CH. V.] THE BLOOD 77 



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 ithe 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 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. 



