CONDITIONS OF 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 Reptiles, 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. 
592. Further, the energy of muscular contraction depends 
n 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 
nen, and even breaking cords and bands that would hold the 
most powerful man in his ordinary state. The strength in 
ach circumstances seems almost preternatural ; but it is not 
_2 Other substances do this with even greater rapidity ; thus a strong 
olution of nitrate of potass (nitre) injected into the blood-vessels, and 
c pered by them to the heart, one. immediate cessation of its 
iction,—the poison finding its way, ugh the vessels of the organ 
elf, into the capillaries of its muscular structure, 
GG2 
