THE CIRCULATION OF THE BLOOD. 279 
to so large an extent up to the present time; but that our widen- 
ing experience shows (what ought to have been expected) that 
the greatest allowance must be made for group if not individ- 
ual variations everywhere. There is also evidence to show that 
the mode of stimulation in experimental cases causes the result 
to vary. From such facts as are stated in paragraph seven, it 
is inferred that there are vaso-motor centers in the spinal cord 
which are usually subordinated to the main center in the me- 
dulla, but which in the absence of the control of the chief cen- 
ter in the medulla assume an independent regulating influence. 
A local vaso-motor mechanism does not seem to us neces- 
sary to explain the changes which the blood-vessels undergo, 
and should not be adopted as an article of physiological faith 
till demonstrated to exist. If we assume that the independent 
contractility of muscle-cells is retained in the blood-vessels, 
and that, when freed from the influence of the central nervous 
system, which becomes more and more dominant as we ascend 
the animal scale, there is a reversion to an ancestral condition, 
a new light is thrown upon the facts. It is a case of old habits 
gaining sway when the check-rein of nervous influence is re- 
moved; and, as we shall show from time to time, this law applies 
to every organ of the body. Moreover, not to go beyond the 
vascular system, this independent rhythmic activity is seen in 
the isolated sections of the pulsatile veins of the bat’s wing, 
devoid,-so far as we know, of nervous cells. .Such facts lend 
some color to the view that, after distention of the vessels by 
the cardiac systole, the return to their previous size is aided by 
rhythmical contractions of the muscle-cells. 
Let us now consider certain other well-known experimental 
facts : 
1. There is a nerve with variable origin, course, etc., in dif- 
ferent mammals, but in the rabbit given off from either the 
vagus, the superior laryngeal, or by a branch from each, 
which, running near the sympathetic nerve and the carotid 
artery, reaches the heart, to which it is distributed. This is 
known as the depressor nerve. 
2. The vagi nerves having been divided, stimulation of the 
central end of the cut depressor nerve is followed by a fall in 
blood-pressure, which may not be accompanied by any altera- 
tion in the cardiac rhythm. 
3. This effect may in great part be prevented if the splanch- 
nic nerves be divided previous to stimulation of the depressor. 
4, If the splanchnic area (region of the main abdominal 
