674 



CIRCULATION. 



lions, and are described under the head of 

 CILIA in this Cyclopaedia, isdefective in so 

 far as neither cilia nor any power of exciting 

 currents has yet been shewn to exist in the 

 interior of the bloodvessels, and they have 

 been examined in circumstances in which we 

 conceive they would have been seen had they 

 been present. In fine we cannot see how any 

 power of spontaneous motion belonging to the 

 blood itself could be a cause of progressive 

 motion of that fluid, unless the direction of the 

 motion were determined by the solid textures 

 containing the blood, and in this case the same 

 objections would apply to this explanation of 

 the cause of motion as to the one to which allu- 

 sion has just been made; and besides, the evi- 

 dence of spontaneous motions of the blood ap- 

 pears upon the whole of a very unsatisfactory 

 kind. 



From these considerations we find ourselves 

 constrained to hold the opinion that, however 

 great the power which the capillary vessels 

 possess of modifying the distribution of the 

 blood, there is not reason to believe that they 

 contribute as a whole to its progressive motion. 



4. Phenomena of the venom circulation. In 

 the natural state of the circulation the flow of 

 the blood is nearly quite uniform in the veins, 

 as may be seen when a vein is opened in the 

 common operation of venesection. In those 

 rare instances in which the flow from a vein is 

 accelerated after each beat of the heart, in the 

 same way as the arterial jet, it may be supposed 

 either that the intermitting impulses of the 

 heart are, from some circumstance or other, 

 transmitted more freely and to a greater dis- 

 tance than usual through the capillary vessels, 

 as is known occasionally to happen, or, what is 

 more probable, that the larger branch of the 

 vein receives the successive impulses directly 

 from neighbouring large arteries, which are 

 more than usually dilatable. 



As the size of the veins is generally greater 

 than that of the corresponding arteries at the 

 same distance from the heart, and as they are 

 also more numerous, the velocity of blood is 

 less in these parts of the veins than of the arte- 

 ries ; and as the whole venous system contains 

 considerably more blood than the arterial, the 

 velocity of the blood taken as a whole must be 

 less in the veins than in the arteries. The same 

 quantity of blood must be brought by the venae 

 cavae to the right auricle as issues from the left 

 ventricle, (making allowance for the expendi- 

 ture by secretions, &c.) and consequently the 

 velocity of the blood entering and of that issuing 

 from the heart must be equal. Again, the ve- 

 locity of the blood must be gradually on the 

 increase in its progress from the small to the 

 larger veins, because the capacity of the vessels 

 into which it flows is gradually becoming less. 



In the systemic veins, excepting the venae 

 portae, the direction of the flow of blood is de- 

 termined by the structure of the valves, which 

 permit of the return of blood from the extremi- 

 ties of the veins towards the heart, but oppose, 

 by the filling of their pouches and the apposi- 

 tion of their free edges, a complete obstacle to 

 the reflux of the blood in another direction. 



The principal cause of the progressive flow 

 of the blood in the veins is unquestionably the 

 force of impulsion of the heart continued 

 through the arteries and small vessels, as ap- 

 pears from the flow from the remote part of an 

 opened vein and the simple experiments of 

 Hales, Magendie, and Poiseuille already re- 

 ferred to. Hales ascertained, by introducing 

 tubes into the larger veins of the horse, that the 

 pressure on the blood from behind, or vis a tergo, 

 is sufficient to raise the blood in the tube to a 

 considerable height above the level of the heart, 

 and is consequently more than sufficient to re- 

 turn the blood to the auricle of the heart. The 

 blood did not, in Hales' experiments, in ge- 

 neral at first rise in the tube connected with 

 a vein more than six inches, but this he 

 shewed to proceed from the easy escape of 

 the blood by lateral communicating vessels, 

 for when the other large veins were tied, or 

 when they became fully distended with blood, 

 that fluid sometimes rose in the tube connected 

 with a large vein to a height of three or four 

 feet. M. Poiseuille* demonstrated, in a still 

 more satisfactory manner, the action of the 

 pressure of the heart on the blood in the veins 

 by means of the bent tube with which he mea- 

 sured the pressure of the arterial blood : and 

 this fact is proved in an equally convincing 

 manner by Magendie's experiment of isolating 

 the principal artery and vein from the other 

 parts of the limb of an animal, in which it was 

 found that the flow of blood from the vein is 

 immediately stopped by pressure or ligature of 

 the artery. It is scarcely necessary, in order to 

 obtain a proof of this fact, to have recourse 

 to the vivisection of animals, for in common 

 bleeding from the arm, the flow of blood from 

 the vein will be found to be immediately influ- 

 enced by the state of the artery, and even with- 

 out the division of a vein, it is easy to observe 

 the action of this force of impulsion which 

 drives the blood onwards towards the heart in 

 any of the superficial veins of the arm by the 

 application of external pressure, a mode of 

 illustration successfully adopted by Harvey in 

 his explanation of the course of the blood. 

 These very simple experiments are looked upon 

 by some as quite sufficient to demonstrate the 

 proposition that the blood is moved in the 

 veins by an impulsion from behind, and that 

 that impulsion is derived from the action of the 

 heart; while others, not satisfied with this ex- 

 planation, have endeavoured to point out addi- 

 tional forces as contributing to the progressive 

 motion of the blood in the veins. 



The larger veins are, like the arteries, highly 

 elastic, and they are generally regarded as 

 stronger proportionally to the thickness of their 

 coats than the arteries. This elasticity belongs 

 chiefly to the external cellular coat, for a mid- 

 dle fibrous coat is not apparent in most of the 

 larger healthy veins, and in those rarer in- 

 stances in which it is apparent, it is very 

 much thinner than in the arteries. The smaller 

 or capillary veins appear also to be possessed 

 of some degree of irritability, for they have been 



* Magendie's Journ. vol. x. 



