84 BIOLOGY. [BOOK n. 



maintain themselves in what we call a suitable state of purity, 

 that is to say, in a state of composition sufficiently equilibrated 

 for the histological elements to find therein at every instant their 

 food. We shall see in the course of this exposition that in the 

 complex organisms special apparatus of exhalation, of secretion, 

 and of excretion are charged to keep up incessantly across 

 these media renovating currents, exactly as other apparatus, 

 for instance, the digestive apparatus and certain glands, pour 

 in suitable nutriments. 



These general data accepted, we can now analyse the acts, the 

 phases of nutrition. We know that this biological property 

 exercises itself in all living substances, figurate or not, as well in 

 the plasmas and the blastemas as in the figurate elements. In 

 both it depends in part on physical conditions of endosmosis, 

 exosmosis, and diffusion, and in part on the physical affinities of 

 substances brought into relation. In all this there is not the 

 smallest place for a metaphysical agent. We have simply to do 

 with physical and chemical phenomena producing themselves in 

 conditions of complexity and simultaneousness entirely special, 

 yet narrowly bound to the variations of the ambient medium. 

 We see in effect these phenomena intensified or enfeebled accord- 

 ing as the air is more or less oxygenised, according as the 

 temperature is higher or lower, and so on. 



Though the nutritive phenomena are simultaneous and unin- 

 terupted, we can, for the convenience of exposition, divide them 

 into phenomena of assimilation and phenomena of disassimi- 

 lation. 



It is by endosmosis that the immediate principles reach the 

 substance of the anatomical elements, reach the living liquids. 

 The principles of the first class, that is to say, the mineral 

 substances, often arrive without modification by simple dissolution, 

 and it is thus, for example, that the chlorures and the alkaline 

 sulphates arrive. Certain of these substances combine with 

 the organic matters, as, for instance, the phosphate of lime com- 

 bines with osse'ine in the bones ; but then, in opposition to the 



