STRUCTURE OF MUSCULAR TISSUE. 315 



portional to the functional activity of the Muscle ; and their generation, which 

 occurs as constantly when the nutrient operations proceed in their regular course, 

 is probably accomplished by a development from these centres, at the expense 

 of the blood with which the muscle is copiously supplied. 



311. The ordinary rate of Nutrition of the Muscular tissue, depends upon 

 its functional activity. Every exertion of its vital Contractility involves there 

 is good reason to believe a waste or disintegration of a proportional amount of 

 its substance ; the vital force, which previously manifested itself in the acts of 

 growth and development, being now expended in the form of mechanical power ; 

 and the living tissue being consequently reduced to the condition of dead mat- 

 ter, and being thus rendered subject to decomposition, which it previously re- 

 sisted. Of this there is a great variety of evidence. The increase of the 

 demand for food, occasioned by Muscular activity, is an indication that the nu- 

 tritive operations are excited by it; and the purpose of these can scarcely be 

 anything else, than the reparation of the loss which the Muscle has sustained. 

 Again, it will be hereafter shown ( 324), that the presence of Oxygen is essen- 

 tial to the continued development of the contractile force ; and there is evi- 

 dence that, in this development, a chemical change is effected in the substance 

 of the Muscle, which is of a nature destructive to its integrity as an organized 

 tissue. For, in the first place, the researches of Helmholtz, just referred to 

 ( 307), indicate such a change, from the comparative results of Chemical ana- 

 lysis of the muscle, before and after the violent excitement of its contractility ; 

 and this is more definitely shown by that increase in the lactic acid ( 49), and 

 in the creatine and creatinine ( 61), obtainable from Muscle, which is conse- 

 quent upon its functional activity. But still more decided evidence is given 

 to the same effect, by the increase in the excretions which is observable after 

 Muscular exertion ; and especially by the augmentation of the Carbonic acid 

 set free from the respiratory organs, and by that of the Urea eliminated by the 

 kidneys. The amount of the latter, indeed, may be regarded, cseteris paribuSj 

 as an approximate indication of the quantity of Muscular tissue which has 

 undergone disintegration ; being increased or diminished, in precise proportion 

 to the degree of exertion to which the Muscular system has been subjected. 

 It cannot but be regarded as a probable inference from these facts, that the 

 development of the Contractile force is in some way dependent upon the Che- 

 mical change, which seems to be so essential a condition of it; just as the de- 

 velopment of the Electric force of the galvanic battery is dependent upon the 

 new Chemical arrangements, which take place between the bodies brought to act 

 upon one another in its trough. 



312. The frequently-renewed exercise of Muscles, by producing a determina- 

 tion of blood towards them, occasions an increase in their nutrition ; so that a 

 larger amount of new tissue becomes developed, and the muscles are increased 

 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 examples in those, who practise a sys- 

 tem 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 nutri- 

 tion 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 manifest 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 too constant ; for it 

 is during the intervals of repose, that the reparation of the muscular tissue 

 occurs ; and the Muscular system, like the Nervous ( 360), may be worn out 



