Part 2

The mechanism of the human body is so wonderfully, so curiously, and so wisely contrived, that the more we examine it, the more we must with admiration acknowledge, that nothing but an ALMIGHTY BEING could be its author. This incomprehensible BEING let us ever remember to reverence and adore, when we examine into the stupendous contrivance of our fabric; HE is the fountain, the benevolent upholder of nature, and all the creation; in whom we live, breathe, and have our being.

SECT. I.

_Of the Composition and close Connection of Parts in the Human Body._

The human body is composed of solids and fluids; these are so closely, and so intimately combined with each other, that we find upon a physical enquiry and examination, they are scarce separable. They aid and give action to each other, and life depends not on each singly, but on the united, and on the separate spur, and on the soul of activity each gives to the other: Their substances form the body, and their action produces the operation of the mind.

Whilst every thing moves free and agreeable to the talk prescribed by nature, the automaton is in a state of health; body and mind enjoy peace and tranquility. So closely are body and mind in the most perfect bond of friendship, that they share pleasure and pain sympathetically with each other; and reciprocally contribute to each other’s welfare.

With what satisfaction is the mind continually engaged to give pleasure to the body! and with what alacrity are for ever the Members ready to obey the will! Happy harmony, by which we so wonderfully exist!

The _solids_ are the canals and springs of life, and the _fluids_, which circulate in those canals, nourish them, and set the springs in action.

By the _solids_ we understand,

1st The _bones_; the frame and support of our wonderful structure.

2dly. _Gristles_ or _cartilages_; which cover the ends of the bones, in order to make them move with ease and agility.

3dly. _Muscles_; the fleshy parts, which by a nervous sensation, are induced, either voluntarily or involuntarily, to contract or dilate themselves, in order to give motion to the various parts of our body, to which they are fastened or destined to move.

4thly. _Tendons_; the chords as it were, by which the muscles are fastened to the bones, so as to make them follow the contraction of the muscles.

5thly. _Ligaments_; the chords by which the bones are connected with each other, and which give a firmness to their contact and articulation.

6thly. _Arteries_; the canals which transport the blood from the heart, to all the parts of the body.

7thly. _Veins_; the canals which carry the blood from the remotest arteries (in the most minute parts of the body) back again to the heart, for a new rotation of the blood.

8thly. _Lymphatic Vessels_; which are small cellular canals, that convey the lymph in different parts of the body.

9thly. _Glands_; which are kernels formed to _secrete_ or separate various humours from the blood.

10thly. _Nerves_; these are medullary branches springing from the brain, and distributed in the most minute parts of the body; ordained to give the act of sensation, as the soul of animal activity.

11thly. _Hair_ and _nails_; these are a horny substance, and appropriated either for defence or warmth of the different parts where they are placed.

The _fluids_; are, 1st. _Blood_; which is circulated in the arteries and veins, and is the nutriment and source of life.

2dly. The different humours separated by the glands from the blood; of which there are many, as the _lymph_, _serum_, _spittle_, _seed_, _chyle_, _urine_, _gall_, &c. appropriated to different uses, as hereafter will be described.

3dly. _Fat_, and _marrow_ in the bones; which are designed not only as a store of nourishment, but to lubricate and warm the parts where they are lodged.

These are the principal materials which compose our wonderful machine. There are various other parts mentioned by Anatomists, both solids and fluids; but as this is designed only to give a universal idea of the most material parts of the structure of this stupendous fabric, we will let this definition suffice, and now consider with what order the principal organs of life act, either in consort, or in opposition to each other.

SECT. II.

_Of the Brains and Nerves._

The brains are by all Physiologists esteemed the most mysterious organs of life, being the fountain of the nerves, the organs of sensation, and hence allowed by all to be the seat of the soul, or the _sanctum sanctorum_ of the human understanding.

They are distinguished into the common or great brain called _cerebrum_; the small part called _cerebellum_; and the _spinal marrow_, which projects down the back-bone. The great brain is divided into two great lobes, seated in the upper and fore part of the skull, of which it takes in the greatest cavity. It is so exceedingly sensible, that the least pressure puts an immediate stop to the sensation of the animal; and which is generally termed a _apoplexy_.[2]

Footnote 2:

Hence in a fracture of the skull, or any other heavy contusion, where the brain is compressed either by the fractured bone, or the extravasated blood, the patient will remain in a state of stupefaction, till either the fracture is reduced, or the extravasated blood removed by trepannation; after which he immediately comes to himself, as if suddenly revived from a trance.

This part of the brain, though it is so exquisitely sensible, seems to be designed by nature as preparatory to the _cerebellum_, the immediate root of the nerves.

