168 



THE MUSCLE CELLS 



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JZL 



ing glucose and other crystalloids. The length of time necessary 

 to make the muscle fit to perform again its full complement of 

 work depends, under ideal conditions, on the amount of work it 

 has been called on to do in the immediate past. As indicated 

 above, these are the findings of experiments in which the condi- 

 tions were not natural. 



A muscle doing work in the body is supplied with adequate 

 means (as we shall see in future chapters) of keeping up its store of 

 energy. Like the starting-lighting battery of a motor car, it 

 stores and uses energy sinmltaneously. It has, moreover, at least 

 one advantage over the car battery : it is self-attending and self- 

 adjusting. It regulates (by means of the balance of hydrophilic 

 colloids and crystalloids) its own water level, and, by the " buffers " 

 in its complex, maintains its H^ concentration. It also carries 



out (by means of circulatory changes) its 

 own cleaning and repairs. Further, if it 

 is asked to pro\'ide for a heavy discharge 

 fairly regularly, it meets this demand by 

 adding to the size and number of its cells. 



This wonderful transformer has been the 

 subject of many investigations — as to its 

 structure and mechanism. 



Structure. The units of the accumulator 

 are long cells, which, in muscles like the 

 semi-membranosus and sartorius, are prac- 

 tically as long as the muscle (excluding 

 tendinous attachments). In other muscles 

 {e.g. gastrocnemius) they are just a little 

 over half the length of the muscle. These 

 fibres, consisting of fibrils which lie side by side immersed in sarco- 

 plasm, though they, of necessity, all undergo the same amount 

 of shortening together, do not necessarily all develop tension 

 during this process. If the muscle is feebly stimulated only 

 a few fibres actively shorten, the others passively adjusting 

 themselves to keep their due place by their fellows. The 

 stronger the stinudus, the more fibres are involved till, with 

 a certain strength of stimulus, all the fibres are activated. No 

 further increase of stimulus can then produce any further effect. 

 That is, other factors remaining constant, the power exhibited by a 

 inuscle depends on the number of muscle fibres involved. Muscles 

 are, as we have just seen, of different lengths. If a maximal 

 contraction is induced, and each muscle is able to shorten to, say, 

 half its length, then obviously the longer the muscle, the greater 

 will be the distance through which it can pull its load (Fig. 41). 



Fig. 41. — Influence of tlie 

 length of a muscle upon the 

 work done. A muscle one 

 Inch long (left-hand figure) 

 in contracting to half its 

 length lifts a weight over half 

 an inch. A muscle of two 

 inches, on the other hand, is 

 capable of Ufting the weight 

 over one inch. 



(Noel Paton's Essentials of 

 Human Physiology.) 



