THE THIRD HUXLEY LECTURE 529 



the frog's web wiien 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. Kolliker 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 capil- 

 laries 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 lining membrane. 

 A more efficient mechanism for their constriction could hardly be con- 

 ceived.^ 



As regards the regulation of the arterial contractions, Bernard's classical 

 experiment of inducing turgescence of the vessels of the ear by section of the 

 sympathetic in the neck, and Waller's converse observation that galvanic stimu- 

 lation of the distal end of the divided nerve made the distended vessels shrink 

 and the ear assume unwonted pallor, had demonstrated the great fact of nervous 

 control over the local circulation ; but it was still a subject of discussion what 

 part of the nervous centres exercised the function.-^ 



In entering on the inquiry, I first divided the sciatic nerve, a proceeding 

 which had been stated on high authority to cause relaxation of the arteries of 

 the web. The immediate effect of its division was, indeed, some enlargement 

 of their channels, but this very soon passed off. I then resorted to operations 

 on the roots of the spinal nerves and on the medulla. And, not to weary you 

 too much with details, I may say that removal of all that part of the spinal 

 cord which gives off branches for the hind legs, caused the arteries of the web 

 to relax completely and to remain permanently of about the same calibre as the 

 veins. But if any portion that furnished nerves to the posterior extremities 

 was allowed to remain, whether a little caudal segment or a small anterior part, 

 the arteries resumed, after temporary dilatation, their ordinary and varying 

 dimensions.^ 



It was thus shown that the cerebro-spinal axis is the centre that presides 



' Vide Trans, of the Royal Society of Edinburgh, vol. xxi, part iv" (see vol. i, p. 15). 



^ For authorities on this question, vide Phil. Trans., ibid., pp. O07 ct seq. (vol. i, p. 2~). 



' Vide Phil. Trans., ibid. 



