248 THE CIRCULATION OF THE BLOOD 



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 acjjj^jnu&el^sdilate, 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 (see page 431). Irritants applied to the surface 

 of the body, such as liniments, etc., cause local dilatation of the super- 

 ficial and perhaps of the immediately underlying vessels, probaTHy by 

 axon reflexes (page 898), and it is believed reflex constriction of those 

 elsewhere in the body. Hot applications (poultices, etc.,) have a similar 

 effect. Application of cold to local areas of skin causes local vasocon- 

 striction. 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. In this case 

 therefore reciprocal action is not obtained. Chilling of the skin as by a 

 '[draught of air also causes constriction of the vessels of the nasal mucosa ; 

 iiot dilatation, as might be expected. On the other hand, stimulation of 

 the central end of the great auricular nerve of the ear in a rabbit, causes 

 dilatation of the vessels of the ear at the same time as a rise in arterial 

 blood pressure (Loven reflex). Similarly, when the central end of one 

 of the sensory 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 (Bay- 

 liss). These last two experiments demonstrated reciprocal innervation. 



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 



