LECTURE I.

ANAnatomical and Physiological DescriptionOF THEHUMAN BODY.

Anatomical and Physiological Description

OF THE

HUMAN BODY.

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 anAlmighty Beingcould be its author. This incomprehensibleBeinglet 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.

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!

Thesolidsare the canals and springs of life, and thefluids, which circulate in those canals, nourish them, and set the springs in action.

By thesolidswe understand,

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

2dly.Gristlesorcartilages; 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 tosecreteor 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.Hairandnails; these are a horny substance, and appropriated either for defence or warmth of the different parts where they are placed.

Thefluids; 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 thelymph,serum,spittle,seed,chyle,urine,gall, &c. appropriated to different uses, as hereafter will be described.

3dly.Fat, andmarrowin 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.

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 thesanctum sanctorumof the human understanding.

They are distinguished into the common or great brain calledcerebrum; the small part calledcerebellum; and thespinal 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 aapoplexy.[2]

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 thecerebellum, the immediate root of the nerves.

Thecerebellumis 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 thiscerebellumprolongates the marrow in the back-bone, calledmedulla 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, thegreat brain, thesmall 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 smallestramification. The whole in a living subject is in a continual motion peculiar to itself.[3]

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 thecerebellum, andmedulla 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 themedulla 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 qualityfeeling, 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 commonsensorium, 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 ofvibration, 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.

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, calledventricles, which are separated length-ways by a wall, and distinguished into the right and left.[4]

4.They might with equal propriety, be termedanterior, or foremost, andposterioror hindermost.

Each of these cavities orventricles, have particular valves, call’d ears, orauricles; which perform the function somewhat similarto 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 ofinspiration, 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 theexpirationagain, 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 isrespirationperformed 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 humanautomaton, and a principal organ of life, in the animal creation in general.

The upper part of the wind-pipe, which is in thefaucesof the mouth, is called thelarynx. 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 passit over, and hereafter say as much as relates to its preservation, and giving it relief when disordered.

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 orventricles, distinguished by the right and left; each of these chambers have also a valvular cavity call’dauricle, or ear. Eachventricleopens itself into anarteryandauricle; and eachauricleopens itself into aventricleandvein. 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 rightventricleis the longest and greatest, but weakest: From this ventricle springs forth thepulmonary artery, which spreads itself throughout the Lungs.

The rightauricleis also the largest; in which opens itself the large vein, calledvena cava; as the common trunk that receives the blood from all its branches spread over the whole human body.

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

The leftauricleis also somewhat smaller than the right; and in that opens itself thepulmonary 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 rightventriclebeing full of blood, contracts itself. This contraction is called thesystole, by which it forces the blood through thepulmonary arteryinto 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 thesearteriesjoin the ramifications of thepulmonary vein, which receives the blood from the ramifications of the arteries, byanastomosation; which, by little quantities, these veins gather gradually into little branches, and at last become one commontrunk, which empties the blood into theleft auricleof the heart.

From theleft auricleit is let, by means of a peculiar constructedvalve, into theleft ventricle; which, by a dilation called thediastole, receives it from itsauricle.

Immediately upon this reception, thesystoleor 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.

Thisarteryspreads 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.

Thesystolic contraction, and thediastolic 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.

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 infinitesmall 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 thevena cava, (or the great universal vein); and this empties itself again into theright auricle; from whence it is let again into theright 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.

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 extendedad infinitum, and so curiously distributed, that though there is not a part in the whole system exempt from their spreading, yet theyare 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 withsystolicanddiastolicmotions of the heart, in exchanging fresh and foul air for the support of this wonderful animal structure. This is the function of breathingrespiration, namelyinspiration, or the drawing in the fresh air; andexpiration, 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. Thearteriesare 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 thesystole, even to the remotest and most minute ramifications; by which the blood is pushed along to the minutest extremities. This is immediately succeeded with adiastolicreception of a fresh quantity of blood; so that on succession, thediastoleandsystoleis perpetuated not only in the heart, but throughout all the arteries, in a friendly correspondence over the whole system: And this constitutes thepulse.

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 veinsperform 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 thelymphatic 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.

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, thebrains, thelungs, and theheart,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.

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 reducedto blood, and the remaining dross is expelled the body as useless.

The animal functions towardschylifactionandnutrition, aremastication, or chewing thefood;deglutition, or swallowing;digestion;chylifaction;nutrition; and theexcretionof thefæces.

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

In the first place then,smellingis that sensation which nature has given to every animal, that has a choice of food, as the first safe-guard to inform it of any thing agreeable or disagreeable, useful or pernicious to its body; its seat is in the nose chiefly, but we find by experience, that it has a friendly connection with our palate and stomach; for the effluvia of any thing will either create a desire, or give us the greatest aversion to every substance that throws out a flavour.

It is performed by means of a subtile spreading of nerves, peculiarly delicate; which continues through the membrane of the nose, the roof of the mouth, gullet, and the very stomach.

Tasteis the next sensation, which nature has given us, not only as a distinguisher of proper food, but a sensation from which we receive many luxurious pleasures; and tothe indulgence of this sensation, most evils and plagues to mankind, take their origin.

