Yet least; by comparing the air which is breathed into them. The forcing cavities, when it is from the lungs, with the air which enters the their turn to contract, compel the same blood lungs, it is found to have lost some of its pure into the mouths of the arteries. part, and to have brought away with it an ad- The account here given will not convey to dition of its impure part. Whether these ex- a reader, ignorant of anatomy, any thing like periments satisfy the question, as to the need an accurate notion of the form, action, or use which the blood stands in of being visited by of the parts, (nor can any short and popular continual accesses of air, is not for us to in- account do this;) but it is abundantly suffi quire into, nor material to our argument: it cient to testify contrivance; and although imis sufficient to know, that, in the constitution perfect, being true as far as it goes, may be of most animals, such a necessity exists, and relied upon for the only purpose for which we that the air, by some means or other, must be offer it, the purpose of this conclusion. introduced into a near communication with the "The wisdom of the Creator," saith Ham, blood. The lungs of animals are constructed burgher, “is in nothing seen more gloriously for this purpose. They consist of blood-ves-than in the heart." And how well doth it sels and air-vessels, lying close to each other; execute its office! An anatomist, who under. and whenever there is a branch of the trachea stood the structure of the heart, might say beor windpipe, there is a branch accompanying forehand, that it would play; but he would exit of the vein and artery, and the air-vessel is pect, I think, from the complexity of its me always in the middle between the blood-ves- chanism, and the delicacy of many of its parts, sels.* The internal surface of these vessels, that it should always be liable to derangement, upon which the application of the air to the or that it would soon work itself out. blood depends, would, if collected and expanded. be, in a man, equal to a superficies of fif. teen feet square. Now, in order to give the blood in its course the benefit of this organization (and this is the part of the subject with which we are chiefly concerned,) the follow. ing operation takes place. As soon as the blood is received by the heart from the veins But farther from the account which has of the body, and before that is sent out again been given of the mechanism of the heart, it is into its arteries, it is carried, by the force of evident that it must require the interposition the contraction of the heart, and by means of of valves; that the success indeed of its ac a separate and supplementary artery, to the tion must depend upon these; for when any lungs, and made to enter the vessels of the one of its cavities contracts, the necessary tenlungs; from which, after it has undergone the dency of the force will be to drive the enclosed action, whatever it be, of that viscus, it is blood, not only into the mouth of the artery brought back by a large vein once more to the heart, in order, when thus concocted and prepared, to be thence distributed anew into the system. This assigns to the heart a double office. The pulmonary circulation is a system within a system; and one action of the heart is the origin of both. shall this wonderful machine go, night and day, for eighty years together, at the rate of a hundred thousand strokes every twenty-four hours, having, at every stroke, a great resistance to overcome; and shall continue this action for this length of time, without disorder and without weariness. where it ought to go, but also back again into the mouth of the vein from which it flowed. In like manner, when, by the relaxation of the fibres the same cavity is dilated, the blood would not only run into it from the vein, which was the course intended, but back from the artery, through which it ought to be mov For this complicated function, four cavities ing forward. The way of preventing a reflux become necessary; and four are accordingly of the fluid, in both these cases, is to fix valves, provided: two, called ventricles, which send which, like flood-gates, may open a way to the out the blood, viz. one into the lungs, in the stream in one direction, and shut up the pasfirst instance; the other into the mass, after sage against it in another. The heart, constiit has returned from the lungs; two others tuted as it is, can no more work without valves also, called auricles, which receive the blood than a pump can. When the piston descends from the veins; viz. one, as it comes imme-in a pump, if it were not for the stoppage by diately from the body; the other, as the same the valve beneath, the motion would only blood comes a second time after its circulation thrust down the water which it had before through the lungs. So that there are two re-drawn up. A similar consequence would frusceiving cavities, and two forcing cavities. The trate the action of the heart. Valves therestructure of the heart has reference to the fore, properly disposed, i. e. properly with relungs; for without the lungs, one of each spect to the course of the blood which it is newould have been sufficient. The translation cessary to promote, are essential to the contriof the blood in the heart itself is after this vance. And valves so disposed, are according manner. The receiving cavities respectively communicate with the forcing cavities, and, by their contraction, unload the received blood * Keill's Anatomy, p. 121. ly provided. A valve is placed in the communication between each auricle and its ventricle, lest, when the ventricle contracts, part of the blood should get back again into the auricle, instead of the whole entering, as it ought to do, the mouth of the artery. A valve is also fix- the action of the part, or by the action or presed at the mouth of each of the great arteries sure of adjoining parts. Thus the bend and which take the blood from the heart; leaving the internal smooth concavity of the ribs may the passage free, so long as the blood holds its be attributed to the equal pressure of the soft proper course forward; closing it, whenever the bowels; the particular shape of some bones blood, in consequence of the relaxation of the and joints, to the traction of the annexed musventricle, would attempt to flow back. There cles, or to the position of contiguous muscles. is some variety in the construction of these But valves could not be so formed. Action valves, though all the valves of the body act and pressure are all against them. The blood, nearly upon the same principle, and are des- in its proper course, has no tendency to pro. tined to the same use. In general they con- duce such things; and in its improper or resist of a thin membrane, lying close to the side flected current, has a tendency to prevent their of the vessel, and consequently allowing an production. Whilst we see, therefore, the use open passage whilst the stream runs one way, and necessity of this machinery, we can look but thrust out from the side by the