clxxii BLOOD-VESSELS. 



The vital contractility of small-sized arteries is easily demonstrated in the trans- 

 parent parts of cold-blooded animals. If the point of a needle be two or three times 

 drawn quickly across one of the little arteries (not capillaries) in the web of a frog's 

 foot placed under the microscope, the vessel will be seen slowly to contract, and the 

 stream of blood passing through it becomes smaller and smaller, and, by a repetition 

 of the process, may be made almost entirely to disappear. After persisting in this 

 contracted state for some minutes, the vessel will gradually dilate again to its original 

 size. The same effect may be produced by the application of ice-cold water, and also 

 by electricity, especially the interrupted electric current. Moreover, if one of the 

 small arteries in the mesentery of a frog or of a small warm-blooded animal, such as a 

 mouse (Poiseuille), be compressed so as to take off the distending force of the blood 

 from the part beyond the point where the pressure is applied, that part will diminish 

 in calibre, at first no doubt from its elasticity, and therefore suddenly, but afterwards 

 slowly. This gradual shrinking of an emptying artery after its elasticity has ceased 

 to operate, may be shown also by cutting out the frog's heart or dividing the main 

 trunks of the vessels ; it is obviously due to vital contraction. The contractility of the 

 smaller arteries, as well as its subjection to the influence of the nervous system, is 

 beautifully shown in the experiment of cutting and afterwards stimulating the cervical 

 sympathetic nerve in a cat or rabbit. Immediately after the section, the vessels of 

 the ear become distended with blood from failure of their tonic contraction ; but, on 

 applying 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 maintained by the spinal cord operating 

 through the branches of the cervical part of the sympathetic nerve ; it has been found, 

 moreover, that direct stimulation of the spinal cord causes contraction of other arteries, 

 probably through branches of spinal nerves. 



The contractility of the middle-sized and larger arteries is not so conspicuous, and 

 many excellent 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 irritation or electric stimulation of these arteries in recently-killed 

 animals. To render this effect more evident, my former colleague, Dr. C. J. B. Wil- 

 liams, adopted a method of experimenting which he had successfully employed to test 

 the irritability 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 appli- 

 cation of galvanism caused a narrowing of the artery, the reality of which was made 

 manifest by a rise of the fluid in the tube. Contraction is said also to follow the 

 application of chemical stimulants, but, as these may directly corrugate the tissue by 

 their chemical action, the evidence they afford is less satisfactory. Cold causes con- 

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

 as in the smaller arteries, a gradual shrinking in calibre ensues in these vessels, when 

 the distending pressure 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 Dr. 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 point, at which it is 

 finally maintained by its elasticity. 



VEINS. 



Mode of distribution. The veins are ramified throughout the boily, like 

 the arteries, but there are some differences in their proportionate number 

 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 altogether 

 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 