The tongue is the principal instrument of that peculiar quality; but if we examine somewhat closer into this affair, we shall find that the soul of pleasure and pain of that sensation, as well as that of smelling, has its seat in the stomach; for that which will taste pleasing and good at the first approach, will soon lose its relish when the stomach is gratified: and, if any thing tastes disagreeable, the stomach receives it with reluctance, and will ever incline to discharge it again.

The tongue is an instrument (if I may be allowed the term) very curiously constructed; it is moved by a variety of muscles, and serves not only for tasting, but also as a labourer, to shovel and to turn our meat between our grinders; so that nothing may escape being well masticated, and intermixed with that fine digesting balsam, the spittle, in order that it may be easily swallowed.

Besides this, it makes the most requisite instrument for the noble and excellent faculty of speaking; which forms one of the principal characteristics that distinguishes man from the brute creation.

The gullet oroesophagus, is the canal which conveys drink and food from the mouth to the stomach; this canal is a muscular, tendinous, and vascular tunic.

The commencement of it is in the mouth, and is called thepharynx; a curious structure, that receives the food, and by its contractive motion, and the help of the tongue, forces the aliment into the stomach.

The stomach is much like the bag of a Scotch bag-pipe; it lies immediately under thediaphragmormidriff, covered partly on the right side with the liver, and on the left side with the spleen. The left and superior part, is continued with theoesophagus; and the right and inferior part, or orifice, commences theintestines.

The first orifice is called the mouth of the stomach; and the second thepylorus, orporter: At the porter there is a curious valve which lets the aliment out by small parcels into the intestines, where it undergoes its various other changes.

The stomach has three teguments, amuscular, atendinous, andnervous coat; thisnervous coathas another slimy one, but this in reality, is a delicate lining, interwoven with nerves, and the ramifications of fine blood vessels.

Theintestinesor guts, are a continuation of the stomach, they are a canal which is generally reckoned six times as long as the subject it is taken from; it is distinguished in smalltenuia, and widecrassa.

Each again is divided into three parts: thetenuia, or small narrow intestines, are theduodenum, or twelve finger-gut; thejejunum; theileum: The wide orcrassa, is divided into thecæcum; thecolon; and therectum.

Throughout the whole canal of intestines are numbers of little vessels, calledlacteals, which lead the chyle, extracted from the aliment, into areceptacle, which is lodged in themesentery, and from thence, by another duct call’d thethoracic duct, is carried along the back-bone upwards, and joins to the left subclavian vein, where thechylegradually commences to be blood.

This short description we will let suffice, and now enter upon the action itself.

The morsel now, which is designed for food, is taken into the mouth, masticated with the teeth, turned about with the tongue; and as the mouth is at work, thesalivaor spittle is squeezed from the salival glands, and thus intermixed with the aliment; when enough chewed and moistened with this saliva, it is conveyed to thepharynx, or swallow, which receives it, and, by its contraction, forces it into theoesophagus; and by a repeated contraction, is carried down into the stomach.

There it is again moistened withsaponaceous liquid, orpancreatic juice; by whichand by a perpetual motion of the stomach, it is brought into a state of digestion; then by small degrees entered through thepylorusor porter, into the first division of the gut, theduodenum.

This gut is about twelve fingers long; and whilst the aliment is there, it is intermixed with the gall, which is a liquor separated by the liver, and contained in the gall bladder; this liquor, the gall, is carried into theduodenum, by a small duct, called theductus cysticus; where also enters another kind of liquor called thepancreatic juice.

When the aliment is thus prepared, and fit for a particular state of dissolution, it is carried into, and through thejejunum. This gut is in length about twelve or thirteen hands breadth, and its motion somewhat brisk; through which the aliment passes pretty quick, and hence, generally is somewhat empty.

As it passes through this part, the chyle is separated from it by the lacteals, which are small vessels that separate the chyle from the aliment, and abound there more than in any other part of the gut.

From thence it comes into theileum; that is the longest of all the divisions of the guts, and is in length about twenty-one hands breadth; it has a great many circumvolutions, and next to thejejunum, has many lacteals to separate the chyle.

Now the aliment comes into the wide gut, and gradually becomes fæces; and first, thecæcum: This part is rather an appendix only, and hangs from the main part like a finger to a glove. The use of this gut has been much controverted by anatomists; it seems however, very probable, that this appendix is designed to keep the aliment in for further digestion, as it now begins to putrefy, and becomes fæces or excrement.

From thence it enters thecolon, which is a long, and very winding intestine; it runs up along and about the liver, touches the gall bladder, and the spleen; from thence it descends again to theos sacrum. It has but few lacteals, and is, as it were, the last drainer of the fæces: It is this intestine which is the seat of the Colic, and of most other complaints of the belly.

Next to this comes the last and straightest, therectum; this gut is closely adherent to thesacrum, and ends in the fundament; which is provided with muscles to open and shut theanus, in order to contain the fæces, and discharge it.