The _cerebellum_ is the lesser, but the more material part of the brains. It is seated in the hinder cavity of the skull, (which for the sake of protecting this exquisite sensible organ, is remarkably strong and firm) and from thence as the fountain of life itself, the nerves, the instruments of sensation in the whole animal frame, take their commencement: For whereas the other is so sensible, as that the least pressure will cause a stupor; this is so far superior in point of exquisite sensibility, that a pressure on that part, not only occasions an apoplexy, but even stops life itself.

From this _cerebellum_ prolongates the marrow in the back-bone, called _medulla spinalis_, which is but a continuation of the same, and is equally sensible, and of the same substance with the former.

The whole brain, that is, the _great brain_, the _small brain_, _spinal marrow_, together with all the nerves which spring from thence, are covered with two coats, called the external stronger coat, _dura mater_; and the thinner and inner coat, _pia mater_; which continue with each nerve, even to its smallest ramification. The whole in a living subject is in a continual motion peculiar to itself.[3]

Footnote 3:

This motion in the brain is not unlike the peristaltic motion; this is an observation too seldom taken notice of by anatomists, though evidently plain in living animals, and is of singular service in physiological enquiries.

From the _cerebellum_, and _medulla spinalis_, all the nerves take their commencement and origin. Ten pair spring forth from the part in the skull; the principal are those which form the various senses, tasting, smelling, hearing, and seeing, and that common to all, feeling. From the _medulla spinalis_, or marrow in the back-bone, proceed thirty pair of nerves, which spread themselves variously over the whole system.

All nerves have this in common with each other, namely, exquisite sensibility; but in other respects they differ however widely; particularly the nerves destined for some of the external senses.

It is from that common quality _feeling_, that they become the subtile springs of life; and give an edge to all the animal functions.

There is nothing so plentifully, and nothing more curiously interwoven with every other substance, as the nerves; and so amazingly quick is their sensation, that the very instant a single nerve, even the smallest and remotest in the system, is affected, the soul, or the common _sensorium_, is sensible of the identical part so affected.

It has been asserted, that this amazing quick conveyance of the sensation of the nerves, is owing to a subtile nervous fluid; but on a rational reflection, and a just enquiry into the animal œconomy, it must appear, that such conveyance of a fluid is inconsistent with probability and experience: For, neither are the nerves tubulous, nor has there ever been discovered any circulating humour in them. And secondly, is it consistent with reason, that a conveyance of a fluid from the toe to the finger, should be so momentarily quick, as is sometimes experienced in the gout, and other the like ailments? That the quick transposition of sensation of the nerves, is rather effected on a principle of _vibration_, appears more probable, and will admit of a more convincing experiment; which may be exemplified in the following manner: Suppose (in the language of a seaman) a rope runs from the top-mast-head, or any other part of the rigging, to the deck, and is kept tought and free from any interception; then it will be evident, that the least touch at one end will instantaneously be conveyed to the other end; in like manner the nervous sensation may be communicated, perhaps not unlike electricity; which similarly will in an instant be conveyed to any part, by a direct communication, but equally subject to be intercepted.

Hence, a paralytic limb will lose, together with its irritable sensibility, its circulation of fluids, and its nourishment; and recover again when the pressure or interruption is ceased. Hence also, spasmodic contractions will be affected at the irritation of some principal parts, and from this instantaneous irritability, the motion of every part is thought to obey the will.

SECT. III.

_Of the Heart and Lungs._

The heart is a muscular body, situated in the breast or chest, somewhat between the lobes of the lungs, and formed by nature to be the principal organ for the circulation of the blood and other fluids.

Its structure is wonderful, and of a most curious piece of mechanism. It has two principal cavities, called _ventricles_, which are separated length-ways by a wall, and distinguished into the right and left.[4]

Footnote 4:

They might with equal propriety, be termed _anterior_, or foremost, and _posterior_ or hindermost.

Each of these cavities or _ventricles_, have particular valves, call’d ears, or _auricles_; which perform the function somewhat similar to the suckers of a pump: By these contrivances the heart receives the blood from the veins, and expels it again by the arteries, to every part of the human body; in a manner that I shall hereafter describe.

The lungs are organs immediately concerned in the circulation of the blood also; their function is to receive the air, to purify it, to circulate, and to distribute it in due proportion in the sanguineous mass, and to extract and expel that part of air already made use of; and thus act the part of ventilators in the animal œconomy.

They are divided into two lobes. These lobes are a continuation of little bladders, that have an immediate continuation with the wind-pipe, _aspera arteria_. The wind-pipe from the larynx down to the very lungs, is protected externally with semicircular gristles, in order to keep it always open, and so shelter it from external injury; when it enters the lungs it spreads itself into numberless branches like a tree, and sends ramifications into every little air-bladder in the lungs.

