28 INTRODUCTION. 



bathing the surface of the skin. The respired, or purified blood is 

 properly qualified for restoring the composition of all the parts, and 

 to effect what is properly called nutrition. This facility, which the 

 blood possesses, of decomposing itself at every point, so as to leave 

 there the precise kind of molecule necessary, is indeed wonderful ; 

 but it is this wonder which constitutes the whole vegetative life. 

 For the nourishment of the solids we see no other arrangement than 

 a great subdivision of the extreme arterial ramifications, but for the 

 production of fluids the apparatus is more complex and various. 

 Sometimes the extremities of the vessels simply spread themselves 

 over large surfaces, whence the produced fluid exhales ; at others it 

 oozes from the bottom of little cavities. Before these arterial ex- 

 tremities change into veins, they most commonly give rise to parti- 

 cular vessels that convey this fluid, which appears to proceed from 

 the exact point of union between the two kinds of vessels; in this 

 case the blood vessels and these latter form, by interlacing, particu- 

 lar bodies called conglomerate or secretory glands. 



In animals that have no circulation, in Insects particularly, the 

 parts are all bathed in the nutritive fluid : each of these parts draws 

 from it what it requires, and if the production of a liquid be neces- 

 sary, proper vessels floating in the fluid take up by their pores the 

 constituent elements of that liquid. 



It is thus that the blood incessantly supports the composition of 

 all the parts, and repairs the injuries arising from those changes 

 which are the continual and necessary consequences of their func- 

 tions. The general ideas we form with respect to this process are 

 tolerably clear, although we have no distinct or detailed notion of 

 what passes at each point, and for want of knowing the chemical 

 composition of each part with sufficient precision, we cannot render 

 an exact account of the transmutations necessary to effect it. 



Besides the glands which separate from the blood those fluids that 

 are destined for the internal economy, there are some which detach 

 others from it that are to be totally ejected, either as superfluous, 

 or for some use to the animal, as the ink of the cuttle-fish, and the 

 purple matter of various Mollusca, &c. 



There is a process or phenomenon, infinitely more difficult to 

 comprehend than that of the secretions the production of the germ. 

 We have even seen that it is to be considered as almost incompre- 

 hensible ; but the existence of the germ being admitted, the subject 

 presents no particular difficulties. As long as it adheres to the 



