248 THE CIRCULATION OF THE BLOOD 



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 011 account of the local increase in hydro- 

 gen-ion concentration (see page 431). 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 296) 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, dilatation of the ves- 

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

 sure (Loven reflex). Similarly when the central end of one of the sen- 

 sory roots of the leg of a dog is stimulated, there is a rise in arterial 

 blood pressure and an increase in the volume of the limb. 



THE INFLUENCE OF GRAVITY ON THE CIRCULATION 



If the arterial blood pressure is measured in the arm and leg in a man 

 standing erect, a difference corresponding to the hydrostatic effect of 

 gravity will be found between the two readings. In comparison with 

 the high pressure normally existing in the arteries, this difference is, 

 however, of little significance. On the other hand, in the veins, where 

 the average pressure is low, gravity would cause serious embarrassment 



