CHAP, iv.] THE VASCULAR MECHANISM. 287 



and this increased constriction, like the dilation of blushing, is 

 effected through the agency of the central nervous system and the 

 cervical sympathetic. Blushing and its opposite pallor are most 

 marked in the face; but other parts of the body may blush (or 

 grow pale) the change being brought about by appropriate nerves. 



The vascular condition of the skin at large affords another 

 instance. When the temperature of the air is low the vessels of the 

 skin are constricted, and the skin is pale ; when the temperature of 

 the air is high the vessels of the skin are dilated, and the skin is 

 red and flushed. In both these cases the effect is mainly a reflex one, 

 it being the central nervous system which brings about augmen- 

 tation of constriction in the one case and inhibition in the other ; 

 though possibly some slight effect is produced by the direct local 

 action of the cold or heat on the vessels of the skin. Moreover 

 the vascular changes in the skin are accompanied by corresponding 

 vascular changes in the viscera (chiefly abdominal) of a reverse 

 kind. When the vessels of the skin are dilated those of the 

 viscera are constricted, and vice versa; so that a considerable 

 portion of the whole blood ebbs and flows, so to speak, according 

 to circumstances from skin to viscera and from viscera to skin. 

 By these changes, as we shall see later on, the maintenance of the 

 normal temperature of the body is in large measure secured. 



We shall see later on that the secretion of urine is in a peculiar 

 way dependent on the flow of blood through the kidney. A very 

 favourable condition for this flow is a dilated condition of the renal 

 arteries coincident with a high general blood pressure, and this 

 condition as we shall see leads to a copious secretion of urine. 

 The high general blood pressure in this case is largely caused 

 by very general arterial constriction, leading to great increase 

 of peripheral resistance, while the dilated state of the renal arteries 

 appears to be due to a lack of the usual tonic constrictor impulses ; 

 though these constrictor impulses are increased in respect to other 

 arteries, they are diminished in respect to the renal arteries 

 themselves. 



When food is placed in the mouth the blood vessels of the 

 silivary glands as we have seen are flushed with blood as an 

 adjuvant to the secretion of digestive fluid ; and as the food 

 passes along the alimentary canal each section in turn, with 

 the glandular appendages belonging to it, welcomes its advent by 

 flushing with blood. As we have already said, we have, at present, 

 no satisfactory evidence, except in the case of the salivary glands, 

 that this flushing is carried out by special vaso-dilator nerves. Along 

 the rest of the alimentary canal the widening of the arteries and 

 thus the increased flow seems to be brought about by diminution 

 of vaso-constrictor impulses, so far at least as it is ensured by the 

 intervention of the central nervous system. We say ' so far 1 

 because as we shall see we have evidence that the vessels of the 

 kidney may change in calibre independently of any influences 



