INTRODUCTION. 19 



upon them, either by penetrating through vessels, or by simply 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 particle necessary, is indeed wonderful ; but it is this 

 wonder which constitutes the whole vegetative life. For the nourish- 

 ment 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 extremi- 

 ties 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 extremities change into veins, they most 

 commonly give rise to particular 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 the latter, form 

 by interlacing, particular 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 necessary, proper 

 vessels floating in the fluid take up by their pores the constituent ele- 

 ments 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 functions. 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 

 other fluids 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 parent, 



c 2 



