Page:The third Huxley lecture.pdf/38

34 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 contractions 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