438 



MANNER OF MUSCULAR CONTRACTION. 



acid juice and the alkaline plasma of the blood are doubtless the cause 

 of the production of those electric currents which have been demonstrated 

 in the muscles. It does not follow, therefore, that these currents occur 

 in the natural state : they may be the result of the experimental arrange- 

 ment for their own detection, since it has long been known that an acid 

 and an alkaline juice, separated from each other by a conducting organic 

 body, will form an effective voltaic circle. 



Of the contractile element of muscular fibre, syntonin, it may be re- 

 marked that it can be dissolved by the aid of dilute hydrochlo- 

 ric acid, and that it differs from fibrin of blood not only in that 

 respect, but also both in its ultimate composition and physical and chem- 

 ical qualities. In certain cases it seems to degenerate into fatty sub- 

 stance. In the growth of a muscle, the constituent fibrils increase in 

 number and in length, their diameter remaining, however, nearly the 

 same as in the early periods of life. The thickening of a muscle is, 

 therefore, not so much due to the thickening of its constituent fibrils as 

 to their increase of number. 



The contraction of a muscular fibre does not take place throughout its 

 Manner of con wno ^ e l en gth at once ; it generally begins at the end, a change 

 traction of a of aspect arising from the approach of the opaque centres of 

 Hie cells to one another, and this occurring simultaneously 

 across the whole fibre. This approach may, however, ensue in different 

 parts of the length at the same time, the sarcolemma being raised up in 

 bulla3 as the contraction takes- place. This effect is shown in Fig. 223, 



Fig. 223. 



Contracting muscle of Dytiscus. 



in which the thickened portion of the contracted middle space of the mus- 

 cle is surrounded with the sarcolem- 

 mic bullas. 



Fig. 224. 



The same is demon- 

 strated in Fig. 224, which repre- 

 sents the border of a muscular fas- 

 ciculus in a young crab, with a spot 

 of contraction, and the sarcolemma 

 elevated along the edge. In these 

 cases the contraction is brought on 

 by the action of water, which, in 

 some measure, may exaggerate or disturb the phenomena. Fig. 225 

 exhibits, under the same circumstances, a fasciculus from the eel, a being 

 the uncontracted, b the contracted part, on the edge of which the sarco- 



Sarcolemma raised in bullse. 



