lo Sept., 1908.] Elements of Animal Physiology. 5-3 



The blood throughout this arterial system is under pressure, and there- 

 fore, as it cannot escape from the aorta back again into the ventricle 

 owing to the semilunar valves, it is forced through the arterioles, which are 

 always more or less constricted, into the smallest vessels of the body 

 or CAPILLARIES which form a dense network round the living cells of 

 the body. The flow of blood in this network of capillaries can be 

 beautifully seen by observing the web of the foot of a living frog under 

 an ordinary microscope. The diameter of a capillary vessel is about 

 that of a red blood-corpuscle; its frail wall is composed of thin and very 

 fiat cells, one deep. The flow of blood in the capillaries is slow, the 

 rate being usually given as an inch per minute (though it must be remem- 

 bered that in no region of the body could the blood pass through more 

 than a small fraction of an inch of capillaries), but its most remarkable 

 character is that the flow is constant and not intermittent and jerky as it is 

 in the arteries. This change is brought about by the elastic character of 

 the arteries, just as the elastic bag in a cosmetic spray-producer changes 

 the intermittent pumping of the air by the hand into a continuous outflow. 

 The blood as it passes through the capillaries alters in character ; amongst 

 other changes it loses some oxygen and gains some carbon dioxide, so 

 that it becomes purplish (or venous) in colour. 



The blood now leaves the capillaries passing into a number of thin- 

 walled venules, each a little larger than a capillary ; these venules unite 

 into small veins and these into larger veins in a branching system resembling 

 the arteries, only that here the current passes from branch to stem 

 and narrows as it travels heartwards. The venous blood from the 

 abdominal alimentary canal, the pancreas and the spleen, passes into 

 the portal vein, as has been stated, and this vein breaks up into branches 

 and these finally into capillaries in the liver so that the blood passing 

 out of the aorta into the abdominal digestive organs has to pass through 

 two systems of capillaries. But from other parts of the body, including 

 the liver itself, the blood passes from twig to branch along the veins 

 until finally it enters into one of two great veins or vena, ccrcce which 

 open into the right auricle. The veins are thin-walled compared with 

 the arteries, and also very much more distensible. An increase in pressure 

 which would make no apparent increase in an artery might increase 

 tenfold the calibre of a vein. The causes that operate in the flow of 

 blood in the veins are somewhat complex. First there is some back 

 pressure due originally to the heart. Secondly in every movement of 

 the body the muscles during their contraction press on the veins and urge 

 their contents heartwards. This is one of the reasons why exercise is 

 so beneficial and why animals, if they are kept on their feet too long 

 and bereft of natural exercise, show swelling of the veins of the leg or 

 even dropsy. Thirdly there is the suction which is exerted by the chest 

 in breathing. When the thorax expands not only is air drawn into the 

 lung but also blood is drawn towards the thorax along the great veins 

 leading into it. At the same time the midriff or diaphragm compresses 

 the contents of the abdomen and drives heartwards the blood contained 

 therein. 



The blood when it reaches the right auricle is subjected to the same cycle 

 as when in the left heart. In the right ventricle however the muscular 

 walls are much thinner and less powerful than in the left, and the artery 

 arising from it, called the pulmonary artery, has within it a much 

 smaller pressure than in the aorta. The right ventricle contracts at the 

 same time as the left ; the tricuspid valve is closed tight and pulled 

 down ; a quantity of blood equal to that expelled from the left ventricle 



