INTRODUCTION. 23 



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 cavi- 

 ties. 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 ves- 

 sels and these latter form, by interlacing, particular bodies call- 

 ed 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 elements of that liquid. 



It is thus that the blood incessantly supports the composi- 

 tion 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 dis- 

 tinct 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 eject- 

 ed, either as superfluous the urine, for instance, which is 

 produced by the kidneys; or for some use to the animal, as the 

 ink of the cuttle-fish, and the purple matter of various mol- 

 lusca, &c. 



