STRIATED MUSCLE 295 



stance. But whether the finest artificial fibrils into which 

 dead muscle may be broken up are identical with these 

 apparently natural fibrils, it is not at present certainly 

 determined. In some cases the artificial fibrils seem 

 smaller than the natural ones, as if the latter, like the 

 fibre itself, were capable of longitudinal cleavage. 



These are the most important structural appearances 

 presented by ordinary striated muscle. But it may 

 further be noticed that the dim bands exert a powerful 

 influence on polarised light. Hence when a piece of 

 muscle is placed in the field of a polarising microscope 

 and the prisms are crossed so that the field is dark, these 

 bands appear bright. The granular lines have a similar 

 but very much less marked efl'ect. 



In the embryo the place of the adult tissue is occupied 

 by a mass of closely applied, undifferentiated nucleated 

 cells. As development proceeds, some of these cells are 

 converted into the tissues of the perimysium, but others 

 increasing largely in size gradually elongate and take on 

 the form of more or less spindle-shaped rods or fibres. 

 Meanwhile the nucleus of each cell repeatedly divides, 

 and thus each rod becomes provided with many nuclei, 

 so that each fibre is really a multi-nucleate cell. Along 

 with these changes the protoplasmic substance of the 

 original cell becomes, for the most part, converted into 

 the characteristically striated muscle substance, only a 

 little remaining unaltered around each nucleus as a muscle 

 corpuscle. 



The many-nucleated cell tlius changed into a muscular 

 fibre is nourished by the fluid exuded from tlie adjacent 

 capillaries, and it may be said to i-espire, insomuch as its 

 substance undergoes slow oxidation at the expense of the 

 oxygen contained in that fluid, and gives off carbonic 

 acid. It is, in fact, like the other elements of the tissues, 

 an organism of a peculiar kind, having its life in itself, 

 but dependent for the permanent maintenance of that 

 life upon the condition of being associated with other 



