PHYSIOLOGY. 
Ihilependently of the chain, which unites 
these two powers, and of the double appa- 
ratus of organs wiiich they require, tliey 
produce also several modifications in the 
faculties common to alt organised bodies ; 
and these modifications, joined to the two 
peculiar powers, constitute more particu- 
larly the essential nature of animals. Thus, 
in respect to nutrition, vegetables being at- 
tached to the earth, absorb nutritive fluids 
directly by their roots ; these almost infi- 
nitely subdivided, penetrate the smallest 
intervals of the soil, and, if we may use the 
expression, travel to a distance in quest of 
nourishment for the plant to which they be- 
long; their action is quiet and constant, 
being liable to interruption only when 
drought deprives them of the necessary 
juices. Animals on the contrary, fixed to no 
spot, but frequently changing their abode, 
required the power of transporting with 
them the provision of fluids necessary for 
their nutrition ; they have therefore an inte- 
rior cavity to receive their food ; and on its 
inner surface there are the openings of ab- 
sorbing vessels, which, to use the energetic 
language of Boerhaave, are real internal 
rooU. The size of this cavity, and of its ori- 
fices allowed in several animals the intro- 
duction of solid substances. These re- 
quired instruments for their division, and li- 
quors for their solution ; in a word, nutrition 
was no longer performed by the immediate 
absorption of matters in the state in which 
the earth or atmosphere furnished them ; it 
was necessarily preceded by various prepa- 
ratory operations, which, taken altogether, 
constitute digestion. 
Thus digestion is a function of a secon- 
dary class, peculiar to animals. Its exist- 
ence, as well as that of the alimentary ca- 
vity in which it takes place, is rendered ne- 
cessary by the power which animals have of 
voluntary motion ; but it is not the only 
consequence of that power. 
Vegetables, having few faculties, are 
simple in their organization ; being com- 
posed almost entirely of parallel or slightly 
diverging fibres. Moreover, their fixed po- 
sition admitted of the general motion of 
their nutritive fluid being kept up by simple 
external agents ; thus it ascends by means 
of suction in their spongy or capillary 
texture, and also through the influence of 
evaporation, from the surface ; it is rapid in 
a direct ratio to this evaporation, and may 
even become retrograde, when that process 
ceases, or when it is changed into absorp- 
tion by the moisture of the atmosphere. 
It was necessary that atiimals should liavd 
within themselves an active principle of 
motion for their nutritive fluid, not only be- 
cause they were destined to constant 
changes of situation and temperature ; but 
also from their more numerous and highly 
developed faculties requiring a much greater 
complication of organs. Hence the com- 
ponent parts became very intricate in their 
composition, and often very distant, and 
possessed in many instances a power of 
changing their relative position ; conse- 
quently the means of carrying the nutritive 
fluid through such multiplied intricacies 
must be more powerful than in vegetables, 
and differently arranged. It is contained, 
in most animals, in innumerable canals, 
which branch out from two trunks, that 
communicate together in such a way, that 
the fluid urged into the branches of one is 
received by the roots of the other, and car- 
ried back to a common centre, from which 
it is propelled a fresh. 
At the point of communication between 
the two great trunks is placed the heart, 
whose contractions impel the nutritive fluid 
into all the branches of the arterial trunk ; 
for the orifices of the heart possess valves 
disposed in such a way that the circulating 
juices can only move in the directions now 
described, viz. from the heart towards all 
parts by the aiteries, and from all parts to 
the heart in the veins. 
In this rotatory motion consists the cir- 
culation of the blood, which is another se- 
condary function peculiar to animals, chiefly 
performed and regulated by the heart. This, 
however, is not so essentially connected to 
the faculties of sensation and motion as the 
business of digestion ; for w hole classes of 
animals (as insects) possess no circulation, 
and are nourished, like vegetables, by the 
mere imbibing of fluids prepared in the in- 
testinal canal. 
The blood seems to be merely a vehicle, 
receiving constantly from the intestines, 
skiivand lungs, different substances, which 
it incoi porates intimately, and by which its 
losses arising from the preservation and 
growdh of parts, are supplied. The nutri- 
tion of the' body is performed during the 
course of the blood in the minute extremi- 
ties of the arteries ; here the fluid changes 
its nature and colour ; and it is only by the 
addition of the various substances just 
pointed out, that the venous blood again be- 
comes proper for the purposes of nutrition, 
or, in one word, again becomes arterial. 
The venous blood receives the supplies 