All along these ramifications and the air-bladders, are arteries, veins, and lymphatic vessels, which have an immediate communication with the heart, by means of the pulmonary arteries and veins.

Now at the instant of _inspiration_, these little bladders in the lungs are filled with air, which infuses itself by the small arteries into the whole mass of blood; and at the _expiration_ again, the air which has been made use of, and drawn from the neighbouring little veins, is again expelled by the wind-pipe through the mouth: And thus is _respiration_ performed by alternate motions of the lungs; receiving the fresh, and alternately expelling the foul air again; thence justly may they be deemed the ventilators of the human _automaton_, and a principal organ of life, in the animal creation in general.

The upper part of the wind-pipe, which is in the _fauces_ of the mouth, is called the _larynx_. This is the organ of voice.

In this animal mechanism is contained, all the variety of formation requisite to every kind of sound that can be performed upon any instrument whatever. Its parts are chiefly of a gristly substance, at the same time exquisitely delicate, with nerves and glands; from whence it is subject to diseases on the slightest indisposition.

In men it is larger than in women; hence their voice is rougher; which generally increases by exercise of the voice also. Its formation is very complex, and too tedious here to enter upon; we will therefore pass it over, and hereafter say as much as relates to its preservation, and giving it relief when disordered.

SECT. IV.

_Of the Circulation of the Blood._

We shall now consider the circulation of the blood; the grand source of our existence. At the contemplation of which we must be lost in admiration, at the wonderful wisdom of its contrivance; and with uplifted eyes adore the cause of its motion!

The Heart, as I before observed, has two chambers or _ventricles_, distinguished by the right and left; each of these chambers have also a valvular cavity call’d _auricle_, or ear. Each _ventricle_ opens itself into an _artery_ and _auricle_; and each _auricle_ opens itself into a _ventricle_ and _vein_. These openings have valves, which open and shut alternately, and by this, prevent any confusion or hindrance in the influx and efflux of the blood.

The right _ventricle_ is the longest and greatest, but weakest: From this ventricle springs forth the _pulmonary artery_, which spreads itself throughout the Lungs.

The right _auricle_ is also the largest; in which opens itself the large vein, called _vena cava_; as the common trunk that receives the blood from all its branches spread over the whole human body.

The left _ventricle_ is shorter, but the walls are considerably stronger than that of the right _ventricle_. Here begins the _aorta_, or the great artery, by which the blood is sent to all the parts of the human body.

The left _auricle_ is also somewhat smaller than the right; and in that opens itself the _pulmonary vein_, which returns the blood from the lungs, after it has received its necessary assistance from the vesicles of air in the lungs.

This premised and understood, the circulation is performed in the following manner:

The right _ventricle_ being full of blood, contracts itself. This contraction is called the _systole_, by which it forces the blood through the _pulmonary artery_ into every part of the lungs, even into every vesicle; where it receives a portion of the air, drawn in by the wind-pipe.

At the end of these _arteries_ join the ramifications of the _pulmonary vein_, which receives the blood from the ramifications of the arteries, by _anastomosation_; which, by little quantities, these veins gather gradually into little branches, and at last become one common trunk, which empties the blood into the _left auricle_ of the heart.

From the _left auricle_ it is let, by means of a peculiar constructed _valve_, into the _left ventricle_; which, by a dilation called the _diastole_, receives it from its _auricle_.

Immediately upon this reception, the _systole_ or contraction takes place, and the blood is forced into the great universal artery, _aorta_; by which it is farther transported into every part of the human body.

This _artery_ spreads as it goes, and continually decreases in magnitude, as it becomes numerous in branches; until it terminates into the smallest ramification in the remotest part of the body.

The _systolic contraction_, and the _diastolic dilation_, which alternately take place in the heart, continue throughout the arteries from the great trunk to the remotest capillary ramifications,[5] by which means the blood is transported with more certainty and facility to the minutest part.

Footnote 5:

Concerning this great act of continued systole and diastole, Anatomists have greatly varied; but from the structure of the arteries, and the continued pulsation, this way of transporting the blood is confirmed both by reason and experience.

At the extremity of these arteries the veins take their commencement, in ramifications equally small with the former; these take up the blood from the arteries, by many infinite small quantities, and carry it gradually back from those extremities to the heart again, for a new rotation.

These veins commencing infinitely small, but gradually gathering, become branches, and at length form one general trunk, called the _vena cava_, (or the great universal vein); and this empties itself again into the _right auricle_; from whence it is let again into the _right ventricle_, whence it came; then again forced into the lungs; thence back again into the left ventricle; from thence all over the whole body, and continues the Circulation.

SECT. V.

_Of the Joint Connection of the Brain, Heart, Lungs, &c._

These are the principal actions in the grand movement of the human automaton.

