822 
ZOOLOGY. 
ZOO'LOGY, s. [from the Gr. £ 00 ?, life, and \oyo;, 
doctrine.'] The doctrine of life; the laws of animated ex¬ 
istence. 
Introduction. 
In the body of this work, when treating of the different 
genera into which living beings have been divided, we have 
spoken so largely on the habits and forms of animals, that 
we have nothing further to supply under the present head, 
than some account of their anatomy or internal structure. 
This, also, has been, in a slight degree, anticipated in the 
article Anatomy (comparative). But the accessions 
that have been made to Zoological science since that was 
written, are so numerous and important, and the article itself 
was so strictly confined to an exposition of those facts that 
serve especially to illustrate human anatomy, that it is impe¬ 
rative on us, in this place, to investigate as fully as our 
prescribed limits allow, into the structure and functions of 
animals. 
In attempting to define the elements of which animals are 
composed, we are, at first sight, much embarrassed by the 
discovery of this fact. That the chemical constitution and 
the physical appearances of living parts vary not only with 
every animal, but with every part of the same animal; and 
the gradations that exist, are not only thus numerous, but 
are so changeable that we are utterly lost in the multiplicity 
that surrounds us. 
Yet the simpler movements into which the more compli¬ 
cated motions of animated beings are resolvable are so few, 
that if we take them as a basis of investigation, and endea¬ 
vour to determine, 1st, what are life’s primary actions, and, 
secondly, what are the structures to which these actions are 
united, we shall find the whole of the wonderful processes of 
animated existence effected in a manner beautifully perfect 
by three elementary parts. 
We have stated, in the article Physiology, that in 
plants, which form the lowest grade of existence, the chief 
vital acts that are performed, are the imbibition , circulation, 
and conversion into their own substance of the air and water 
which surrounds them. Further, we said that the part which 
performs this office is a tube which is of such small di¬ 
mensions, that it may be denominated capillary, (indeed, 
that term is also appropriate on account of its mode of 
action, which is analogous to capillary attraction,) and 
vital, because it is not merely a capillary tube which can 
imbibe and circulate the fluid presented to it; but one 
which is only effected by particular fluids, and which 
changes them as they pass along it, either by decomposing 
their elements or by effecting new combinations. The 
existence of the Vital Capillary Tube in all parts of 
most animals, has been rendered certain by the injections 
of skilful and laborious anatomists; but their existence re¬ 
quired no such demonstration. For if a seed or an ani¬ 
mal grow, this must arise evidently from some addition of 
matter; and since it is certain, this addition arises not 
from accretion, but from an internal growth, it follows that 
there must be some canal to carry it along the interior of 
the plant or animal; but since, when it is in the interior, it 
does not remain in its original state, but is changed into a 
peculiar substance, it follows that this canal has not only the 
power of imbibing and carrying matter, but also of sub¬ 
jecting this to chemical decomposition. Now it matters not 
what the form of this canal may be. It may be a cell, 
large in itself, communicating by small openings with others, 
forming thus a cellular tissue, or by large openings, an 
arrangement which produces a reticular texture. But ordi¬ 
narily, from the flower to man, beings that are endowed 
with life, are manifestly composed of slender tubes running 
in various directions and separable into different kinds, ac¬ 
cording as they receive fresh materials for growth or expel 
redundancies, according as they are of one colour or another, 
and according as they are clearly defined or contorted and 
convolved on themselves. 
The fact here stated, admits not, in our present state of 
knowledge, and indeed will scarcely ever admit further ex¬ 
planation. The power that fine tubes of glass or metal 
possess of imbibing fluids and carrying them upwards against 
the force of gravity, bears some resemblance to the im¬ 
bibing and circulating powers in question ; and the circum¬ 
stance that glass attracts water more than metal, inasmuch as 
it tends to shew that the phenomena of capillary attraction 
depend on a chemical attraction existing between the fluid 
imbibed and the imbibing vessel, serves to give some hint 
that the assimilating power of living vessels depends on the 
specific powers of attraction that their coats possess. But it 
will require study for ages to discover the laws of these at¬ 
tractions. In the mean time, we content ourselves with the 
undeniable fact, that a seed thrown into the earth, doth, by 
the innate powers of its vessels, imbibe the surrounding water 
and air, and has these circulated throughout its substance by 
vessels that produce successively the root, the stem, the 
leaves, flower, and the fruit. We must content ourselves 
with knowing, that in our own persons these vessels open 
into our stomachs and lungs, thence imbibe the materials 
for our existence, and deposit perishing materials, even as 
an acorn swells with imbibed moisture and grows; and 
that in the whole race of animals, from the thin bag of 
membrane that is termed the hydatid, through all the grada¬ 
tions of creatures that swim through the waler, fly through 
the air, or walk upon the earth—to Man—the perfection of 
the whole—all are composed of these vessels; that every 
sustenance those are capable of deriving, are by these assimi¬ 
lated ; and that all the products, or, as they are technically 
termed, secretions, these pour forth, are formed by the same. 
From the black ink, which defends like a cloud the cuttle¬ 
fish, to the glowing hues that adorn the peacock’s tail; from 
the disgusting odour of the pole-cat, to the balmy breath of 
the cow; the warm fur, the impenetrable hide, solid hoofs, 
tough claws, irrefrangible teeth, fair hair, smooth skin, and 
shining eyes, all are the product of secretion; and the won¬ 
drous power of the vital tube is, in animated existence, what'" 
the law of attraction is in the inanimate world. 
It will strike every one, that the knowledge of the parti¬ 
cular powers of particular vessels must be highly important, 
and would properly form the next step of our enquiry; but, 
beyond the general exposition we have made, we have no¬ 
thing to add. These vital tubes are different with every part 
they occupy. A bundle of them, denominated the liver, 
separates from the same materials a fluid totally different 
from the fluid separated by those that form the kidney. 
One set of vessels makes hair, another teeth, &c.; but no 
one has formed even a conjecture as to the causes on which 
these changes depend. All that can be said is, that every 
part has its particular powers, unknown and unknowable; 
and the vascular frame-work of the body is constant only in 
this law, that it always imbibes and circulates, partly assi¬ 
milates, and partly rejects. 
The next part which, in the article above referred to, we 
said was found distinct in power and in appearance from 
the general slructure of animals, is a contracting fibre. 
This part exists plainly, almost uniformly, wherever the 
slightest motion is visible. There are only a few gelatinous 
animals which shorten themselves when touched, and in 
whom contracting structure and vital tubes seem to co-exist 
without apparent difference, but blended as it were into an 
homogeneous mass. With these few exceptions, animals are 
furnished with parts composed of fibres that shorten them¬ 
selves when stimulated. The word stimulation, which re¬ 
quires some explanation, implies simply this fact, that fibres 
of a given length shorten themselves when touched by a 
foreign body, or when a part that enters between these fibres 
and is called a nerve, is touched. As this phenomenon bears 
no similitude to the impulse that is given by one moving 
body to another, but depends upon some innate quality of the 
fibre itself, it is called stimulation. All enquiries inlo the 
means whereby contraction is effected, or into the mode by 
which touching a muscle or its nerve acts so as to stimulate 
them, are futile; no one knows any thing of the matter, nor 
probably can know. It is enough, however, to be in posses¬ 
sion of the facts; for we have here a power of infinite ad¬ 
vantage 
