212 THE VASCULAR MECHANISM. 



coat of the artery, the capillaries fed by the artery and the veins into which 

 these lead become less filled with blood and 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, the constriction 

 or dilatation 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 disturb- 

 ing 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 experi- 

 mental 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 peripheral 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 the 

 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 vasomotor nerves. 



150. If the ear of a rabbit, preferably a light-colored 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 dila- 

 tation. 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 repeated at 

 somewhat irregular intervals of a minute, more or less. The extent and 

 regularity of the rhythm are usually markedly increased if the rabbit be 

 held up by the ears for a short time previous to the observation. Similarly 

 rhythmic variations in the calibre of the arteries have been observed in 

 several places, e.g., in the vessels of the mesentery and elsewhere; probably 

 they are widely spread. 



Sometimes no such variations are seen ; the artery remains constant in a 

 condition intermediate between the more extreme widening and extreme 

 narrowing just described. In fact, we may speak of an artery as being at 

 any given time in one of three phases. It may be very constricted, in which 

 case its muscular fibres are very much contracted ; or it may be dilated, in 



