PROPERTIES OF MUSCULAR FIBRE. 281 



time than those of adults ; on the other hand, those of Birds lose their con- 

 tractility sooner than those of Mammalia. Hence, as a general rule, the dura- 

 tion of the contractility is inversely to the amount of respiration. Muscular 

 contractility is deadened by many substances, especially by those which have 

 a narcotic or sedative action on the nervous s^tem. In carbonic acid gas, 

 hydrogen, carbonic oxide, or sulphurous acid gas, muscles contract very feebly, 

 or not at all, when stimulated ; whilst in oxygen they retain their contractility 

 longer than usual. Narcotic substances, such as a watery solution of opium, 

 when applied directly to the muscles, have an immediate and powerful effect 

 in diminishing or even destroying their contractility ; this effect is also pro- 

 duced, though in a less powerful degree, by injecting these substances into 

 the blood. In the same manner, venous blood, charged with carbonic acid, 

 and deficient in oxygen, has the effect of a poison upon muscles ; diminishing 

 their contractility, when it continues to circulate through them, to such a 

 degree, that they sometimes lose it almost as soon as the circulation ceases, as 

 is seen in those who have died from gradual and therefore prolonged Asphyxia. 

 The unfavourable influence of venous blood is also shown in the Morbus 

 Cceruleus ; patients affected with which are incapable of any considerable 

 muscular exertion. Most of the stimuli which occasion muscular contraction, 

 when directly applied to their fibres, operate also when applied to their motor 

 nerves ; but the same does not hold good in regard to those agents which 

 diminish contractility. It is a fact of some importance, in relation to the dis- 

 puted question of the connection of muscular contractility with the nervous 

 system, that when, by the application of narcotic substances to the nerves, 

 their vital properties are destroyed, the contractility of the muscle may remain 

 for some time longer ; and the latter must, therefore, be independent of the 

 former. Hence we should conclude, that contractility must be a property 

 really inherent in Muscular tissue, which may be called into action by various 

 stimuli, and which may be weakened by various depressing agents, applied 

 to itself ; and that the nerves have the power of conveying the stimuli, which 

 call the property into action, but have little or no other influence on it. 



377. It seems to be a general law of Muscular Contraction, that it shall alter- 

 nate with relaxation at no long intervals. This is most evident in the action 

 of the Heart, and in the peristaltic movements of the Alimentary canal. In these 

 parts, the whole or a large part of the fibres seem to contract together, and then 

 shortly to relax ; but this is probably no less true, as has been just shown, of 

 the individual fibres of those muscles which are kept in a state of contraction 

 by an effort of the will ; since none of them appear to remain in a contracted 

 state for any length of time. The peculiar contractility of Muscular tissue, 

 like the vital properties of other parts, is diminished by want of action ; and 

 in this, as in other cases, it is quite clear that the cause of its loss is to be 

 found in the alteration of the nutritive processes, which results from the ces- 

 sation of the usual operations of the part ( 221). In persons whose lower 

 extremities have, from any cause, been long disused, not only does the bulk of 

 the muscles much diminish, but their characteristic structure in great part 

 disappears, degenerating into fat mixed with ordinary fibrous tissue. On the 

 other hand, a frequently-renewed exercise of muscular contractility increases 

 the power, by stimulating the increased nutrition of the muscles, which be- 

 come more developed, and consequently more powerful; this is welt seen in 

 the arms of the Smith or Waterman, and in the legs of the Opera-dancer. 

 But the exercise must not be too constant ; for it appears to be during the 

 intervals of rest, that the increased nutrition chiefly takes place ; and if the 

 action have been of a violent character, the contractility of the muscle is for a 

 time exhausted, and can only be restored by an interval of inactivity. A great 

 variety of evidence has been, for some time, conducting physiologists to the 



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