Part 3

This aptitude will continue in some animals longer than others, probably owing to the peculiar manner in which they are influenced by stimuli; look at the large class of hybernating animals; though they are in their retreat to all appearance dead, none of the evident symptoms of life shewing existence, yet we may conclude that its influence still pervades their systems and preserves their bodies, composed of a variety of elements disposed by their properties to run into discomposition, from the disorganizing effects of chemical action.

This disposition of animal bodies to action, it may be observed, admits of increase or diminution. When the same substances produce more action, the aptitude may be supposed accumulated, or its energies increased, when less action, we may suppose it diminished; and when no action at all, under any circumstances we may conclude it destroyed, and here the capability for action ceasing altogether, discovers the difference between dead and living matter. Hence remarking its various vicissitudes with respect to energy, and its regeneration when not too much impaired, I am disposed to refer its origin to some source capable of supply, and not to an inherent or insulated quality.

Seeking for the medium through which this vital influence immediately operates, I am induced to turn to the brain as the point where all the powers of the animal appear more completely concentrated, and its continuations the (nervous elongations) as the active agents of life, existing more or less through the whole body. This proposition, I shall indeavour to support by direct and collateral arguments, adduced with as much perspicuity and brevity as possible.

Much may be argued from the importance of the brain in the economy of the system and the rank it occupies in intellectual operations. And though the mode of connexion between mind and matter, and the living principle and it, be not demonstrable to the senses, and will probably forever remain among the arcana of nature, yet we continually witness their effects and may conceive them a quality impressed under particular circumstances on the nervous system: possibly something in the way that bodies are endowed with the power of affinity or principle of gravity. And although I leave it to the researches of the metaphysician to explain how mental phenomena are produced through the agency of matter, and how the sentient principle acts again through the same mean. Still we may trace their proximate cause to the nervous medulla and brain, as the common centre of communication between all parts, and as the direct medium through which external substances act, and which again produce a re-action.

Injuries or inflammations of the brain are attended immediately with derangement of organs, or the most destructive consequences; whereas injuries of other parts, essential to the powers of life, and therefore called vital, appear rather by indirect means to impair the bodily functions; necessary to the proper performance of which, there is a very delicate organization of the whole, existing in close dependence on the circulation, or (the tout ensemble) of organic life.

We may also argue something from the tenacity of life possessed by the nerves. While other parts of the body, even bone, may be destroyed by pressure, the nerves resist its destructive influence, as is evinced in the ligature of the surgeon passed round them in awkward operation, for however tight it may be drawn; it only impairs the communication between the extremity and the sensorium commune, producing no slough or death, as in other parts. Again,

Our evidence of life is most clearly evinced in an alternate state of contraction and relaxation of parts. To which effect a nervous influence appears essentially necessary, for destroy the continuity of their chords, and though all other circumstances may remain the same, the action is prevented from taking place. “When the recurrent nerve on one side of the larynx is cut, the voice becomes sensibly weaker, when both are cut it is entirely destroyed.” From whence it is plain that the moving power is intercepted. But it may be, and I am aware it has been objected, to this inference that muscles may be made to contract by the application of stimuli after excision from the body. Which fact in my estimation only serves to prove the great tenacity and subtilty of the nervous influence. Anatomical research has traced the ramification of the nerve through all the fibre entirely diffused even till lost in pulp, and no one I believe has reason to doubt the complete dependence of sensibility on the presence of the nerves, yet there is no instrument, however sharp, that can touch a single point in a muscle without producing a sensation more or less acute; which circumstance shews the entire distribution of the nerves. And until every part be entirely removed, I presume they may communicate their influence; which upon the application of stimuli will be evinced, though in a feeble and irregular manner.

I therefore conclude we can not concede the important vis nervae, the direct influence of which is supported by demonstration, in favour of an imaginary inherent, or any other property of muscular matter.

