i 14 THE MECHANISM OF THE CIRCULATION. 



in the arterioles be lowered to a certain point. Bernard stimulated the 

 chorda tympani nerve, and found that the blood issued in pulses from 

 the veins of the submaxillary gland. By plunging the hand in very hot 

 water, the pulse may be seen to reach even the turgid veins on the back 

 of the hand. In cases of aortic insufficiency, a capillary pulse is readily 

 obtained in any area of congestion which is produced by scratching the 

 skin. 



The effect of vasodilatation can be observed under the microscope with the 

 greatest ease. On brushing the tongue of a curarised frog, an appearance of 

 intense redness shows that arterial congestion has set in. In the words of 

 Cohnheim, a really magnificent spectacle now presents itself. All the vessels, 

 arteries, capillaries, and veins are wide and strongly distended with blood; 

 innumerable capillaries are perceptible at a glance, where previously a few red- 

 coloured threads were toilsomely sought for ; and in all these vessels, small and 

 large, the blood rushes on with the greatest rapidity — so rapidly that even in 

 the capillaries the eye in vain strives to catch the outline of a single corpuscle. 



By the application of a piece of ice to the tongue of the frog, vaso- 

 dilatation can be converted into constriction. The arteries become narrow, 

 the tongue pale. The eye has difficulty in finding any except the larger 

 vessels ; few capillaries appear to contain blood ; and where a considerable 

 quantity of blood is still present, as in the arteries and veins, the flow is 

 tardy, and even in the arteries the individual corpuscles can now generally be 

 recognised. 



In a warm-blooded animal the results of exposure to an irritant are much 

 more rapidly established; after exposure of the rabbit's ear to water at 56° C, 

 the blood is altogether unable to penetrate the arteries. A change has taken 

 place in the relations betAveen the blood and the vessel wall as regards friction 

 and adhesiveness, and thus complete stasis of the circulation arises. If the 

 change be less intense the porous nature of the vessel is affected, and a quanti- 

 tative and qualitative change in the transudation from the capillaries ensues. 

 The rabbit's ear, with exception of the central artery anil vein, may be entirely 

 separated from the body. After thus dividing all the vasomotor nerves, 

 vascular dilatation will be greatly increased by rubbing the ear, and all the 

 phenomena of inflammation occur in just the same way after the application of 

 an irritant or as a result of iscliamiia. We have here to deal, not with a 

 nervous mechanism, but with a molecular change of the vessel wall. The 

 circulation through the capillaries is possible only so long as the vessel wall 

 is in the normal physico-chemical condition which characterises the living 

 state. 



In spite of considerable variations of internal tension, the capillaries show 

 only slight variations in width. After amputation of the hind-limb of a frog, 

 the capillaries in the swimming-web scarcely become reduced in size. 



Rate of flow. — Hales 1 reckoned the velocity of a blood corpuscle 

 in the capillaries of the frog's muscles to be OT inch in 9 seconds 

 (0 - 28 mm. per second). The method most conveniently used is to 

 employ an ocular micrometer, and to follow the course of a corpuscle 

 during a period of time given by a clock beating one-fifth seconds. 

 The velocity has been thus found by various observers to be 0-25 to 

 0-57 mm. per second in cold-blooded animals. 



Vierordt 2 reckoned the velocity in the retinal capillaries by the 

 entoptic method. He projected the shadow of the corpuscles on an j 



1 "Statical Essays," 1733, vol. ii. p. 63. 



2 " Die Erscheinungen und Gesetze der Stromgesohwindigkeit des Blutes," Frankfurt, 

 1858, S. 41, 111. 



