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 cther 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. 
Tp 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 
suflicient 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 imternal 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. 
