198 VASO- MOTOR NERVES. [BOOK i. 



The small arteries frequently manifest what may be called 

 spontaneous variations in their calibre, and these variations are 

 very apt to take on a distinctly rhythmical character. If a small 

 artery in the web of the frog be carefully watched, it will be seen 

 from time to time to vary very considerably in width, without any 

 obvious change taking place in the heart's beat or any events 

 occurring in the general vaso-motor system. Similar variations may 

 be witnessed in the vessels of the mesentery of a mammal. The 

 most striking and most easily observed instance of rhythmical 

 constriction and dilation is to be found in the median artery of the 

 ear of the rabbit. If the ear be held up before the light, it will be 

 seen that 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 thick 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 

 several times a minute. 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. Similar rhythmic 

 variations in the calibre of the arteries have been observed in 

 several places, ex. gr. in the saphena artery of the rabbit, in 

 the axillary artery of the tortoise, and in the small arteries 

 of the muscles of the frog ; probably they are widely spread. They 

 may be compared with the rhythmic movements of the veins in the 

 bat's wing and of the caudal vein of the eel. 



The extent and intensity of the constriction or dilation which 

 may be observed in the frog's web are found to vary very largely. 

 Irregular variations of slight extent occur even when the animal 

 is apparently subjected to no disturbing causes; 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. Thus section of the 

 sciatic nerve is generally followed by a dilation 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. 



These facts shew that the contractile elements of the minute 

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

 relaxation of causing constriction or dilation of the calibre of the 

 arteries ; and that this condition of constriction or dilation may be 

 brought about through the agency of nerves. 



