OH. V.] THE BLOOD. 83 



different groups of the vertebrates. In mammals they are bi- 

 concave (except in the camel tribe, where they are biconvex) 

 non-nucleated discs, in man 3 ^ 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 hcemoylobin. This is a proteid-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 

 commencements of the veins are, in the tissues, connected by the 

 thin-walled microscopic vessels called capillaries. Tn the capil- 

 laries, leakage of the blood-plasma occurs ; this exuded fluid 

 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 oxygenated 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 haemoglobin, and this removal of 

 oxygen changes the colour of blood to the bluish-red 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. 



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