CHAP, iv.] THE YASCULAR MECHANISM. 307 



the widening, which corresponds to the relaxation of the muscular 

 coat, the same parts are fuller of blood and redder. It is obvious 

 that, the pressure at the entrance into any given artery remaining 

 the same, more blood will enter the artery when relaxation takes 

 place and consequently the resistance offered by the artery is 

 diminished, and less when contraction occurs and the resistance is 

 consequently increased ; the blood flows in the direction of least 

 resistance. 



The extent and intensity of the narrowing or widening, the 

 -constriction or dilation which may thus be observed in the frog's 

 web, vary very largely. Variations of slight extent, either more or 

 less regular and rhythmic or irregular, occur even when the animal 

 is apparently subjected to no disturbing causes, and may be spoken 

 of as spontaneous ; larger changes may follow events occurring in 

 various parts of the body ; while as the result of experimental 

 interference the arteries may become either constricted, in some 

 cases almost to obliteration, or dilated until they acquire double or 

 more than double their normal diameter. This constriction or 

 dilation may be brought about not only by treatment applied 

 directly to the web, but also by changes affecting the nerve 

 of the leg or other parts of the body. Thus section of the 

 sciatic nerve is generally followed by a widening which may be 

 slight or which may be very marked, and which is sometimes 

 preceded by a passing constriction ; while stimulation of the peri- 

 pheral stump of the divided nerve by an interrupted current of 

 moderate intensity generally gives rise to constriction, often so 

 great as almost to obliterate some of the minute arteries. 



Obviously then the contractile muscular elements of the minute 

 arteries of the web of the frog's foot are capable by contraction or 

 relaxation of causing decrease or increase of the calibre of the 

 arteries ; and this condition of constriction or dilation may be 

 brought about through the agency of nerves. Indeed not only in 

 the frog, but also, and still more so, in warm-blooded animals have 

 we evidence that in the case of nearly all, if not all, the arteries of 

 the body, the condition of the muscular coat, and so the calibre of 

 the artery is governed by means of nerves ; these nerves have 

 received the general name of vaso-motor nerves. 



164. If the ear of a rabbit, preferably a light coloured one, 

 be held up before the light, a fairly conspicuous artery will be seen 

 running up the middle line of the ear accompanied by its broader 

 and more obvious veins. If this artery be carefully watched it will 

 be found, in most instances, to be undergoing rhythmic changes of 

 calibre, constriction alternating with dilation. At one moment the 

 artery appears as a delicate hardly visible pale streak, the whole 

 ear being at the same time pallid. After a while the artery slowly 

 widens out, becomes broad and red, the whole ear blushing, and 

 many small vessels previously invisible coming into view. Again 

 the artery narrows and the blush fades away ; and this may be 



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