354 OF THE CIRCULATION OF THE BLOOD. 



of the arterial; the former is usually estimated to contain from two to three 

 times as much blood as the latter, in the ordinary condition of the circula- 

 tion ; and when \veconsiderthe great proportion which the Veins in almost every 

 part of the body bear to the Arteries, we shall scarcely regard even the larger 

 of these ratios as exaggerated. Of course the rapidity of the movement of 

 the blood in the two systems will bear an inverse ratio to their respective 

 capacities; thus if, in a given length, the Veins contain three times as much 

 blood as the Arteries, the fluid will move with only one-third of the velocity. 

 Speaking generally, circumstances causing diminution of blood-pressure by 

 dilatation of the Capillaries, as division of the spinal cord, render the blood- 

 current slower in the Veins. 1 Even at their origins in the capillary plexus, 

 the Veins are larger than the Arteries which terminate in the same plexus; 

 so that wherever the arterial and venous networks form distinct strata, they 

 are readily distinguished from each other. The Veins are remarkable for 

 the number of valves which they contain, formed of duplicatures, or loose 

 folds of the internal tunic, between the component laminre of which contrac- 

 tile fibres are interposed ; and also for the dilatations behind these, which, 

 when distended, give them a varicose appearance. The valves are single in 

 the small veins, the free edge of the flap closing against the opposite wall of 

 the vein; in the larger trunks they are double; and in a few instances they 

 are composed of three flaps. The object of these valves is evidently to pre- 

 vent the reflux of blood ; and we shall presently see that they are of im- 

 portant use in assisting in the maintenance of the venous circulation. They 

 are most numerous in those veins which run among parts affected by mus- 

 cular movement; and they are not found in the veins of the lungs, of the 

 abdominal viscera, or of the brain. 



274. The movement of the blood through the Veins is, without doubt, 

 chiefly effected by the vis a tergo or propulsive force, which results from the 

 action of the heart and arteries; this, as already shown ( 258), is very greatly 

 diminished by the time that it acts on the blood in the veins; but the resist- 

 ance to the onward movement of the blood is now so slight, that a very 

 feeble power is adequate to overcome it. There are some concurrent causes, 

 however, which are supposed by some to have much influence upon it, and 

 of which the consideration must not be neglected. One of these has been 

 found by some physiologists, in the inspiratory movement; this is supposed to 

 draw the blood of the Veins into the chest, in order to supply the vacuum 

 which is created there at the moment of the descent of the diaphragm. 

 That the movement in question has some influence on the flow of venous 

 blood into the chest, is evident from the occurrence of the respiratoi'y pulse, 

 long ago described by Haller, which may be seen in the veins of the neck 

 and shoulder in thin persons, and in those especially who are suffering from 

 pulmonary diseases. During Inspiration, the veins are seen to be partially 

 emptied, whilst during Expiration they become turgid, partly in consequence 

 of the accumulation from behind, and of the check in front; and partly (it 

 may be) in some eases, through an absolute reflux from the veins within the 

 chest ( 237). The fact that in the immediate neighborhood of thr chest 

 the flow of blood towards the heart is aided by inspiration and impeded by 

 expiration, is further proved by Sir D. Barry's experiment, which consisted 

 in introducing one extremity of a tube into the jugular vein of a Horse, and 

 the other into water, which exhibited an alternate elevation and depression 

 with inspiration and expiration ; this has been repeated and confirmed by 

 several Physiologists. On the other hand, the expirotori/ movement, while it 



1 In regard to tins point sec Cyon and Stcinmann, Bulletin do I'Afad. de> Sciences 

 de St. P6tersbourg, t. xvi, 1S71, p. 'Jiii;. 



