824 
ZOOLOGY. 
absorbents, undergoes, while in those vessels, a still further 
change ere it is converted into the structure of the animal. 
In a few of the very lowest animals these vessels carry the 
nutrient matter directly to the rest of the body, building up 
the different organs, as it were, by its own powers. But this 
simple process seems inadequate to the production of the more 
complicated structures, or of parts that require constant acces¬ 
sion of nutriment; and hence we observe that these vessels 
form an homogeneous fluid, of a constant appearance, which 
they pour into larger tubes. These, without materially chang¬ 
ing the fluid, circulate it from one end of the body to another 
with wonderful rapidity—with a rapidity far greater than the 
absorbent vessels could effect. This fluid, then, into which 
all nutrient matters flow, and whence all the structures are 
formed, is called, in the animals of the higher orders, blood, 
sanies in the lower, and may be likened to the sap of plants. 
All these fluids resemble each other in being separable into a 
coagulum, and a fluid that is not solidified by heat or chemi¬ 
cal agents. 
Of the blood-vessel circulation .—The vessels that receive 
blood are too large to act on the principle of capillary attrac¬ 
tion, or any other power analogous thereto. The aid of the 
contractile fibres is here required. The larger vessels are sur¬ 
rounded by rings of these fibres, which, alternately contract¬ 
ing and dilating, propel the blood rapidly through the body. 
In a few animals these fibres exist uniformly along the whole 
blood-system, but in the majority they are collected into a 
large mass at a central point, and here are denominated a 
heart. The heart is very soon divided into two cavities; 
one, which propels the blood through the body, is called the 
ventricle; one, (an auricle) that merely collects the return¬ 
ing blood, while the other is in action, then pours that 
fluid into the latter with a sudden gush, and thus affords 
a due quantity for the full impulse of the ventricle’s fibres. 
The necessity that arises for two cavities, in order to ef¬ 
fect a vigorous circulation, arises from this circumstance. 
The blood is brought to the heart by veins, which are large 
vessels, formed by the union of capillary vessels, and the 
power of the latter forces the blood in the former, even 
to the central point; for veins have no contractile fibres, 
and are too large to possess the attracting power. They are 
furnished, indeed, with valves, that prevent the return of 
blood, but these cannot, of course, contribute to its advance. 
Now, as the supply from the veins is constant and uniform, 
there is always a quantity of blood pressing for entrance into 
the heart. If there had been but a ventricle to receive it 
during the long contraction of this cavity, the veins must be¬ 
come enormously distended; but the auricle prevents this, 
for though, of course, during its contraction, the veins must 
be somewhat distended, yet this is but for a moment, the 
auricle requiring but a very brief contraction to throw its 
contents into the adjacent ventricle. But in perfect hearts we 
ha we four cavities; in order to explain the reason of which 
we must first give an account of the process of aeration, this 
function being intimately connected with the complexity of 
the heart. 
Of the respiratory function .—There is no plant or animal 
that can live without air. The contact of this fluid is as ma¬ 
terial to the formation and growth of the body as food. In 
the article Physiology, we have stated that the change which 
the air effects on the fluids of animals is a union of the carbon 
contained in these fluids with that portion of the air which is 
called oxygen, and the consequent production of carbonic 
acid gas. 2. That this is expelled from the body. 3. That 
there is an evident connexion between this process and the 
temperature of the animal, but that both calorification and 
aeration depend, in a great degree, on the influence of the 
nervous system in all the higher animals. It was hinted, also, 
that besides the more evident change, some finer one, of an 
electrical nature, may be also effected during breathing. 
Though all animals breathe, they have not all pulmonary 
organs. In a few the blood, flowing through the vessels on 
the skin of the body, becomes aerated through pores. In 
others, manifest air-vessels carry the air from the surface to 
the internal parts; these are called trachcce. In others, a 
large internal structure is formed by extended meshes of po¬ 
rous blood-vessels, and over these a quantity of aerating fluid 
is driven. The fluid in question flows in at the mouth, and 
out by another aperture; such respiratory organs are called 
gills. The more perfect of breathing creatures have lungs, 
organs into which pure air is drawn by muscular effort, and 
impure ejected by that power, both kinds passing through 
the same aperture. 
Now in animals that breathe by the skin, it is clear that no 
extra circulating organs are necessary; and even in those 
which have separate respiratory organs, if they do not require 
to throw off carbon in abundance, or to evolve much heat, 
a vessel, branching from the main trunk, may carry to the 
air these organs contain a considerable quantity of blood, 
and this, being aerated, may be returned to the general mass, 
which becomes, therefore, partially purified. Here a heart, 
composed only of an auricle and ventricle, is sufficient. 
But the higher animals require that every particle of the blood 
should be submitted to the influence of the air, and therefore 
they have two circulations, one composed of an auricle and 
ventricle, to circulate impure blood through the lungs, and 
one composed of similar cavities to circulate purified blood 
through the body. 
It will be seen, from the observations preceding the last 
paragraph, that whatever other functions the respiring system 
performs, it is an emunctory—that it gives out from the blood 
matter, which, if retained, would unfit that fluid for its of¬ 
fices. But there are several other structures that pour forth 
secretions from the blood for no other apparent purpose than 
their speedy ejection from the body. Thus the lower part 
of the alimentary canal, in the higher animals, is constantly 
being filled by large quantities of excrementitious matter. 
The skin and the urinary organs are the other main parts for 
excretion. 
The skin and urinary organs .—The surface of the body 
in the lowest animals receives all that is received, and rejects 
all that is rejected, and hence is at once stomach, lungs, and 
every other viscus. In others, covered with hard shells or 
impervious scales, its office is confined to secreting these 
substances. In the mammalia and man it is constantly tran¬ 
spiring, and affords that peculiar odour that distinguishes one 
species from another, and even man from man. Its pro¬ 
ducts are too various to allow us to enumerate them in this 
general view. 
The urinary organs are the parts that separate the water 
from the blood, and with this several salts which it contains. 
The water passes originally into the blood from the alimentary 
canal, through absorbents; not, however, through the same 
absorbents that carry the chyle; on the contrary, aqueous 
fluids may be imbibed from the stomach itself, or from the 
lower end of the intestine. The water, after circulating along 
with the blood, is separated from a great portion of it by an 
artery that branches out into a congeries of capillaries, which 
get the name of kidney. These secrete the urine, and return 
the blood to the circulation in a venous state ; the urine itself 
is poured by a duct, called a ureter, into a receptacle, called 
a bladder, where it collects in large quantities. The kidneys 
are the parts most generally found. 
The locomotive system .—It seems useless to attempt a 
classification of the locomotive system in animals, for this is 
extremely diversified. Two great divisions may be made of 
those which move upon solid matter, and those which use 
the fluids in which they are placed, as resisting media against 
their muscular efforts. Both classes admit innumerable sub¬ 
divisions, according as we consider the various modes in 
which they use these media as means of locomotion. 
“ The most simple progressive motion,” observes Sir 
Everard Home, in the first volume of his Comp. Anatomy, 
“ that can be imagined, is that belonging to those blubbers 
of different descriptions which, from their structure, are 
nearly of the same specific gravity with the sea they inhabit, 
and, from their expanded form, are enabled to float upon its 
surface, moving with the waves on which they are sup¬ 
ported, and in this way passing from one place to another, 
and catching the food necessary for their support. Next to 
this 
