i ORGANISATION AND STRUCTURE OF MUSCLE 39 



proved, inter alia, by the frequent appearance of fat -drops, 

 which are presumably in close, genetic relation with the inter- 

 stitial granules mentioned above (Knoll). Again, we know 

 that certain matters which penetrate the muscle-fibres are further 

 distributed to the sarcoplasm ; e.g. Leo Gerlach (21) found that 

 flie muscle - fibres of frogs which had been treated for several 

 days with indigo-carmine became speckled with blue, particularly 

 towards the tendon end of the fibres, in consequence of the indigo 

 assimilated, and often exhibited a definite, serial arrangement of the 

 pigment. The rows of blue granules lie, like the fat-drops in 

 other cases, between the fibrils in the sarcoplasm; so that the 

 indigo-carmine must be taken up by the sarcoplasm in solution. 

 If, then, it really is the role of the interfibrillar plasma to preside 

 over the nutrition of the contractile substance, the greater abund- 

 ance of sarcoplasm in the muscles which serve the most strenuous 

 and persistent functions is readily intelligible. The frequent 

 pigmentation of the dark, " plasmic " muscle-fibres (as previously 

 cited) seems also to be closely related. Such are in opposition 

 to the body-muscles the deep purple-red buccal muscles of many 

 snails (Chiton, Haliotis, Limnseus, Trochus, Paludina, Littorina, 

 Patella), the cardiac muscles of many invertebrates and all 

 vertebrates, as \vell as the dark -red muscles which contain 

 haemoglobin. 



The finer structure of the single muscle-fibrils (infra), on the 

 other hand, seems to be in relation with quite another property 

 of the muscular elements, i.e. rapidity of contraction. We have 

 already stated that distinct cross-striation of the fibrils is excep- 

 tional in the uninuclear cells of Invertebrates, and where it 

 does appear (e.g. in Medusa?, adductor muscle of Pecten, etc.) 

 exists only in the more swiftly contracting muscles. Thus 0. and 

 Pi. Hertwig (22) observed that the individuals of the Hydroid- 

 colony have smooth muscle-fibres, so long as they remain attached 

 as inert hydroid polyps to the parent, " but acquire striated fibrils 

 directly they swim off as active Medusse." Again, the tentacular 

 muscles of the Ctenophora are usually smooth, only the lateral 

 muscles of Euplocamis, which contract with especial vigour and 

 rapidity, being striated. In Vertebrates, on the other hand, the 

 bulk of the muscles is composed of cross-striated fibres, and only 

 the sluggishly reacting muscles of the intestinal tract, urogenital 

 apparatus, and blood-vessels, are smooth, i.e. exhibit no further 



