Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

J70 A NI M A L
Animal internal structure and organic functions: thus the pos-
Kingdom. session of nerves, muscles, and a stomach, with the con-
sequent attributes of sensation, voluntary motion, and di¬
gestion, will be found to separate animals properly so call¬
ed from all other organized matter. If these leading
characters are not common to all animals, they are at
least proper to them alone; and if the whole of these
characters are not always united in the same animal, we
invariably find at least one of the three. Thus certain
species of polypus, the sensibility and voluntary move¬
ments of which cannot be said decidedly to manifest
themselves, are obviously furnished with a digestive cavity
or stomach; and many of the infusory animals, of the
digestion of which we know nothing, are as perfect in re¬
gard to their varied powers of locomotion as any of the
higher classes. Numerous zoophytical species, indeed, are
of so simple a nature, that in them we cannot perceive
either a distinct tissue or a nourishing fluid; but we can
form some opinion of the nature of their elements from
the character of their properties. Irritability indicates
nerves, motion supposes the existence of muscles, and
the continued maintenance of life attests nutrition. Thus
the materials of animal life, so vaguely constituted in
these creatures, are detected by their general properties.
In relation to their chemical characters, animals may be
said to be principally composed of azote; and vegetables,
with the exception of cruciferae, of carbon. Animals ab¬
sorb oxygen, plants disengage it; the former reject car¬
bon, the latter become impregnated with it. An exchange
of principles is thus effected between the two great divi¬
sions of organized existence; but it has been observed
that plants merely fix or organize carbon, whereas animals
appear to transform into azote, both the air which they
respire, and the aliments by which they are nourished.
It has been asserted that a single mouth, or opening to
the digestive canal, sufficiently characterizes animals from
plants, as the latter always possess innumerable pores,
which with them are the representatives of the mouth,
and conduce to the same ends; but this distinction is in
fact inaccurate, as some species of Fasciola possess two
mouths, certain Tristomae three, and the genus called by
Cuvier Rhisostoma many more ; to say nothing of the in¬
fusorial tribes, many of which have no mouth at all, but
derive their nourishment by imbibition through the me¬
dium of external pores. Nutrition, or the power of de¬
riving nourishment from other bodies, is common alike to
plants and animals, and effects for organized and living
bodies that increase of bulk which inorganic or disorgan¬
ized substances can only attain through the medium of an
affinity of particles, or by mechanical aggregation. The
functions of nutrition, however, as manifested in the ani¬
mal and vegetable kingdoms, are very differently perform¬
ed in each. Fixed for ever to the soil which gave them
birth, plants are rendered incapable of searching after
nourishment by a voluntary change of place, but derive
their chief support from roots, the pores of which absorb
the nutritive portions from the humid soil, and by a uni¬
form and continuous action, which is only interrupted by
an absence of the necessary moisture. The generality of
animals, on the contrary, being possessed of the power of
locomotion, are also endowed with the capacity of trans¬
porting with themselves a supply of necessary nourish¬
ment ; for which purpose they are provided with an in¬
ternal cavity or stomach, the inward surfaces of which
are provided with absorbing pores, which Boerhaave ex¬
pressively named internal roots. “ The magnitude of this
cavity,” observes Cuvier, c< in a number of animals, per-
KINGDOM.
mits them to introduce solid substances into it. It was Animal
necessary then that they should have instruments for di-
viding those substances, and liquors for dissolving them. V^rv'v
In a word, with such animals nutrition does not immedi¬
ately commence upon the absorption of the substances
which the soil or the atmosphere furnishes. It is necessa¬
rily preceded by a vast number of preparatory operations,
the whole of which constitute digestion^1 The motion of
the nutritive fluid in plants, from the simplicity of their
structure, and the fixity of their position, seems to be
preserved by simple external agents. “ It appears to
proceed upwards by the effect of their spongy or capillary
texture; and the evaporation which takes place at their
top, and its motion in that direction, is the more rapid in
proportion as the evaporation is great. It appears also
that the motion of this fluid may even become retrograde,
when it ceases to flow in its usual course, or changes into
absorption by the coldness and humidity of the atmo¬
sphere.”2 In regard to animals, however, the case is dif¬
ferent. Being destined continually to change their locali¬
ties, and to live exposed to a variety of temperatures, they
require an active principle within themselves for the con¬
veyance of their nourishing fluid. This fluid is therefore
contained in a multiplicity of canals, which are ramified
from two trunks, communicating with each other in such
a manner that the roots of the one, called the venous
system, receive the contents which the other, known as
the arterial system, has propelled to the extremity of its
. branches, and restore them to the centre, from which
they are again driven forward. It is this rotation which
constitutes what is called the circulation of the blood. It
may be regarded as a function of a secondary order pro¬
per to animals, but not universal to that kingdom, as it
depends in a great measure on the existence of that cen¬
tral organ called the heart, of which some classes are en¬
tirely destitute. It is therefore less essential to life than
digestion, and not so intimately related to the faculties of
sensation and locomotion. In regard to respiration, ani¬
mals which are unprovided with a regular circulating sys¬
tem respire, like vegetables, over the whole of their surface,
or by various vessels which are placed at different points,
and convey the air to the interior of their bodies. “ No
animals,” saysCuvier, “respire by a particular organ, except
such as have a real circulation ; because in them the blood
coming from one common source, the heart, to which it
constantly returns, the vessels that contain it are so dis¬
posed that it cannot arrive at the other parts until it has
passed through the lungs. This, however, cannot take place
in vegetables, or in those animals in which this fluid is
everywhere diffused in a uniform manner, without being
contained in vessels.” Pulmonary or branchial respira¬
tion is therefore an animal function of a third order, in¬
variably connected with circulation, and one degree re¬
moved from such faculties as are essential to animal life.
When a vegetable dies of old age, it begins to decay in
the centre. We frequently see ancient willow-trees en¬
tirely dead, except in a few slender twigs, or in small por¬
tions of their superficial bark. An animal, on the con¬
trary', first dies in its extremities and circumference, whilst
the heart or central portion continues for a time to per¬
form, however languidly, its accustomed actions.
Among plants both sexes usually occur in the same
individual, or even on the same flower; but in a far
greater proportion of animals the two sexes are repre¬
sented by separate individuals. There is indeed no
genuine hermaphrodital union among mammiferous ani¬
mals, birds, reptiles, fishes, cuttle-fish, Crustacea, or in-
1 Lectures on Comparative Anatomy, lecture i.
* Ibid.