823 
ZOOLOGY. 
vantage to the purposes of life. Possessed of this fibre, an 
animal, expanded to its full dimensions, can, on the ap¬ 
proach of danger, contract itself into a small space, and 
into a covering of safety, or can -move one part of its body 
towards another so as to lay hold on prey, or can push forth 
its extremities so as to use the media in or upon which 
it rests, as fixed points for moving its body from place to 
place. 13ut we shall not, in this place, investigate further 
the various modes in which this simple power may be ap¬ 
plied to complex movements, since this will be better seen 
in the sequel. It remains only to remark in this place, that 
the appearance of the muscular structure is as various as that 
of the vascular. 
The third elementary part, referred to in the article Phy¬ 
siology, was the Nerve. We already somewhat antici¬ 
pated the use of this part, by stating that muscular fibres are 
moved to contract, if the nerves that enter them are touched; 
and we may see at once how useful this circumstance must 
be, inasmuch as the movements of distant muscles may be 
thereby concatenated. But, independent of exciting muscu¬ 
lar actions, nerves communicate to our consciousness an im¬ 
pression of feeling. We stated, in the article before men¬ 
tioned, that it had been discovered by Mr. Charles Bell, that 
these two powers resided in different kinds of nerves, at least 
in general. But there are a third class of nerves, which pre¬ 
side over the assimilating functions, that excite the vessels 
or contractile fibres on which one of their extremities termi¬ 
nate when the other is irritated, and which we are unable to 
divide into the two classes of movent and sentient; indeed, 
to talk of these as sentient appears an error in terms, seeing 
that nothing can be called sentient of which we have no 
consciousness. The difficulty is easily got over by stating 
that these nerves are simply movent, and, of course, like all 
movent nerves, capable of stimulation. Still it may be con¬ 
jectured that this power is analogous to sensation. In dis¬ 
eased states nerves actually become sensitive. 
We find that when the nerve of one part terminates in the 
nerve going to another, a swelling is produced at the point of 
union, called, if small, a ganglion, (or more properly, by 
the Germans, a nerve-knot); but, if large, a brain. In both 
these one change is remarked, that the impression that the 
one nerve has received commands, if we may use the term, 
the other, to set in motion the muscular fibre or secreting 
tube. In the smaller ganglions, this, no doubt, takes place 
instinctively, and in obedience to a law as strict and unde¬ 
viating as that which governs any property of dead matter; 
nor is there probably any difference in the power of the two 
nerves. But in time we observe that this action becomes 
modified by repetition, so that, as it were, habit is produced. 
Next we see obscure dawnings of powers, that bear some 
analogy to those which our internal conviction informs us we 
possess ourselves; for pleasure and pain, memory and asso¬ 
ciation, affection and passion, become apparent. And, 
lastly, we entertain the strongest presumption, that in our¬ 
selves the same structure is inhabited (if such a term be al¬ 
lowed in speaking of the immaterial) by reason and will. 
We shall not carry further in this place our analogies of 
the nervous properties, contented with these facts:—1. That 
nerves set distant vessels and muscles in motion, sometimes in 
consequence of a preceding impression, sometimes without 
this. 2. That ganglia are the centres to which external im¬ 
pressions are carried, and from which moving impulses de¬ 
part. 
The existence of the three preceding parts being allowed, 
we profess to shew that, separately or combined, they and 
their products compose the whole animal kingdom. 
The vital capillary tube forms, in all animals, far greater 
proportions of the systems that assimilate food and air, and 
engender progeny than any other; in a few animals, as in 
nearly all plants, it forms them entirely. Muscular and ner¬ 
vous fibres are added, but even the highest animals only in 
small proportions. The nerves particularly are but thread¬ 
like fibrils; and the highest development of the muscular 
fibres in these systems are seen in the hearts of mammalia 
and the stomachs of birds. 
