CHAP. VIL] THE CAT'S OHGANS OF CIRCULATION. 197 



is continuous with the inner wall of the vein, while the straight 

 edge of the fold hangs freely inwards into the cavity of the vein. 

 Usually two such folds, or valves, are placed one opposite to the 

 other and when these hang down their free edges meet and so close 

 altogether the passage through the vein. Any pressure exercised 

 irom that side towards which the concavities of the valves look 

 tends of course to separate them from the wall of the vein and so 

 close the passage through it, while pressure from the opposite side 

 tends to press the valves against the walls of the veins, and so to 



Fig. 100. DIAGRAMS, SHOWING VALVES OP VKINS. 



A. Part of a vein cut longitudinally and opened 



out, showing two pairs of valves. On 

 account of the vessel being thus opened 

 out, each pair of valves appear as if placed 

 side by side, instead of one opposite the 

 other. 



B. Longitudinal section of a vein, showing the 



two valves of a single pair in their natural 

 position, and showing the apposition of the 



edges of the valves in their closed state. 

 The slightest pressure applied from below 

 would cause these valves to open ; but 

 pressure from above would only press them 

 together more tightly. 



C. Portion of a distended vein, seen externally 

 and exhibiting a swelling in the situation of 

 a pair of valves. 



open the passage to its full width. Now these valves are so placed 

 that their convexities look towards the capillary vessels, from which 

 the vein which contains them springs, while their concavities look 

 towards that point (generally the heart) towards which the stream 

 of venous blood is flowing. Thus the action of these valves is to 

 help on that stream along its course, since they readily yield and 

 allow it to flow along its appointed direction, whilst they descend, 

 unite, and bar the passage, when temporary local pressure or any 

 other cause tends to drive the blood in the reverse direction to that 

 which it ordinarily pursues. 



There are no valves in the venaa cav&, the portal and hepatic 

 veins, those of the kidneys and uterus, nor in the pulmonary veins 

 and those of the interior of the skull and vertebral column. 



6. The veins and arteries are, as has been said, connected 

 together by the intervention of the sanguineous CAPILLARIES. These 

 are microscopic vessels and form a network in most of the tissues a 

 network so rich that sometimes the interspaces between them are not 

 wider than are the capillaries themselves. Their walls are extremely 



