CONDITIONS OP MUSCULAR ACTIVITY. 451 



pendent upon several conditions. In the first place, it requires 

 an active nutrition of the muscles themselves. Firm, plump, 

 and high-coloured muscles act with greater force than those 

 which are pale and flabby, even though the size of the latter 

 may be greater. Again, in all those animals whose activity is 

 greatest, a constant supply of oxygen is requisite for muscular 

 vigour. This, like the nutritive material, is conveyed, in 

 Birds and Mammals, by the blood ( 235) ; in Insects, on the 

 other hand, it actually enters the muscular tissue in the state 

 of atmospheric air ( 321). In Eeptiles, again, the blood 

 goes to the tissues very imperfectly oxygenated ; and their 

 movements are comparatively slow and feeble. But it is a 

 remarkable circumstance, that in the dead bodies of the latter, 

 or in parts separated from the living body, the property of 

 contractility does not depart nearly so soon as it does in similar 

 parts of warm-blooded animals. By experiments on Mammals 

 it has been found that the muscles of the trunk cannot be 

 caused to contract by galvanism for more than two or three 

 hours after death, though the auricles of the heart retain their 

 contractility for some hours later. The muscles of Birds 

 (whose respiration is more active, and whose temperature is 

 higher) lose their contractility yet sooner ; but those of Rep- 

 tiles sometimes retain the power of contracting for several 

 days. When venous or imperfectly-aerated blood is made to 

 circulate through the vessels of warm-blooded animals, it acts 

 like a poison upon them, diminishing or even destroying their 

 contractility. 1 



592. Further, the energy of muscular contraction depends 

 in great degree upon the power of the stimulus which is trans- 

 mitted to it through the nervous system. We often have the 

 opportunity of observing this, in the case of persons who are 

 under the excitement of violent passion or of insanity; a 

 delicate female becoming a match for three or four strong 

 men, and even breaking cords and bands that would hold the 

 most powerful man in his ordinary state. The strength in 

 such circumstances seems almost preternatural ; but it is not 



1 Other substances do this with even greater rapidity ; thus a strong 

 solution of nitrate of potass (nitre) injected into the blood-vessels, and 

 conveyed by them to the heart, causes the immediate cessation of its 

 action, the poison finding its way, through the vessels of the organ 

 itself, into the capillaries of its muscular structure. 

 GG2 



