50 ZOOLOGY. 



The fibrin of the blood also plays an important part, for 

 M. Magendie has shown that when blood deprived of its 

 fibrin is injected into the veins of a dog, the animal dies with 

 symptoms resembling those of putrid fevers. 



88. The influence of the blood over nutrition may also 

 be readily demonstrated. Withdraw the blood more or less 

 from any organ, and it gradually wastes away in proportion 

 to the quantity withdrawn; on the contrary, the greater 

 size of the muscles in those who employ them 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 ; the latter 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 blocd 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 

 movement. 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 both a pulmonary 

 and a systemic circulation. 



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 

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

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



