150 THE VASCULAR MECHANISM. 



the small tube v be opened, the water will simply ooze out or well up, as 

 does blood from a vein in the living body. If the arterial tube be ligatured, 

 it will swell on the pump side and shrink on the peripheral side; if the 

 venous tube be ligatured, it will swell on the side nearest the capillaries and 

 shrink on the other side. In short, the dead model will show all the main 

 facts of the circulation which we have as yet described. 



110. In the living body, however, there are certain helps to the circu- 

 lation which cannot be imitated by such a model without introducing great 

 and undesirable complications ; but these chiefly affect the flow along the 

 veins. 



The veins are in many places provided with valves so constructed as to 

 offer little or no resistance to the flow from the capillaries to the heart, but 

 effectually to block a return toward the capillaries. Hence any external 

 pressure brought to bear upon a vein tends to help the blood to move for- 

 ward toward the heart. In the various movements carried out by the 

 skeletal muscles, such an external pressure is brought to bear on many of 

 the veins, and hence these movements assist the circulation. Even passive 

 movements of the limbs have a similar effect. So, also, the movements of 

 the alimentary canal, carried out by means of plain muscular tissue, promote 

 the flow along the veins coming from that canal, and when we come to deal 

 with the spleen we shall see that the plain muscular fibres which are so 

 abundant in that organ in some animals, serve by rhythmical contractions 

 to pump the blood regularly away from the spleen along the splenic veins. 



When we come to deal with respiration, we shall see that each enlarge- 

 ment of the chest constituting an inspiration tends to draw the blood toward 

 the chest, and each return or retraction of the chest walls in expiration tends 

 to drive the blood away from the chest. The arrangement of the valves of 

 the heart causes this action of the respiratory pump to promote the flow of 

 blood in the direction of the normal circulation ; and, indeed, were the heart 

 perfectly motionless, the working of this respiratory pump alone would tend 

 to drive the blood from the vense cavse through the heart into the aorta, and 

 so to keep up the circulation ; the force so exerted, however, would, without 

 the aid of the heart, be able to overcome a very small part only of the 

 resistance in the capillaries and small vessels of the lungs, and so would 

 prove actually ineffectual. 



There are, then, several helps to the flow along the veins, but it must be 

 remembered that, however useful, they are helps only, and not the real cause 

 of the circulation. The real cause of the flow is the ventricular str.oke, and 

 this is sufficient to drive the blood from the left ventricle to the right auricle, 

 even when every muscle of the body is at rest and breathing is for a while 

 stopped, when, therefore, all the helps we are speaking of are wanting. 



Circumstances Determining the Rate of the Flow. 



111. We may now pass on to consider briefly the rate at which the 

 blood flows through the vessels, and first the rate of flow in the arteries. 



When even a small artery is severed, a considerable quantity of blood 

 escapes from the proximal cut end in a very short space of time. That is to 

 say, the blood moves in the arteries from the heart to the capillaries with a 

 very considerable velocity. By various methods, this velocity of the blood- 

 current has been measured at different parts of the arterial system ; the 

 results, owing to imperfections in the methods employed, cannot be regarded 

 as satisfactorily exact, but may be accepted as approximately true. They 

 show that the velocity of the arterial stream is greatest in the largest arteries 

 near the heart, and diminishes from the heart toward the capillaries. Thus, 