The nerves are the subtile springs of the sensation, by which the whole becomes sensible of irritation, and agitated to its functions. Their ramifications are extended _ad infinitum_, and so curiously distributed, that though there is not a part in the whole system exempt from their spreading, yet they are distributed with such regularity, that they don’t interfere with each other. A regularity absolutely requisite to the order in the animal œconomy: For whereever the vibration of the nerves is obstructed, life or motion must in consequence cease.

The lungs are the ventilators in the system: they receive and let in the air; they sift it from all its gross particles, and extract the useful parts from this universal element of life; thus they give from every little vesicle, a due proportion of extracted air to the blood by the adjacent veins; and by the neighbouring arteries, again extract the foul air from the circulated blood, which by the mouth and nostrils is expelled again; at that instant a fresh quantity of air is drawn in again for a new supply.

Thus by intervals, the lungs, like a perfect ventilatory machine, repeatedly, and without intermission, keep time with _systolic_ and _diastolic_ motions of the heart, in exchanging fresh and foul air for the support of this wonderful animal structure. This is the function of breathing _respiration_, namely _inspiration_, or the drawing in the fresh air; and _expiration_, the expelling the foul air.

The heart may be considered as the grand perpetual pumping engine, constructed on principles, so as to have the least friction possible.

This curious pump, _the heart_, sends the fluid of life, _the blood_, to all the parts, even the remotest in the whole machine. The _arteries_ are the canals of the conveyance; they are strong elastic tubes, whose fibres are of spiral direction, and receive from the heart a power of contraction; which is continued from the heart in the moment of the _systole_, even to the remotest and most minute ramifications; by which the blood is pushed along to the minutest extremities. This is immediately succeeded with a _diastolic_ reception of a fresh quantity of blood; so that on succession, the _diastole_ and _systole_ is perpetuated not only in the heart, but throughout all the arteries, in a friendly correspondence over the whole system: And this constitutes the _pulse_.

The veins, however, are somewhat of a different texture from the arteries; whilst the arteries transport the blood by an elastic and pulse-like contraction, the veins on the contrary, receive it from them at the extremities, in an easy and imperceptible gradation; for whereas the fibres in the arteries are in a spiral direction, elastic and strong, the coat of the veins are tender, and the fibres more longitudinal.

Hence, the wounds in the arteries and veins differ greatly; for whilst the arteries are for ever in a strong agitation, the veins perform their functions with less exercise; and therefore the wounds in arteries are always attended with danger, whilst the veins heal easy. From the same reason also, the arterial blood differs from the venal; for as the arterial blood is sent to all the Parts, as the grand nutriment, from whence the glands secrete their peculiar humours; the veins only transport it back again, to prepare it for a new rotation. Whence the blood in the arteries is of a high colour, and of a more alkalascent nature, than that in the veins.

Collateral with the minute ramifications of the veins and arteries, over the whole system, are the _lymphatic vessels_, which separate from the blood as it passes along, a transparent lymphatic humour, adapted for the different parts, which they are connected with.[6] These vessels are materially different, from either arteries and veins in their structure, and justly to be compared to a continuation of little cells; so constructed, as to transport the humours, without admitting a return.

Footnote 6:

An opinion has lately been broached, that the lymphatics are all absorbing vessels, of the same nature with the lacteals, and that they all join to empty themselves in the subclavian vein to the common mass of the blood; but this to me, seems too general, and contradicts experience.

From hence we see plainly how these three organs, the _brains_, the _lungs_, and the _heart_, are concerned, and mutually assist each other in the actions, and circulation of the fluid of life; nothing could move without the sensation of the nerves; these could not exist without the blood, from which they receive their support; the blood could not circulate without the heart, arteries, and veins; and in them it could not move with a necessary freedom, except its being supplied with a sufficiency of air from the lungs: So that evidently all three are so interested in this grand movement, that the stopping of the one, must needs be the stopping of the whole.

SECT. VI.

_Of Chylifaction and Nutrition._

In the preceding we have briefly considered the state of the fluid of life, and mechanical instruments of their motion in the human machine. In this we will consider how the whole is supported, and the vital lamp, from time to time, becomes supplied.

Under this consideration we will examine the canal of food, and the various changes the morsel undergoes from the time we take it in our mouths, till the nutriment is reduced to blood, and the remaining dross is expelled the body as useless.

The animal functions towards _chylifaction_ and _nutrition_, are _mastication_, or chewing the _food_; _deglutition_, or swallowing; _digestion_; _chylifaction_; _nutrition_; and the _excretion_ of the _fæces_.

In order therefore to understand this clearly, it becomes necessary to describe the organs which nature has formed for the requisite performances thereof.