On the other hand, the systems that hold an animal in 
relation with the surrounding world, the prehensile and loco¬ 
motive organs, are almost entirely composed of muscles and 
nerves. All the limbs of land animals, nearly the whole body 
of fishes and birds, are composed of large masses of muscu¬ 
lar fibres, commonly called lean, and these are set in motion 
by large nerves, that have a communication with each other 
at divers points; so that there remain only so much of the 
capillary vessels as are necessary to keep the preceding struc¬ 
tures in constant repair. 
The very different proportions in which these elementary 
substances are combined form then a marked and leading di¬ 
vision of the systems of an animal. The first class are con¬ 
cerned in the digestion of food, and its assimilation ; in ex¬ 
cretion, nutrition, and generation. The second class com¬ 
prises the organs of prehension, voice, and locomotion, and 
the senses. We premise a general view of these organs pre¬ 
paratory to a particular account of them in different classes. 
Of the digestive system. —The digestive organs are 
more invariably met with in the animal kingdom than any 
other of the compound systems. In some it is a mere re¬ 
ceptacle, open at one end, which receives the fluid in which 
the animal is placed. The innumerable capillaries, which 
open on its inner surface, imbibe the nutritious particles, and 
bear them throughout the animal, to form its substance; 
while the remaining or innutritious portion of the matter re¬ 
ceived is ejected by the opening that gave it entrance. But 
this is not the general type of the alimentary canal. It may 
be universally described as a tube, running throughout the 
animal, supplied with—], one order of capillaries, called se- 
cernents, to secrete a solvent juice; 2, another order of the 
same vessels, denominated absorbents, to imbibe the nu¬ 
trient particles; 3,muscular fibres, variously disposed, w-hich 
carry onwards the food ; 4, a few nerves, in which reside the 
sensation of hunger, and which preside over the movement 
of the vessels and muscles. Various are the appearances this 
alimentary canal exhibits, sometimes beinghalf the length of 
the animal, sometimes coiled in convolutions, which, when 
unravelled, extend to several times its length. The internal 
coat, rarely smooth, is thrown in folds or irregularities, that 
vary from mere rugae to a perfect honeycomb structure, or a 
surface covered by long and numerous projections. In like 
manner the capillary vessels, coiled upon themselves, form 
around or in the neighbourhood of this tube glands that per¬ 
mit a more complete elaboration of the fluids presented to 
them than could probably be effected by tubes of short dimen¬ 
sions. The glands that are most constant have acquired va¬ 
rious names, as the liver, pancreas, &c.; and, in the higher 
animals, we have made some faint approaches towards a dis¬ 
covery of the particular part they play respectively in the di¬ 
gestive function. 
The processes that are carried on in the most perfect speci¬ 
mens of digestion are—1, trituration; 2, solution; 3, the se¬ 
paration of the nutrient from the excrementitious portions of 
the food ; 4, the assimilation of the former, and the ejection 
of the latter. The most superficial reflection must suggest 
that these processes are not all of them necessary in equal de¬ 
grees ; that animals may exist whose food is so delicate that 
it requires no trituration, while others may take nourish¬ 
ment so nearly approaching the constitution of their own 
bodies, that almost all of it may be assimilated, and the re- 
crementitious portion reduced to the smallest proportions. 
The alimentary canal may be divided into various parts, 
according to the part it takes in these processes; thus the first 
portion, which is merely for taking in the food, is called the 
oesophagus; the second, which is the part where it is dis¬ 
solved, is called the stomach ; in the smaller, or first intes¬ 
tines, the separation of nutritious from excrementitious mat¬ 
ter is effected, and the former absorbed. In the large, or 
lower intestines a partial absorption is continued, and the 
excrement propelled to the termination of the whole. The 
process of trituration also, when not performed previous to 
deglutition, is effected in the stomach, as happens in birds, 
lobsters, &c. ^ 
The nutrient portion of the food that is taken up by the 
absorbents, 
