CHAP, iv.] THE VASCULAR MECHANISM. 219 



venous tube be similarly pricked or cut, or 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 shew all the main facts of the circulation which we have as 

 yet described, 



121. In the living body, however, there are certain helps to 

 the circulation 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 con- 

 structed as to offer little or no resistance to the flow from the 

 capillaries to the heart, but effectually to block a return towards 

 the capillaries. Hence any external pressure brought to bear 

 upon a vein tends to help the blood to move forward towards 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 

 enlargement of the chest constituting an inspiration tends to draw 

 the blood towards 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 venae cavaa 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 stroke, 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. 



