INTRODUCTION. 17 
Even among those where the intestinal canal has two orifices, there are 
many in which the nutritive juices, being absorbed by the parietes of the 
intestine, are immediately diffused throughout the whole spongy substance 
of the body: such, it would appear, is the case with all Insects. But 
from the Arachnides and Worms upwards, the nutritive fluid circulates in 
a system of closed vessels, whose ultimate ramifications alone dispense its 
molecules to the parts that are nourished by it; the vessels that convey it 
are called arteries, those that bring it back to the centre of the circulation, 
veins. The circulating vortex is here simple, and there double and even 
triple (including that of the vena-porte); the rapidity of its motion is often 
assisted by the contractions of a certain fleshy apparatus called a heart, 
which is placed at one or the other centres of circulation, and sometimes 
at both of them. 
In the red-blooded vertebrated animals, the nutritive fluid exudes from 
the intestines white or transparent, and is then termed chyle; it is poured 
into the veins, where it mingles with the blood, by a set of peculiar vessels 
called /acteals. Vessels similar to these lacteals, and forming with them 
an arrangement called the lymphatic system, also convey to the venous 
blood the residue of the nutrition of the parts and the products of cutaneous 
absorption. 
Before the blood is fit to nourish the parts, it must experience from the 
circumambient element the modification of which we have previously 
spoken. In animals possessing a circulating system, one portion of the 
vessels is destined to carry the blood into organs in which they spread it 
over a great surface to obtain an increase of this elemental influence. 
When that element is air, the surface is hollow, and is called lungs; when 
it is water, it is salient, and is termed branchie. ‘There is always an ar- 
rangement of the organs of motion for the purpose of propelling the ele- 
ment into, or upon, the organ of respiration. 
In animals destitute of a circulating system, air is diffused through 
every part of the body by elastic vessels called trachee; or water acts 
upon them, either by penetrating through vessels, or by simply bathing 
the surface of the skin. The respired or purified blood is properly quali- 
fied for restoring the composition of all the parts, and to effect what 1s pro- 
perly called nutrition. This facility, which the blood possesses, of decom- 
posing itself at every point, so as to leave there the precise kind of mole- 
cule necessary, is indeed wonderful; but it is this wonder which consti- 
tutes the whole vegetative life. - For the nourishment of the solids we see 
no other arrangement than a great subdivision of the extreme arterial ra- 
mifications; 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 
VOL. I. c 
