STRUCTURE OP MUSCULAR TISSUE. 809 



a large amount of new tissue is developed, and the muscles augment in size 

 and vigor. This is true not only of the whole muscular system when equally 

 exercised, but also of any particular set of muscles which is more used than 

 another. Of the former we have an example in those who practice a system 

 of gymnastics adapted to call the various muscles alike into play ; and of 

 the latter in the limbs of individuals who follow any calling that habitually 

 requires the exertion of either pair, to the partial exclusion of the other, as 

 the arms of the smith or the legs of the opera dancer. But this increased 

 nutrition cannot take place unless an adequate supply of food be afforded ; 

 and if the amount of nutritive material be insufficient, the result will be a 

 progressive diminution in the size and power of the muscles, which will mani- 

 fest itself the more rapidly as the amount of exertion, and consequently the 

 degree of waste, is greater. Nor can it be effected if the exercise be incessant, 

 for it is during the intervals of repose that the reparation of the muscular 

 tissue occurs; and the Muscular system, like the Nervous, may be worn out 

 by incessant use. The more violent the action, the longer will be the period 

 of subsequent repose required for the reparation of the tissue; and the 

 longest time will of course be requisite when (as sometimes occurs) the con- 

 tractility of the muscle is so completely exhausted by excessive stimulation 

 that no new manifestation of it can be excited. It does not appear improb- 

 able that there is a provision in some Muscles, as the Heart and Respiratory 

 muscles, by which the nutrition is carried on with unusual activity during 

 the short period of repose which intervenes between two successive contrac- 

 tions. Moreover, the muscular tissue, like all the softer and more decom- 

 posable portions of the organized fabric, has a limited term of existence ; 

 and hence, even if its contractility be not called into exercise, it undergoes 

 a gradual disintegration, so soon as all the nutritive changes of its fibres are 

 completed. This change seems to be a necessary consequence of the high 

 temperature of the bodies of warm-blooded animals ; for it does not occur 

 with nearly the same rapidity in cold-blooded animals, nor in the hiber- 

 nating condition of certain warm-blooded Mammalia; indeed, when the tem- 

 perature of the body is reduced to within a few degrees of the freezing-point, 

 no chemical change seems possible in muscle its spontaneous decay and its 

 vital activity being alike checked. Now, when a muscle or set of muscles 

 in a warm-blooded animal is reduced to a state of prolonged inactivity, from 

 whatever cause, its supply of blood is diminished, and its spontaneous decay 

 is not compensated by an equally active renewal ; so that in time, the char- 

 acters of the structure are changed, and its distinguishing properties are no 

 longer presented. Thus it was found by Dr. John Reid 1 that in a rabbit, a 

 portion of whose sciatic nerve had been removed on one side, the muscles of 

 that leg were but very feebly excited to contraction by galvanism after the 

 lapse of seven weeks. The change in their nutrition was evident to the eye, 

 and was made equally apparent by the balance. The muscles of the para- 

 lyzed limb were much paler, smaller, and softer than the corresponding mus- 

 cles of the opposite leg, and they scarcely weighed more than half being 

 only 170 grains, whilst the others were 327 grains. It was found also that a 

 perceptible difference existed in the size of the bones of the leg, even after 

 so short an interval had elapsed; the tibia and fibula of the paralyzed limb 

 weighing only 81 grains, whilst those of the sound limb weighed 89 grains. 

 On examining the muscular fibres with the microscope, it was found that 

 those of the paralyzed leg were considerably smaller than those of the sound 

 limb, presenting a somewhat shrivelled appearance, and that the longitudi- 



1 Edinb. Monthly Journ. of Med. Science, May, 1841 ; and Physiological, Anatomi- 

 cal, and Pathological Researches, p. 10. 



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