38 ELECTRO-PHYSIOLOGY 



insects ; they are bounded only by the surrounding, spongy, con- 

 necting-substance, and supported inside by the system of branched 

 tracheae. The tracheal branches thus form, as it were, the 

 skeleton of a fibril - bundle, while the sarcoplasm fills up the 

 cavities that remain between fibrils and tracheal ramifications. 



Glancing back over the facts that relate to mass-disposition 

 of sarcoplasm, vs. contractile substance proper of the fibrils, the 

 general conclusion seems to be justified, that the elements of those 

 muscles which serve the most persistent or most strenuous action are 

 richest in sarcoplasm. (Cardiac and masticatory muscles of 

 invertebrates and vertebrates, float-muscles of Hippocampus and 

 other fishes, some of the lateral body-muscles of fishes, especially 

 those in the tail - region, which govern the movements of 

 direction.) 



Knoll (13, p. 47) pointed out, in this connection, a curious 

 instance of divergence in the tail-muscles of Torpedo and Raja. 

 In the former there is a well - developed stripe of red, dark 

 muscle, which is totally absent in Raja ; there are corresponding 

 differences in the swimming movements of the two animals, since 

 in Torpedo the flexible tail executes a series of rapid sideway 

 movements, which do not occur with Raja. 



The wing -.muscles of birds and insects afford another 

 example. The great pectoral muscle of the best fliers consists 

 exclusively, or almost exclusively, of plasmic, in the weak-winged 

 "fowls" predominantly of a-plasmic, fibres. In the guiding muscles 

 of amphibia, reptiles, and mammals the a-plasmic and plasmic 

 fibres are intermingled ; the last are more abundant in the free, 

 wild species of mammals than in the domesticated animals. 

 In the rodents, e.g. (rabbit), they are entirely absent, or very 

 sparsely distributed, in certain sections of the leg-muscles. In 

 bats, on the other hand, the fibres of every muscle are rich in 

 protoplasm. 



There seems therefore to be a direct relation between the ex- 

 tension and force of the contractile fibrils and the bulk of surround- 

 ing sarcoplasm (Knoll). 



If, as Sachs conjectured, the nutrition and metabolism of the 

 muscle-fibrils (i.e. of the contractile substance) are intrinsically 

 dependent on the bulk of sarcoplasm, this relation is easy to 

 understand. As a matter of fact there can be no doubt that 

 energetic chemical changes do go on in the sarcoplasm, as is 



