674 CIRCULATION. 



tions, and are described under the head of The principal cause of the progressive flow 

 CILIA in this Cyclopaedia, isdefective in so of the blood in the veins is unquestionably the 

 far as neither cilia nor any power of exciting force of impulsion of the heart continued 

 currents has yet been shewn to exist in the through the arteries and small vessels, as ap- 

 interior of the bloodvessels, and they have pears from the flow from the remote part of an 

 been examined in circumstances in which we opened vein and the simple experiments of 

 conceive they would have been seen had they Hales, Magendie, and Poiseuille already re- 

 been present. In fine we cannot see how any ferred to. Hales ascertained, by introducing 

 power of spontaneous motion belonging to the tubes into the larger veins of the horse, that the 

 blood itself could be a cause of progressive pressure on the blood from behind, or visa tergo, 

 motion of that fluid, unless the direction of the is sufficient to raise the blood in the tube to a 

 motion were determined by the solid textures considerable height above the level of the heart, 

 containing the blood, and in this case the same and is consequently more than sufficient to re- 

 objections would apply to this explanation of turn the blood to the auricle of the heart. The 

 the cause of motion as to the one to which allu- blood did not, in Hales' experiments, in ge- 

 sion has just been made; and besides, the evi- neral at first rise in the tube connected with 

 dence of spontaneous motions of the blood ap- a vein more than six inches, but this he 

 pears upon the whole of a very unsatisfactory shewed to proceed from the easy escape of 

 kind. the blood by lateral communicating vessels, 



From these considerations we find ourselves for when the other large veins were tied, or 



constrained to hold the opinion that, however when they became fully distended with blood, 



great the power which the capillary vessels that fluid sometimes rose in the tube connected 



possess of modifying the distribution of the with a large vein to a height of three or four 



mood, there is not reason to believe that they feet. M. Poiseuille* demonstrated, in a still 



contribute as a whole to its progressive motion, more satisfactory manner, the action of the 



4. Phenomena of the venous circulation, In pressure of the heart on the blood in the veins 



the natural state of the circulation the flow of by means of the bent tube with which he mea- 



the blood is nearly quite uniform in the veins, sured the pressure of the arterial blood : and 



as may be seen when a vein is opened in the this fact is proved in an equally convincing 



common operation of venesection. In those manner by Magendie's experiment of isolating 



rare instances in which the flow from a vein is the principal artery and vein from the other 



accelerated after each beat of the heart, in the parts of the limb of an animal, in which it was 



same way as the arterial jet, it may be supposed found that the flow of blood from the vein is 



either that the intermitting impulses of the immediately stopped by pressure or ligature of 



heart are, from some circumstance or other, the artery. It is scarcely necessary, in order to 



transmitted more freely and to a greater dis- obtain a proof of this fact, to have recourse 



tance than usual through the capillary vessels, to the vivisection of animals, for in common 



as is known occasionally to happen, or, what is bleeding from the arm, the flow of blood from 



more probable, that the larger branch of the the vein will be found to be immediately influ- 



vein receives the successive impulses directly enced by the state of the artery, and even with- 



from neighbouring large arteries, which are out the division of a vein, it is easy to observe 



more than usually dilatable. the action of this force of impulsion which 



As the size of the veins is generally greater drives the blood onwards towards the heart in 

 than that of the corresponding arteries at the any of the superficial veins of the arm by the 

 same distance from the heart, and as they are application of external pressure, a mode of 

 also more numerous, the velocity of blood is illustration successfully adopted by Harvey in 

 less in these parts of the veins than of the arte- his explanation of the course of the blood, 

 ries ; and as the whole venous system contains These very simple experiments are looked upon 

 considerably more blood than the arterial, the by some as quite sufficient to demonstrate the 

 velocity of the blood taken as a whole must be proposition that the blood is moved in the 

 less in the veins than in the arteries. The same veins by an impulsion from behind, and that 

 quantity of blood must be brought by the venae that impulsion is derived from the action of the 

 cavse to the right auricle as issues from the left heart; while others, not satisfied with this ex- 

 ventricle, (making allowance for the expendi- planation, have endeavoured to point out addi- 

 ture by secretions, &c.) and consequently the tional forces as contributing to the progressive 

 velocity of the blood entering and of that issuing motion of the blood in the veins, 

 from the heart must be equal. Again, the ve- The larger veins are, like the arteries, highly 

 locity of the blood must be gradually on the elastic, and they are generally regarded as 

 increase in its progress from the small to the stronger proportionally to the thickness of their 

 larger veins, because the capacity of the vessels coats than the arteries. This elasticity belongs 

 into which it flows is gradually becoming less. chiefly to the external cellular coat, for a mid- 



In the systemic veins, excepting the venae die fibrous coat is not apparent in most of the 



portae, the direction of the flow of blood is de- larger healthy veins, and in those rarer in- 



termined by the structure of the valves, which stances in which it is apparent, it is very 



permit of the return of blood from the extremi- much thinner than in the arteries. The smaller 



ties of the veins towards the heart, but oppose, or capillary veins appear also to be possessed 



by the filling of their pouches and the apposi- of some degree of irritability, for they have been 

 tion of their free edges, a complete obstacle to 

 the reflux of the blood in another direction. * Magendie's Journ. vol. x. 



