop- 

 ment of special contractile fibrillae. In the slowly contracting 

 unstriated muscle of the vertebrate intestine, the longitudinal striation 

 is with difficulty made out , but as the muscle rises in the scale of 

 efficiency, the longitudinal striation becomes more apparent, and in 

 the striated muscle of vertebrates, and still more in the wonderful 

 wing-muscles of insects, which can perform three hundred complete 

 contractions in a second, the longitudinal is associated with and often 

 apparently subordinated to a transverse striation, due to the regular 

 segmentation of the contractile fibrillae or sarcostyles. Every muscular 

 fibre, which presents any trace of histological differentiation, may be 

 said to consist of contractile fibrillse (sarcostyles), each composed of 



FIG. 37. Transverse sections of the pectoral muscles of a, the falcon, b, 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.) 



a series of contractile elements (sarcous elements or sarcomeres), 

 and embedded in a granular material known as sarcoplasm. The 

 great divergence in the aspect of muscular fibres from different 

 parts of the animal kingdom is largely conditioned by the varying 

 relations, spatial and quantitative, of the sarcoplasm to the sarco- 

 sfcyles. Thus in the higher vertebrates, two types of voluntary muscular 

 fibre are distinguished, according to the 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 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 sarco- 

 styles. The connection between structure and activity of the muscle- 

 fibres is well shown by Fig. 37. 



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 



