ACTIVE FORCES OF THE ANIMAL BODY. 
of the blood and other parts ; and the fat, or animal oil, placed in the 
cellular tissue to render it flexible. In fact, all the solids and fluids of 
the animal body are composed of chemical elements contained in the 
blood. It is only by possessing some elements, of which the otltcrs are 
deprived, or by a diflcrcnco in the proportions in which they combine, 
that [in general] they can be distinguished. From this it appears that 
it only requires, for their formation in the body, to abstract the entire, or 
a part, of one or more elements of the blood ; or, in a few cases, to add a 
foreign clement, procured from another source. 
Some substani'oji, differing very much in ehar.ictcr, seem, however, to possess nearly 
tile same chemical compo-sition ; we must therefore consider the peculiar arrangement 
of tho particles as an essential distinction among animal fluids and solids, as well as 
tlieir composition, and the proportions of their elements. 
We might, without impropriety, assign the term secretion to denote the 
various operations by which the blood nourishes and renovates the solid 
and fluid parts of the body. But, we shall restrict the term to tho pro- 
duction of Jluiils only; while, we shall apply the term nutrition, to signify 
the production and deposition of the materials, necessary for the growth 
and maintenance of the solids. To each solid organ, and to every fluid, 
is assigned that peculiar composition which is suited to its place in the 
system ; and, by the renovating power of the blood, their composition is 
preserved during hcaltli, and the continual waste repaired. Tims, by 
affording continual supplies of nutriment, the blood would undergo a 
perpetual deterioration, were it not restored by the new matter obtained 
from the digestion of the food; by respiration, vvhich relieves it of the 
superfluous carbon and hydrogen; by perspiration, and various other 
means, which deprive it of any excess of other principles. 
These continual changes in the chemical composition of the several 
parts, are as essential to the vital action, as the visible motions of the old 
particles, and the constant influx of new ones: indeed, they seem to be 
tho final object for which tho latter motions wore designed. 
SECT. V. — ACTIVE FORCES OF THE ANLUAL BODY. 
Muscular Fibre — Nerves — lltjpoihesis of a Nervous Fluid. 
The muscular fibre is not confined, in its functions, to be merely the 
organ of voluntary motion. We have shown, that it is one of the most 
powerful agents employed by Nature, in efTecting such necessary motions 
and transference of particles in the bodies of animals, as arc possessed by 
them in common with vegetables. Thus, the muscular fibres of tho in- 
testines produce tlie peristaltic motion, which renders those canals per- 
vious to the aliment; and the muscular fibres of tho heart, with the 
arteries, ai-e the agents in the circulation of the blood ; and thus, ultimate- 
ly, of all the secretions. 
The Will contracts certain portions of the muscular fibre through the 
medium of the nerves. Certain other fibres, such as those to which we 
have just alluded, are independent of the Will, and yet are animated by 
nerves extending through them. We may therefore conclude, from ana- 
*°Sy> that those nerves arc the causes of their involuntary contraction. 
The nerves are composed of several distinct filaments, resembling each other in 
every respect; and they appear to ho formed of tho same soft pulpy material, commonly 
ealled marrow, or medullary substance, surrounded hy a cellular membrane. The fila- 
ments are again enveloped in a tube of this membrane, forming a continued nerve, 
extending from tho br.ain to various parts of the muscles and sUn. Yet the functions 
of the several filaments of the same nerve are very different. One filament is designed 
for voluntary muscular motion, another for sensation, and a third for involuntary rao- 
tmn. gif (Jiiarles Bell, to wliora we owe this remarkable discovery, divides all the 
nervous filaments of tho body into four general systems; namely, of voluntary mo- 
bon, of sensation, of respiration, and of involuntary motion. Tho last of these per- 
forms tho functions of nutrition, growth, and ultimately, of decay. Besides these, 
fliero arc nerves de.stinod to particular functions of sensation; such as sight, smell, 
xiid hearing. 
^Vhen the sensitive filament of a nerve is injured in any port of its course, pain and 
bot motion is the result; and the p.ain is referred by the animal to that part of tho 
skin where the remote extremity of tho filament is distributed into minute fibres. A 
patient, whose leg has been amputated, will fool a pain, whieli long-continued habit has 
jaught him to refer to the extremity of the toes; when, in reality, tlie injury has been 
biflicted upon that portion of the nervous filament which terminates at the stump. 
hi the remainder of this section, our author proposes to explain tho pheno- 
biena of the nerves upon tho hyjiothcsis of a nervous fluid, acted upon by certain 
vheraical affinities. We are aware that several, almost insurmountable, objections 
’bay bo urged against this theory, and indeed against every other which attempts to 
explain the complicated functions of life. Yet, if an hypothesis correspond pretty 
■’ccurately with observed facts, it may have its nsrs, by fixing the phenomena in the 
memory, provided we always recollect, that it is hut an hypothesis, to he modified .as 
knowledge extends. Thus the phenomena of heat are referred to the imponderable 
fluid caloric ; of light, to the vibrations of a highly elastic medium ; of electricity, to the 
electric fluid ; — none of which can be demonstrated to have a real existence in nature. 
