CONTRACTILITY OF ARTEEIES. 171 



contractile movements. The nerves come chiefly from the sympathetic, 

 but also from the cerebro-spinal system. They form plexuses round the 

 larger arteries, and run along the smaller branches in form of fine 

 bundles of fibres, which here and there twist round the vessel, and 

 single nerve-fibres have been seen closely accompanying minute arteries. 

 The fine branches destined for the artery penetrate to the middle coat, 

 in which they are chiefly distributed. They lay aside their medullary 

 sheath and form a plexus of pale fibres, the finest of which are without 

 nuclei. 



Minute ganglia have been described by Beale and others as connected 

 with the arteries, or even in the case of the larger ones, situated in the 

 external coat. From these, fine nerves proceed to be distributed, chiefly 

 in the form of plexuses, to the muscular tissue of the middle coat. 



Contractility. Besides the merely mechanical property of elasticity, arteries 

 are endowed in a greater or less degree with vital contractility, hy means of 

 which they can narrow their calibre. This vital contractility, which has its 

 seat in the plain muscular tissue of the middle coat, does not cause rapid con- 

 tractions following in rhythmic succession like those of the heart ; * its opera- 

 tion is, on the contrary, slow, and the contraction produced is of long endm-ance. 

 Its effect, or its tendency, is to contract the area of the arterial tube, and to 

 offer a certain amount of resistance to the distending force of the blood ; and as 

 the contracting vessel will shrink the more, the less the amount of iiuid con- 

 tamed in it, the vital contractility would thus seem to adjust the capacity of the 

 arterial system to the quantity and force of the blood passing through it, bracing 

 U13 the vessels, as it were, and maintaining them in a constant state of tension. 

 In producing this effect, it co-operates with the elasticity of the arterial tubes, 

 but it can be shown that after that property has reached its limit of operation 

 the vital contraction can go fui-ther in narrowing the artery. The Adtal or mus- 

 cular contractility of the arteries, then, counteracts the distending force of the 

 heart and seems to be in constant operation. Hence it is often named " tonicity," 

 and so far justly ; but at the same time, like the contractility of other muscular 

 structures, it can, by the application of various stimuli, be artificially excited to 

 more -vivid action than is displayed in this natiu'al tonic or balanced state ; and, 

 on the other hand, it sometimes relaxes more than the habitual degree, and then 

 the vessels, yielding to the distending force of the heart, become unusually dilated. 

 Such a remission in their contractile force (taking place rather suddenly) is doubt- 

 less the cause of the turgescence of the small vessels of the skin which occurs 

 in blushing : and the arteries of erectile organs are probably affected in the same 

 manner, so as to permit an augmented flow of blood iato the veins or venous 

 cavities when erection begins. 



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

 transparent parts of cold-blooded animals. If the point of a needle be two or 

 three times dra-^m quickly across one of the little arteries iu the web of a frog's 

 foot placed under the microscope, the vessel wiU 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 per- 

 sisting 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 



* Arteries may, however, exhil^it slow rhjrtlimic contractions : this is especially marked 

 in some of the lower animals, e. g. the rabbit, in the arteries of the ear of which it may 

 readily be observed. It is probably dependent on the presence of minute ganglia in con- 

 nection with the vessels. 



