CHAP, iv.] THi: VASCULAR MECHANISM. 171 



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. 



103. 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. 



The flow along the large veins of the abdomen is assisted by 

 the pressure rhythmically brought to bear on them through the 

 movements of the diaphragm in breathing, as well as, at times, by 

 the forcible contractions of the abdominal muscles. Again, 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 study 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 has an opposite effect, and, if powerful 

 enough, may drive the blood away from the chest. The arrange- 

 ment 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 cava3 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 