But, in adopting an hypothesis, we must never forget that it is a temporary, not a 
final, theory ; — a motive fur seeking further analogies, or, as Dr Thomas Brown rightly 
observes, “ a reason for making one experiment rather than another.” 
iiyroTiiEsis OF a kervous fluid. 
Every contraction, and, in general, every change in the dimensions of 
inorganic matter, is occasioned by a change of chemical composition; 
either, hy the absolute addition or abstraction of some solid matter, or by 
the flux or reflux of an imponderable fluid, such as caloric. In this way 
the most violent convulsions of nature arise, such as explosions, con- 
flagrations, &c. 
It is therefore probable that the nerve acts similarly upon tlio muscular 
fibre, by means of an imponderable fluid, especially as it has been proved 
th.Tt the impulse is not mechanical. 
The medullary matter of the entire nervous system is formed through- 
out of the same material; and, blood-vessels accompanying all its ramifi- 
cations, it is thus enabled to exercise, in every part, the functions be- 
longing to its nature. 
All the animal fluids being secreted from the blood, there is every rea- 
son to infer that the nervous fluid is derived from a similar source, and 
that the medullary substance is tlie agent in the secretion. On the other 
hand, it is certain that the medullary substance is the sole conductor of 
tho nervous fluid; all the other organic elements are non-conductors, 
and arrest it, as glass opposes the progress of the electric fluid. 
AH the external causes, capable of producing sensation, or of occasion- 
ing contractions in the muscular fibre, are chemical agents, possessing a 
power of decomposing, such as light, caloric, salts, odorous vapours, &c. 
It is tlicrefore extremely probable, that these causes act in a chemical 
manner upon the nervous fluid, hy altering its composition ; and this 
view appears to he confirmed by the fact, tliat the action of the nerves is 
enfeebled by long continuance, as if the nervous fluid required a supply 
of new materials to restore its composition, and enable it to undergo a 
further alteration. 
An external organ of sense may be compared to a kind of sieve, 
which only permits those agents to pass through it, and act upon the 
nerve, that it is fitted to receive at that place; but it often accumulates 
the nervous fluid so as greatly to increase its effect. Thus, the tongue 
has spongy papilla?, which imbibe saline solutions ; the car is furnished 
with a gelatinous pulp, violently agitated hy the sonorous vibrations of 
tlie air; and the eye is supplied with transparent lenses, which concen- 
trate tlie rays of liglit. 
Tliose substances which liave obtained the name of irritants, from tlieir 
power of occasioning contractions in the muscular fibre, probably exer- 
cise this action through the medium of the nerves ; and they influence 
thorn in the same manner as the Will docs, that is, by affecting the nervous 
fluid, in the manner necessary to alter the dimensions of the muscular 
fibre under its influence. Yet the Will is not concerned in producing these 
effects; often the mind is totally unconscious of their action. Even 
when tlie muscles are separated from the body, they are susceptible of 
being irritated, so long as that portion of the nerve, which accompanies 
them, retains its power of acting. In this case, tho phenomena are to- 
tally removed from the influence of the Will. The state of the nervous 
fluid is altered by muscular irritation, as well as by sensation and volun- 
tary motion : there e:dsts, tliereforc, the same necessity for restoring its 
original composition. Irritants occasion those movements and transfer- 
ences of particles necessary to the functions possessed in common by ani- 
mals and plants; thus, the aliment stimulates the intestine; the blood 
irritates the heart. These motions are all performed independent of the 
influence of the Will, and, in general, while health continues, witliout the 
consciousness of the animal. To effect these objects, the nerves which 
produce the motion luive, in most cases, an arrangement entirely differ- 
ent from those affected by sensation or controlled by the 'Will. 
The nervous functions, by wdiich we moan sensation and muscular ir- 
ritability, are exorcised with more or less vigour upon every point, in pro- 
portion as the nervous fluid is more or less abundant there ; and as this 
fluid is produced by secretion, its quantity ought to depend jointly upon 
the quantity of the medullary matter secreting it, and upon the supplies 
of blood received by this medtillary substance. In animals possessing a 
circulating system, the blood is distributed to all parts of the body, tlirough 
the arteries, by means of their irritability and the action of the heart. 
