ORGANISATION AND STRUCTURE OF MUSCLE 



19 



ably in the two cases. While in the cells of the adductor and 

 pedal muscles the formative plasma is insignificant in comparison 

 with the contractile substance, in the cardiac and masticatory 

 muscles it preponderates. Knoll, who first investigated these 

 differences systematically, used the terms a-plasmic (" clear ") and 

 plasmic (" dark ") to designate the two cases, the cells of the 

 cardiac and masticatory 



muscles being by far the 





o 



\d 



c 



richest in protoplasm, and 

 less transparent. These 

 comparative investiga- 

 tions prove beyond doubt 

 that the structural ratio 

 is, like cross-striation, of 

 functional significance, as 

 also appears from obser- 

 vations on the muscles 

 of higher animals to be 

 discussed below. The 

 cardiac and buccal muscles 

 have obviously more 

 work, and more persistent 

 work, to do than the 

 adductor muscle of mol- 

 luscs or the pedal muscle 

 of snails, which are used 

 less frequently, and since 

 the formative plasma 

 stands, as will be shown, 

 in close relation with the 

 nutrition of the contractile substance, we can readily appreciate 

 the proportions given. 



This theory is substantially confirmed by the " float " muscles 

 of Carinaria. That portion of the foot which is used for floating, 

 and is in constant movement, corresponds both in the cross- 

 striation of the fibrils, and the abundance of its sarcoplasm, to 

 the type of dark "plasmic" muscle-cells characteristic of the 

 buccal and cardiac muscles of Mollusca. Along with the greater 

 richness of sarcoplasm there is often a more or less definite 

 coloration of the muscular elements. The cardiac and masti- 



FIG. 10. Transverse section of muscle-cells of Mollusca. 

 (Knoll.) , Heart of Aplysia punctata; 6, heart of 

 Aplysia limacina; c, masticatory apparatus of Cari- 

 naria; d, longitudinal view of muscle -cells from 

 buccal mass of Aplysia punctata. 



