CH. V.] THE BLOOD 61 



f Serum 



-ri i f Plasma > Fibrin | nl , 



^Corpuscles J Clot 



The corpuscles are of two chief kinds, the red and the white. 

 The white corpuscles are typical amoeboid, nucleated cells. 



The red corpuscles are much more numerous than the white, 

 averaging in man 5,000,000 per cubic millimetre, or 400 to 500 red 

 to each white corpuscle. It is these red corpuscles that give the red 

 colour to the blood. They vary in size and structure in different 

 groups of the vertebrates. In mammals they are biconcave (except 

 in the camel tribe, where they are biconvex) non-nucleated discs, in 

 man ^V^ inch in diameter ; during foetal life nucleated red corpuscles 

 are, however, found. In birds, reptiles, amphibians and fishes they 

 are biconvex oval discs with a nucleus: they are largest in the 

 amphibia. The most important and abundant of the constituents 

 of the red corpuscles is the pigment which is called hcemoglobin. 

 This is a protein-like substance, but is remarkable as it contains a 

 small amount of iron (about 0'4 per cent.). 



The blood during life is in constant movement. It leaves the 

 heart by the vessels called arteries, and returns to the heart by the 

 vessels called veins ; the terminations of the arteries and the com- 

 mencements of the veins are, in the tissues, connected by the thin- 

 walled microscopic vessels called capillaries. In the capillaries, 

 leakage of the blood-plasma occurs ; this exuded fluid (lymph) carries 

 nutriment from the blood to the tissue-elements, and removes from 

 them the waste products of their activity. The lymph is collected by 

 lymphatic vessels, which converge to the main lymphatic, called the 

 thoracic duct. This opens into the large veins near to their entrance 

 into the heart ; and thus the lymph is returned to the blood. 



But blood is also a carrier of oxygen, and it is the pigment 

 haemoglobin which is the oxygen carrier ; in the lungs the haemoglobin 

 combines with the oxygen of the air, and forms a loose compound of 

 a bright scarlet colour called oxyhcemoglobin. This arterial or oxy- 

 genated blood is taken to the heart and thence propelled by the 

 arteries all over the body, where the tissues take the respiratory 

 oxygen from the oxyhsernoglobin, and this removal of oxygen changes 

 the colour of blood to the darker tint it has in the veins. The 

 veins take the blood (minus a large quantity of oxygen and plus a 

 large quantity of carbonic acid received in exchange from the tissues) 

 to the heart, which sends it to the lungs to get rid of its surplus 

 carbonic acid, and replenish its store of oxygen ; then the same round 

 begins over again. It should, however, be noted that haemoglobin 

 is not a carrier of carbonic acid; that gas is carried mainly as 

 carbonates in the blood-plasma. 



