CHAPTER IT 



CHANGE OF FORM IN MUSCLE DURING ACTIVITY 



SOME of the manifestations concomitant with activity in the 

 muscle, e.g. the resulting changes in optical properties, have been 

 described in the previous section. The chemical constitution of 

 muscle -substance, and its alterations in activity, can best be 

 studied in recent text-books of Physiological Chemistry. There 

 remains for consideration the most striking manifestation of 

 muscular activity, i.e. change of form (contraction). The most 

 essential feature contraction in a longitudinal direction with simul- 

 taneous expansion (increase of cross-section) appears, as a matter 

 of course, in all cases where the contractile particles of proto- 

 plasm lie permanently or temporarily in a definite direction. 

 This has already been pointed out re the more or less rapid, 

 sometimes instantaneous, shortening' and thickening of certain 

 forms of pseudopodia (Myopodien, Myophryskeri), as well as in the 

 myoid layer, or myonema, of certain infusoria, which must be 

 regarded as true muscle. Indeed, the same changes of form may 

 be observed, as it were, in an elementary stage in single fibrils, 

 or bundles of fibrils, that reappear in the highly-complex, multi- 

 cellular organs which it is usual to designate as muscles in more 

 highly-organised animals. In every case the mechanical effect of 

 change of form in a muscle depends upon its shortening in a 

 longitudinal direction never upon its thickening. Hence it is 

 customary only to speak of shortening, or contraction, with refer- 

 ence to muscle-activity. 



We have already stated in discussing the manifestations of 

 activity in the myoideum that a single stimulus of very short 

 duration, e.g. a single, quick alteration of density in an electrical 

 current, or the shortest possible mechanical impact, produces an 



