Page:The third Huxley lecture.pdf/35

31 redness had almost entirely disappeared; most parts that were before apparently intensely inflamed being now pale." The irritating agents acted directly on only a minute portion of tissue; but they induced widespread active congestion, which subsided at once on their removal. Such results could only have been brought about through the agency of the nervous system.

As I before had occasion to remark, active congestion takes place throughout the frog's web when an irritant is applied to any part of it. It was, therefore, possible to study the phenomenon upon that animal. It was not at that time clearly known by what mechanism the constriction of the arterioles was effected, or by what part of the nervous system it was regulated. Kölliker had recently made his great discovery of the fibre-cells of involuntary muscle, and had described them as existing along with elastic tissue in the middle coat of the larger arteries; but the trustworthiness of his observations had not yet been by any means universally recognised, and the structure of the ultimate arterioles, as compared with the capillaries, had not been ascertained.

I found, on dissecting out the vessels from between the layers of the frog's web, and examining them with a high magnifying power, that whereas the capillaries showed only a thin, apparently homogeneous wall beset with occasional nuclei, the finest arteries exhibited three coats, of which the middle one was composed of muscular fibre-cells wrapped spirally round the