46 ZOOLOGY. 



actively, and hence draw to them a larger amount of blood, 

 is well known. 



89. The blood, by thus acting on the organs, loses its 

 nourishing properties. It reaches them of a bright vermilion 

 colour ; as it leaves them it is dark and sombre-coloured, and 

 has lost its qualities of maintaining life. But the blood thus 

 altered has its vital properties restored by being exposed to 

 the atmosphere. This important function is called Respira- 

 tion. The blood which has been exposed to the air is called 

 arterial: that which has already acted on the organs is 

 called venous; it is chiefly distinguished by its dark colour. 



CIRCULATION OF THE BLOOD. 



90. The circulation of the blood in man and mammals 

 was discovered by Harvey in 1619. 



In order to nourish all the parts of the body, it is neces- 

 sary that the blood should be conveyed to these parts by means 

 of vessels ; and that, to circulate in these vessels, there should 

 be a power or organ equal to the production of such a move- 

 ment. But it is also necessary, in the higher animals, that 

 the blood should be passed through the respiratory organs, in 

 order to be exposed to the action of the air; hence the necessity 

 of a circulation of the blood through the lungs,as well as through 

 the body, and hence a pulmonary circulation and a systemic. 



91. Apparatus of the Circulation. In certain of the 

 lower animals, the air penetrates into the tissues through 

 pores situated on the surface of the body ; but in all the 

 higher animals, and in many of the lower, there exists a very 

 complex apparatus for the circulation of the blood : 1. A sys- 

 tem of tubes or canals, destined to convey the blood into the 

 various parts of the body. 2. An organ destined to put this 

 fluid in motion. In other words, bloodvessels and a heart. 



The heart is the centre of the circulatory apparatus ; it is 

 a pouch or bag, more or less complex, into which the blood is 

 returned by the veins, and from which it passes to the body, 

 thus constantly circulating. It is, in fact, a forcing-pump 

 placed at the centre of the circulatory system. 



Most animals, from man to the spider, have a heart ; but 

 the arrangement differs in the various classes of animals. 



The bloodvessels are of two kinds : 1. Arteries, which 

 convey the aerated blood into the various parts of the body 

 '2. \\-iiis, which re-couvey the blood to the heart from the 



