CHAP, iv.] THE VASCULAR MECHANISM. 217 



intermittent into a continuous flow. Hence, the dilated arteries, 

 their elasticity not being called into play so much as before, will 

 not contribute their usual share towards destroying the pulsations 

 which -reach them at the cardiac side. The pulsations will travel 

 through them less changed than before, and may, in certain cases, 

 pass right on into the veins. This is frequently seen in the sub- 

 maxillary gland, when the chorda tympani is stimulated. The 

 channels being wider, resistance being less, and the force of the 

 heart behind remaining the same, more blood than before passes 

 through the area in a given time ; or, put differently, the same 

 quantity of blood passes through the area in a shorter time. The 

 blood, consequently, as it passes into the veins is less changed than 

 in the normal condition of the area. Usually the flow is so rapid 

 that the oxy-hsemoglobin of the corpuscles is deoxidised to a much 

 less extent than usual, and the venous blood still possesses an 

 arterial hue. On the other hand, since more blood passes in 

 a given time, there is an opportunity for an increase in the total 

 interchange between the blood and the tissue. Thus the total 

 work may be greater, though the share borne by each quantity of 

 blood is less. 



The general effects of dilation are briefly these. Supposing 

 that the total quantity of blood issuing from the ventricle remains 

 the same, that is to say, supposing that the quantity of blood put 

 into circulation is constant, the surplus passing through the dilated 

 area must be taken away from the rest of the circulation. Con- 

 sequently the fulness of the dilated area will lead to an emptying 

 of the other areas. This is seen very clearly when the dilated area 

 is a capacious one. At the same time, local dilation causes a local 

 diminution of peripheral resistance. This in turn causes a lower- 

 ing of the general arterial pressure ; to this we have already called 

 attention. 



The effects of local constriction, similarly local and general, are 

 naturally the reverse of those of dilation. In the vascular area 

 directly affected, less blood passes through the capillaries in a given 

 time, and in consequence less total interchange between the blood 

 and the tissues takes place, though each unit volume of blood which 

 does pass through is more deeply affected. The blood-pressure in 

 the corresponding arteries is increased, and, if the area be large, the 

 pressure in even distant arteries may be heightened. 



Thus, to indicate results in a general manner, local dilation en- 

 courages a copious flow of blood through the area where the 

 dilation is taking place, and, by reducing the blood-pressure, 

 hinders the flow of blood into other areas. Local constriction, on 

 the other hand, lessens the flow of blood in the particular area, and 

 by heightening the blood-pressure tends to. throw the mass of the 

 blood on to other areas. Hence the great regulative value of the 

 vaso-motor system. By augmenting or inhibitory influences (con- 

 strictor or dilating) applied either to peripheral mechanisms or to 



