THE CONTROL OP THE CIRCULATION 243 



severing the dilator but not the constrictor path, and again depressor 

 stimulation caused the volume of the limb to increase, indicating that 

 an inhibition of constrictor tone must have occurred. 



Reciprocal Innervation of Vascular Areas 



It must not be imagined that changes in the caliber of the blood ves- 

 sels occurring in one vascular area are necessarily occurring all over 

 the body. On the contrary, a -most important reciprocal relationship 

 exists in the blood supply to different parts. After food is taken, for 

 example, more blood is required by the digestive organs than when they 

 are at rest, and this is insured by dilatation of their own vessels along 

 with reciprocal constriction of those of other parts of the body. On 

 account of the relatively great capacity of the abdominal vessels, their 

 dilatation during digestive activity is usually greater than the reciprocal 

 constriction of the other vessels, so that the diastolic blood pressure falls, 

 necessitating a more powerful cardiac discharge in order to maintain 

 the mean pressure. After taking food, the systolic pressure does not 

 as a rule fall so much as the diastolic, if it falls at all; and the pres- 

 sure pulse therefore becomes greater and causes a greater live load to 

 be applied to the vessels with each heartbeat. During the sudden strain 

 that is thrown on them, weakened arteries may give way, especially in 

 the brain. 



Another example of reciprocal action of the vascular system is seen 

 in muscular exercise. The vessels of the active muscles dilate, while 

 those elsewhere constrict. The local dilatation in this case is, however, 

 not entirely at least a nervous phenomenon, being caused in fact, as we 

 shall see, by hormone action on account of the local increase in hydro- 

 gen-ion concentration 4 (see page 414). There can be little doubt that 

 local irritants to the surface of the body, such as hot applications, lini- 

 ments, etc., act in the same way; they cause local dilatation of the super- 

 ficial and perhaps of the immediately underlying vessels and constric- 

 tion of those elsewhere in the body. Application of cold to local areas 

 of skin similarly causes local constriction accompanied by reciprocal 

 dilatation elsewhere. This action of cold is very marked in some parts of 

 the body, such as the hands, where by Stewart's method (page 283) it 

 can be shown, not only that the bloodflow of the hand to which the cold 

 is applied is greatly curtailed, but also that of the opposite side. 



Experimental demonstration of reciprocal vascular innervation is fur- 

 nished by numerous experiments. If the central end of the great auric- 

 ular nerve of the ear is stimulated in a rabbit, a blanching of the ves 

 sels of the ear occurs at the same time as a rise in arterial blood pres- 