The mechanism of chylifaction in the human body differs from the brute creation in general, except that most contemptible of the whole, the hog; to which it bears a very near resemblance, insomuch that there is very little distinction.

Both have that advantage over the generality of terrestrial animals, that they are confined to no particular food; which favours greatly the luxury of the one, and the beastiality of the other.

The brute creation are generally distinguished intocarnivorousandgranivorous: The first is that kind which feeds upon flesh; and the latter upon grain and vegetables. Upon examination however, we find, that the stomach and guts are peculiarly adapted to their food; and that grass agrees no more with the dog, than mutton does with the horse.

But man is so happily made, that any thing which is food, is proper for him, and he may become used to it; and thus is either carnivorous or granivorous.

The whole canals, from the stomach to the anus, is in a continual vermicular motion, which is called theperistaltic motion; by this the aliments are dissolved, and disunited; and as they pass along, are drained by the lacteals, of theirnutrimentorchyle.

These lacteals are, by means of a membrane (with which they are surprisingly interwoven, and connected to the whole canal) called themesentery, lead regularly into one common cistern, lodged almost in the middle of the intestines, in that membrane, called thereceptacle of chyle; and from thence thechyleis carried by a duct up along the back-bone, called theductus thoracicus, into the leftsubclavian vein, where it gradually commences to be blood.

By this mechanism we are nourished, and the substance of our food converted into blood, and transported through the whole animal machine, for the support of every part of its wonderful composition.

As all animals which feed upon flesh, are more subject to diseases, nature has provided them with these advantages: that when any thing is obnoxious to their nature, and received into their stomach, or their being over loaded, it can discharge itself of so troublesome a burthen, by vomiting, which is effected thus: when the inner coat of the stomach, which is irritable and nervous, is stimulated by whatever is obnoxious, it will cause in the whole stomach, a contraction; and by that, force its contents to the shortest direction of evacuation, namely, by the canal of the oesophagus, through the mouth. This expulsion is peculiar to carnivorous animals only.

Purging or discharging by the fundament is common to all animals of whatever kind; and is performed by an irritation in the intestines, by which the peristaltic motion is increased; to this I must add a reversion of the secretion of the lacteal vessels, by whichthe humours are increased, the motion accelerated, and the fæces discharged, without giving any nourishment to the body, and consequently the system diminished.

Digestion is that act by which the aliment or food is prepared, so as to produce a good chyle, and consequently good blood, for the nourishment of the body. Though no animal has a more delicate stomach than man, yet it must be observed, that none has a stomach better adapted for all kinds of food.

Whence therefore in the common course of life, temperance and gentle exercise is what nature requires to maintain health. But nothing becomes more obnoxious to that blessing, than gluttony, voluptuousness, and idleness.

The bones may be considered as the timber-work of the human frame; by which this wonderful fabric is supported, and kept in its due form, that the whole may be brought into its various movements, without confusion or obstruction to each other.

The bones are the most solid parts of the human body, composed of hard and indurated fibres, striated over each other, in a manner peculiar to that substance. The bony fibres are in themselves insensible, yet as the parts are variously distributed with arteries and veins, and that the nerves must necessarily have a share in their formation, they have a peculiar sensibility, which is perceptible in some parts more than others; they cannot strictly speaking, be deemed quite insensible.

The whole bony frame is covered with a tendinous and nervous tegument, calledperiosteum; except such parts of the teeth as are designed for mastication, which are provided with a peculiar enamel, that is harder than the rest of the bony substance.

Theperiosteumis exquisitely sensible, and is the safe-guard to the substance of the bone, which is delicately tender notwithstanding it is not so sensible as the skin that covers it; insomuch that it will becomecariouson the least exposure to the air, or the attack of any foreign body of matter whatever; whence in wounds and fractures in general, great attention should be paid to the substance of the bone, being very subject to becomecariousand toexfoliation, which is of the greatest consequence; but of this I shall say more in another place.

The marrow is principally designed for the nourishment of the bones; which is evident from its being plentiest in young people, when the bones are strongest; and that when it is deficient, they become brittle, and lose their tenacity.

The marrow is contained in a cellular substance, partly in vesicles of a nervous texture, and partly bony cells. At the ends of the long bones the texture is more spungy than in the middle, where the cavity is less, but the substance is most compact. Though anatomically there is no perceptible circulation in the bones, yet, that a circulation is actually existing, is evident, from a liquor oozing out from the ends of a fractured bone in the living animal; by which a fractured bone again unites, and this is called thecallus; and whilst in its liquid state, resembles the white of an egg, which gradually ossifies, and becomes as hard as the main substance of the bone.

The number of bones differ somewhat in various subjects; ordinarily they amount to two hundred and fifty-two: In the head sixty-three; in the trunk seventy; in the arms and hands sixty; and in the legs and feet sixty.

As the bones are the support of the animal fabric, I have inserted in the next page, a catalogue of the human skeleton, which occasionally may be referred to:

A Skeleton of the Human Body.

Bonesin theHead.TheSkull.The Forehead.

Bonesin theHead.

TheSkull.

The Forehead.

The Hindhead.

The Sides of the Head.

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