172 BLOOD-VESSELS. 



elasticity, and therefore suddenly, but afterwards slowly. The contractility of 

 the smaller arteries, as well as its subjection to the influence of the nervous sys- 

 tem, is beautifully shown in the experiment of cutting and afterwards stimulating 

 the cervical sympathetic nerve in a cat or rabbit. Immediately after the sec- 

 tion, the vessels of the ear become distended with blood from failxire of their 

 tonic contraction ; but, on ajiplying the galvanic stimulus to the upper portion of 

 the nerve, they immediately shrink again, and on interrupting the stimulation 

 they relax as before. The tonic contraction of these vessels appears to be main- 

 tained by the medulla oblongata operating through the branches of the cervical 

 part of the sj-mpathetic nerve ; it is found, moreover, that stimvdation of this 

 so-called '' vaso-motor " centre causes marked contraction of the arteries of the 

 body generally, especially the smaller ones. 



The contractility of the large arteries is not so conspicuous, and many excel- 

 lent observers have failed to elicit any satisfactory manifestation of such property 

 on the application of stimuli to these vessels. Others, however, have observed a 

 sufficiently decided, though by no means a striking degree of contraction slowly 

 to follow mechanical nritation or electric stimulation of these arteries in recently- 

 killed animals. To render this effect more evident, C. J. B. Williams adopted a 

 method of experimenting which he had successfully employed to test the irrita- 

 bility of the bronchial tubes. He tied a bent glass tube into the cut end of an 

 artery, and filled the vessel, as well as the bend of the tube, with water ; the 

 application of galvanism caused a narrowing of the aitery. the reality of which 

 was made manifest by a rise of the fluid in the tube. Cold causes contraction of 

 the larger arteries, according to the testimony of various inquirers ; and, as in 

 the smaller arteries, a gradual shrinking in calibre ensues, when the distending 

 pressui-e of the blood is taken off, by the extinction or impairment of the force 

 of the heart on the approach of death. From the experiments of C. Parry, it 

 would appear that the contraction thus ensuing proceeds considerably beyond 

 what would be produced by elasticity alone, and that it relaxes after death, 

 when vitality is completely extinct, so that the artery widens again to a certain 

 poiat, at which it is finally maintained by its elasticity. 



Mode of distribution. — The veins are ramified throughout the 

 "body, lilvc the arteries, but there are some differences in their propor- 

 tionate nmiiber and size, as well as in their arrangement, which require 

 to be noticed. 



In most regions and organs of the body the veins are more numerous 

 and also larger than the arteries, so that the venous system is alto- 

 gether more capacious than the arterial, but the proportionate capacity 

 of the two cannot be assigned with exactness. The pulmonary veins 

 form an exception to this rule, for they do not exceed in capacity the 

 pulmonary arteries. 



The veins are arranged in a superficial and a deep set, the former 

 running immediately beneath the skin, and thence named subcutaneous, 

 the latter commonly accompanying the arteries, and named vemc comites 

 vel micllitcs arieriarwn. The large arteries have usually one accom- 

 panying vein, and the medium-sized and smaller arteries two ; but 

 there arc exceptions to this rule ; thus, the veins within the skull and 

 si)inul canal, the hepatic veins, and the most considerable of those 

 belonging to the bones, run apart from the arteries. 



The communications or anastomoses between veins of considerable 

 size, are more frequent than those of arteries of equal magnitude. 



Structure. — The veins have much thinner coats tlian the arteries, 

 and collapse when cut across or emptied ; whereas a cut artery presents 

 a patent orifice. ^ But, notwithstanding their comparative thinness, the 



