CHAP, iv.] THE VASCULAR MECHANISM. 353 



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 

 salivary 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' 

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

 kidney may change in calibre independently of any influences 

 proceeding from the central nervous system, after for instance all 

 the nerves going to the kidney have been divided ; in such cases 

 the changes in the calibre of the renal vessels seem to be due to 

 some direct local action ; and it is possible that the flushing of the 

 alimentary canal when food enters it is similarly, in part or at 

 times, the result of some local action on the blood vessels. 



180. Vaso-motor nerves of the Veins. Although the veins are 

 provided with muscular fibres and are distinctly contractile, and 

 although rhythmic variations of calibre due to contractions may 

 be seen in the great veins opening into the heart, in the veins of 

 the bat's wing, and elsewhere, our knowledge as to any nervous 



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