An experiment of Billinies, since repeated with success by Doctor Monro, goes far to establish a positive effect exercised by the nerves in muscular motion. “After opening the thorax of a living dog, catch hold of, and press both the phrenic nerves with the fingers, the diaphragm ceases immediately to contract. Let go the hold of the nerves and the muscle acts again, pinch a second time the nerve or nerves, some way above the diaphragm, the muscle again ceases to act, keep firm hold of the nerves, with the fingers of the other hand strip or milch it down from the griping fingers towards the diaphragm, and the muscle is made to contract; and for three or four strippings or milchings, the action follows or obeys the motion of the fingers which strip it down, then it becomes disobedient and contracts no more, strip as you will, unless the finger, griping the nerves let go their hold and pinch farther up, when the muscle may again be made to contract, by stripping down towards the diaphragm.”――Also, an experiment performed by Dr. Whytt, in which he injected a strong solution of opium into the stomach and intestines of a living frog, after his heart was taken out; “in thirty minutes he appeared quite dead, and neither pricking or tearing its muscles produced any motion in them, or the members to which they were attached” but the doctor tells us, on irritating the spinal marrow with a probe, the limbs contracted feebly. Here there could have been no vis insita, or it would have evinced itself on the application of the mechanical stimulus to the muscles. And indeed the whole experiment rather serves to prove the entire dependence of the muscles for irritability on the nerves, for when rendered incapable of being excited, they were again brought into action by the remains of nervous energy, in the medulla oblongata, (called forth by the probe.)

These considerations will I judge establish the direct influence of the nerves in muscular motion. And though its peculiar nature and qualities be unknown, an impenetrable veil shrouding it from observation, we may remain satisfied with a knowledge of the existence of an effectual cause. And as far as we may be allowed to infer from the general plans and regulations of nature surrounding us, seeing she delights in simplicity and uniformity, producing the greatest number of effects by the varied combination of a few elementary principles, we may rationally refer all the apparent complexness of the animal economy to the diversified influence of this nervous agent, particularly modified and applied to the variety of operations in the body, yet all tending to a unity of effect. And as simplification in our particular science appears the order of the day, I may be justified in supposing the irritability, sensibility, &c. of authors, but varieties in the action of this principle.

Thus far we move with reason, beyond this point all is conjecture “and shadows, clouds and darkness rest upon it.”

When we compare the two conditions of life and death, and see that all the corporeal or material parts remain in the latter state, and know at the same time that in life there was some thing that produced the characteristick thereof, we may rationally conclude that the endowments of life was some very subtile or spiritous principle which resided in, and influenced those parts which remain even after it has vanished.

In conclusion I will only observe, it is to the brain and its nervous system we should attribute the residence of vitality, which completely distributed and influenced by proper circumstances produce all the powers of the living system. It is there we find a remote cause and need seek no farther, but if it were necessary to approximate nearer to an efficient cause, I would take a general survey of the extensive field of nature, and observing the design and order that pervades all her regulations, refer their operations to immutable laws arranged in consummate wisdom, and intelligence, pervading all matter, and particularly modified in the human frame to fill the scheme of divine intention, whatever that may be.――Such general laws diffused through all extent, are the immediate attributes of a God――

All are but parts, &c.――POPE.

Before I close these desultory observations, permit me to express a wish for the success and prosperity of the institution from which I am receiving the honors of physick, and the general obligations I am under for many advantages and improvements derived from the lectures of its enlightened professors. Satisfied that comparisons are always indelicate, and might in the present instance prove particularly offensive, I forbear to particularize advantages, but must, however, indulge in an expression of the gratification I feel in seeing the art of medicine once more assume to itself the form and character of a science; order to take the place of confusion; and system, the first effect of genius, triumph over the extravagances of whim, and love of innovation.

FINIS.

Transcriber’s Notes:

――Text in italics is enclosed by underscores (italics).

――Obvious punctuation and spelling inaccuracies were silently corrected.

――Archaic and variable spelling has been preserved.

――Variations in hyphenation and compound words have been preserved.