212 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



tation of its contractile power. Every act of contraction, then, may be 

 said to involve the death of a certain amount of muscular tissue ; and, 

 on the principles formerly laid down (CHAP. I., Sect. 3), we may look 

 upon the development of contractile power as an expenditure of the 

 vital force which that tissue previously possessed, and which ceases 

 to exist as such, when the elements of the tissue enter into new 

 combinations. 



362. On the other hand, the muscular substance is repaired by an 

 act of nutrition, at the expense of the fibrine supplied to it by the 

 circulating fluid. There are certain muscles, as the heart, and the 

 muscles of respiration, whose action is necessarily constant ; and their 

 reparation must take place as unceasingly as their waste. In these 

 muscles, no sense of fatigue is ever experienced. But in the muscles 

 which are usually put in action by the will, this is not the case. Any 

 prolonged exertion of them induces fatigue ; and this fatigue is an evi- 

 dence of their impaired condition, and of the necessity of rest to impart 

 to them a renewal of vigour. The rest of muscles is essential to the 

 recovery of their powers ; and this recovery is due to the nutritive 

 operations, which then take place unchecked, and which repair the 

 losses previously sustained. The permanently-increased flow of blood 

 to a muscle, which takes place when it is continually being called into 

 vigorous action, is thus on the one hand occasioned by the demand for 

 oxygenated blood created by its use, whilst on the other hand it tends 

 to increase the power of the muscle by an augmentation of its nutrition. 

 Hence it is, that, the more a muscle is exercised, the more vigorous 

 and more bulky does it become. This is equally the case, whether 

 the exercise of the muscle be voluntary or not. We see examples of 

 it in the arms of the smith and in the legs of the opera-dancer ; and 

 we have a still more striking manifestation of it in those cases, in 

 which an obstruction to the exit of urine through the urethra, has called 

 for increased efforts on the part of the bladder, the continuance of 

 which gives rise to an extraordinary augmentation in the thickness of 

 its muscular coat. 



363. Thus we see that the property of Irritability is a vital endow- 

 ment peculiar to muscular tissue, and dependent for its existence upon 

 due nutrition of that tissue ; that it may be called into exercise by cer- 

 tain^stimuli, applied either to the muscle itself, or to the nerve supply- 

 ing it, provided that the muscle be also permeated with oxygen ; that 

 it may be exhausted by repeated stimulation, but is then recovered by 

 rest, provided that there be no obstacle to the nutrition of the muscle ; 

 that the nutrition of the muscle is impaired by continued repose, and 

 that ^its irritability diminishes in the same proportion ; that the nutri- 

 tion is increased by frequent use, and that the power of the muscle then 

 augments in like degree ; and finally, that the departure of muscular 

 power, which ensues upon the general death of the system, is depen- 

 dent in part upon the cessation of the supply of oxygen, and in part 

 npon changes in the composition of the muscle itself, which are no 

 longer compensated by the functions that keep it in its normal condition 

 during life. The rapidity of these changes is the greatest in warm- 

 blooded animals, in which also the muscular irritability is most depen- 



