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 others are 

 deprived, or by a difference 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 element, procured from another source. 



Some substances, differing very much in character, seem, however, to possess nearly 

 the same chemical composition; we must therefore consider the pecuhar arrangement 

 of the particles as an essential distinction among animal fluids and sohds, as well as 

 their 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 the pro- 

 duction oi fluids 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 tlie continual waste repaired. Thus, by 

 affording continual supplies of nutriment, the blood would undergo a 

 perpetual deterioration, were it not restored by the new matter obtained 

 6om the digestion of the food ; by respiration, which relieves it of the 

 superfluous carbon and hydrogen ; by perspiration, and various other 

 means, which deprive it of any e.xcess of other principles. 



Tliese 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 

 the final object for which the latter motions were designed. 



SECT. V. — ACTIVE FORCES OF THE ANIM.^L BODY. 



Muscular Fibre — Nerves — Uifpothesis of a Xcrcous Fluid. 



The muscular fibre is not confined, in its functions, to be merely the 

 oi^an of voluntary motion. We have shown, that it is one of the most 

 powerful agents employed by Nature, in effecting such necessary motions 

 and transference of particles in the bodies of animals, as are possessed by 

 them in common with vegetables. Thus, the muscular fibres of the in- 

 testines produce the peristaltic motion, which renders these canals per- 

 vious to the aUment; and the muscular fibres of the heart, witli the 

 arteries, are 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- 

 log}', that these nerves are the causes of their involuntary contraction. 



The nerves are composed of several distinct filaments, resembhng each other in 

 every respect ; and they appear to be formed of the same soft pulpy material, commonly 

 called marrow, or medullary substance, surrounded by a cellular membrane. The fila- 

 ments are ag^n enveloped in a tube of this membrane, forming a continued nerve, 

 extending from the brain to various parts of the muscles and skin. Yet the functions 

 of the several filaments of the same nerve are very different. One filament is designed 

 for volimtary muscular motion, another for sensation, and a third for involuntary mo- 

 tion. Sir Charles Bell, to whom we owe this remarkable discovery, divides all the 

 nervous filaments of the body into four general systems ; namely, of voluntary mo- 

 tion, of sensation, of respiration, and of involuntary motion. The last of these per- 

 forms the functions of nutrition, growth, and ultimately, of decay. Besides these, 

 there are nerves destined to particular functions of sensation; such as sight, smell, 

 and hearing. 



When the sensitive filament of a nerve is injured in any part of its course, pain and 

 not motion is the result ; and the pain is referred by the animal to that part of the 

 skin where the remote extremity of the filament is distributed into minute fibres. A 

 patient, whose leg has been amputated, will feel a pain, which long-continued habit has 

 taught him to refer to the extremity of the toes; when, in reality, the injury has been 

 inflicted upon that portion of the nervous filament which terminates at the stump. 



In the remainder of this section, our author proposes to explain the pheno- 

 mena of the nerves upon the hypothesis of a nervous Jiuid, acted upon by certain 

 chemical afiinities. "W'e are aware that several, almost insurmountable, objections 

 may be urged against this theory, and indeed against every other which attempts to 

 explain the complicated functions of Ufe. Yet, if an hypothesis correspond pretty 

 accurately with observed facts, it may have its uses, by fixing the phenomena in the 



memory, pro^^ded we always recollect, that it is but an hypothesis, to be 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 for seeking further analogies, or, as Dr Thomas Brown rightly 

 observes, " a reason for making one experiment rather than another." 



HYPOTHESIS OF A NERVOUS FLUID. 



Every contraction, and, in general, every change in the dimensions of 

 inorganic matter, is occasioned by a change of chemical composition; 

 either, by 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 

 tlie most violent convulsions of nature arise, such as explosions, con- 

 flagrations, &c. 



It is therefore probable that the nerve acts similarly upon t!ie muscular 

 fibre, by means of an imponderable fluid, especially as it has been proved 

 that 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, tliere is every rea- 

 son to infer that the nervous fluid is derived from a similar source, and 

 tliat the medullary substance is the agent in the secretion. On the other 

 hand, it is certain that the medullary substance is the sole conductor of 

 the nervous fluid; all the other organic elements are non-conductors, 

 and arrest it, as glass opposes the progress of the electric fluid. 



All 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, 8ic. 

 It is therefore extremely probable, that these causes act in a chemical 

 manner upon the nervous fluid, by altering its composition; and this 

 view appears to be confirmed by the fact, that 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 

 fiirther 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 papillae, which imbibe saline solutions; the ear is furnished 

 with a gelatinous pulp, violently agitated by tlie sonorous vibrations of 

 the air ; and the eye is supplied with transparent lenses, which concen- 

 trate the rays of light. 



Those substances which have obtained tlie name of irritants, from their 

 power of occasioning contractions in the muscular fibre, probably exer- 

 cise this action through tlie medium of the nerves ; and they influence 

 them in the same manner as the WiU does, that is, by affecting the nervous 

 fluid, in the manner necessary to alter the dimensions of the muscular 

 fibre under its influence. Yet the WiU is not concerned in producing these 

 effects ; often the mind is totally unconscious of tlieir action. Even 

 when the 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, the 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 exists, therefore, 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, without the 

 consciousness of the animal. To effect these objects, the nerves which 

 produce the motion have, in most cases, an arrangement entirely differ- 

 ent from those affected by sensation or controlled by the Will. 



The nervous functions, by which we mean sensation and muscular ir- 

 ritability, are exercised 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 medullary substance. In animals possessing a 

 circulating system, the blood is distributed to aU parts of the body, through 

 the arteries, by means of their irritability and the action of the heart. 



