530 THE THIRD HUXLEY LECTURE 



over the contractions of the arteries of the foot, and that the function is exercised 

 by the entire posterior lialf of the cord. 



The very transient effect of section of tlie sciatic proved that, as is the 

 case with the pigmentary functions, one nerve can take the place of others in 

 the duty. And I found that even after division of all the soft parts of the 

 thigh except the main artery and vein, the vessels of the webs soon recovered 

 their contractile power ; showing that the control of the cord over the arteries 

 can be kept up in an extensive region through quite insignificant nervous twigs. j 



We can thus understand how a flap of skin raised in a plastic operation in the 

 human subject may have its circulation duly regulated through a narrow neck 

 of attachment. 



An experiment that greatly pleased Dr. Sharpey would seem to indicate 

 that the vaso-motor function for the hind legs is also discharged by the most 

 anterior part of the cord. A frog having been arranged as before described, 

 the point of a fine needle, curved at the end, was introduced into the wound 

 behind the occiput, so as to prick for an instant the divided medulla. The eye 

 of the observer being over the microscope while this was done, the arteries of 

 the web were seen to contract to complete obliteration of calibre, as in the first 

 experiment with warm water. 



The contraction of the vessels caused on that occasion by the application 

 of gentle heat to the web was now explained as a ' reflex action ' through the 

 spinal cord. Their dilatation under irritation remained to be accounted for. 

 In describing that first experiment, I stated that the extreme constriction of 

 the arterioles was followed by relaxation to a larger calibre than they had before 

 the water was applied ; suggesting the idea of fatigue after exertion. I have 

 now to add that, if the warmth was longer continued, the subsequent relaxation 

 was more marked and of longer duration ; and if the water was made somewhat 

 hotter the contraction that preceded the dilatation was so transient as to be 

 barely discernible.^ 



We seem to have here an exact parallel to what occurred as the result of 

 the action of heat upon the ciliated epithelium. And the natural view seemed 

 to me to be that the ganglion cells of the cord concerned in the arterial con- 

 tractions were affected by the nervous impulse conveyed to them by the afferent 

 fibres according to the same law that governed the direct action of heat upon 

 the epithelium cells ; increased activity or suspension of function being induced 

 according to the degree of energy of its operation. I incline still to believe 

 that this was a correct interpretation of the phenomena of active congestion. 



^ Brief contraction of the vessels, followed by dilatation, had been previously observed by others 

 as the result of the application of irritants to the frog's web. 



