Chap, iv.] THE VASCULAR MECHANISM. 263 



therefore paler. During 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, of 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 nerves of 

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

 leg 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 peripheral stump 

 of a divided nerve by an interrupted current of moderate in- 

 tensity 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 a very large number of, 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. 



§ 142. 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 



