202 BIOLOGY. [Book ii. 



power accumulates after a fashion at special points, where a 

 muscular tissue more or less rich appears. Generally, then, 

 there is a respiratory apparatus more or less complete, and as a 

 result there are two species of blood more or less distinct, a blood 

 oxygenised, and a blood which has need to be so. These two 

 species of nutritive liquids finally separate, while there is pro- 

 gressive improvement, and though the second always comes, from 

 the first, it no longer blends with it. Then the pi-opulsive agents 

 are multiple. They are the nutritive call of the anatomical 

 elements, the reflex impressed by them on the waste substances, 

 the general contractility of the vessels, contractility more 

 accentuated in the arteries, that is to say, in the canals which 

 convey the vivified blood. Lastly, there exists a central pro- 

 pulsive organ, a hollow muscle, first with two, then with three, 

 then with four cavities, communicating with each other two by two, 

 but separated by valves which hinder the reflex. Each pair of 

 these cardiacal cavities is, truth to speak, a special heart. One 

 of these hearts is venous, that is to say, charged to impel rapidly 

 the black blood returning from the tissues toward the respira- 

 tory apparatus, where it obtains a supply of oxygen. The other 

 heart is arterial, and has for functio;n to impel toward the 

 tissues the oxygenised blood which is indispensable to them. 



Naturally, the nutritive system is the better specialised, the 

 better closed and the better differentiated the system is which 

 contains it. First of all it is not distinguished from the aliment 

 itself, then from the chyme; then it becomes an elaborated 

 chyme ; finally we meet with, therein, floating figurate elements. 

 These elements, which at the outset are simple epithelial cells, 

 become special globules, characterised by a particular form, a 

 particular colour,^— hsematia, — as greedy for oxygen as they are 

 prompt to relinquish it to the anatomical elements. 



A. Of the Heart. — "We have seen that the heart of the superior 

 vertebrates is, in the animal series, bound to the contractile 

 sanguiferous organs by a long chain of gradations. Moreover 

 the same seriation is individually reproduced in the embryological 