fluid get- to no other account of its origin or formation ting behind it, and opposing the passage of than the intending mind of a Creator. Nor the blood, when it would flow the other way. can we without admiration reflect, that such Where more than one membrane is employed, thin membranes, such weak and tender instru. the different membranes only compose one ments, as these valves are, should be able to valve. Their joint action fulfils the office of hold out for seventy or eighty years. a valve; for instance, over the entrance of Here also we cannot consider but with grathe right auricle of the heart into the right titude, how happy it is that our vital motions ventricle, three of these skins or membranes are involuntary. We should have enough to are fixed, of a triangular figure, the bases of do, if we had to keep our hearts beating, and our the triangles fastened to the flesh; the sides stomachs at work. Did these things depend, we and summits loose; but, though loose, connect-will not say upon our effort, but upon our bided by threads of a determinate length, with ing, our care, or our attention, they would certain small fleshy prominences adjoining. leave us leisure for nothing else. We must The effect of this construction is, that, when have been continually upon the watch, and the ventricle contracts, the blood, endeavour- continually in fear; nor would this constituing to escape in all directions, and amongst tion have allowed of sleep. other directions pressing upwards, gets between It might perhaps be expected, that an orthese membranes and the sides of the passage; gan so precious, of such central and primary and thereby forces them up into such a posi-importance as the heart is, should be defended tion, as that, together, they constitute, when by a case. The fact is, that a membraneous raised, a hollow cone (the strings, before spoken purse or bag, made of strong, tough materials, of, hindering them from proceeding or separa- is provided for it; holding the heart within its ting farther;) which cone, entirely occupying cavity; sitting loosely and easily about it; the passage, prevents the return of the blood guarding its substance, without confining its into the auricle. A shorter account of the motion; and containing likewise a spoonful or matter may be this: So long as the blood pro-two of water, just sufficient to keep the surface ceeds in its proper course, the membranes of the heart in a state of suppleness and moiswhich compose the valve are pressed close to ture. How should such a loose covering be genethe side of the vessel, and occasion no impedi-rated by the action of the heart? Does not the ment to the circulation: when the blood would enclosing of it in a sack, answering no other regurgitate, they are raised from the side of purpose but that enclosure, show the care that the vessel, and, meeting in the middle of its has been taken of its preservation? cavity, shut up the channel. Can any one One use of the circulation of the blood prodoubt of contrivance here; or is it possible to bably (amongst other uses) is, to distribute shut our eyes against the proof of it? nourishment to the different parts of the boThis valve, also, is not more curious in its dy. How minute and multiplied the ramifistructure, than it is important in its office. cations of the blood-vessels, for that purpose, Upon the play of the valve, even upon the pro-are; and how thickly spread, over at least the portional length of the strings or fibres which superficies of the body, is proved by the single check the ascent of the membranes, depends, observation, that we cannot prick the point of as it should seem, nothing less than the life it- a pin into the flesh, without drawing blood, self of the animal. We may here likewise re- i. e. without finding a blood-vessel. Nor, inpeat, what we before observed concerning some ternally is their diffusion less universal. Bloodof the ligaments of the body, that they could vessels run along the surface of membranes, not be formed by any action of the parts them- pervade the substance of muscles, penetrate selves. There are cases in which, although the bones. Even into every tooth, we trace good uses appear to arise from the shape or through a small hole in the root, an artery to configuration of a part, yet that shape or con- feed the bone, as well as a vein to bring back figuration itself may seem to be produced by the spare blood from it; both which, with the addition of an accompanying nerve, form a ly resembling milk than any other liquor with a thread only a little thicker than a horse-which it can be compared. For the straining hair. off this fluid from the digested aliment in the Wherefore, when the nourishment taken in course of its long progress through the body, at the mouth has once reached, and mixed it- myriads of capillary tubes, i. e. pipes as small self with, the blood, every part of the body is as hairs, open their orifices into the cavity of in the way of being supplied with it. And every part of the intestines. These tubes, this introduces another grand topic, namely, which are so fine and slender as not to be visithe manner in which the aliment gets into the ble unless when distended with chyle, soon blood; which is a subject distinct from the pre-unite into larger branches. The pipes, formceding, and brings us to the consideration of ed by this union, terminate in glands, from another entire system of vessels. which other pipes of a still larger diameter II. For this necessary part of the animal arising, carry the chyle from all parts, into a economy, an apparatus is provided, in a great common reservoir or receptacle. This recepmeasure capable of being, what anatomists call tacle is a bag of size enough to hold about two demonstrated, that is, shown in the dead body; table-spoons full; and from this vessel a duct and a line or course of conveyance, which or main pipe proceeds, climbing up the back we can pursue by our examinations. part of the chest, and afterwards creeping along First, the food descends by a wide passage the gullet till it reach the neck. Here it meets into the intestines, undergoing two great pre- the river: here it discharges itself into a large parations on its way: one, in the mouth by vein, which soon conveys the chyle, now flowing mastication and moisture,-(can it be doubt-along with the old blood, to the heart. This ed with what design the teeth were placed in whole route can be exhibited to the eye; nothe road to the stomach, or that there was thing is left to be supplied by imagination or choice in fixing them in this situation ?) the conjecture. Now, beside the subserviency of other, by digestion in the stomach itself. Of this structure, collectively considered, to a mathis last surprising dissolution I say nothing; nifest and necessary purpose, we may remark because it is chemistry, and I am endeavour-two or three separate particulars in it, which ing to display mechanism. The figure and show, not only the contrivance, but the perposition of the stomach (I speak all along with fection of it. We may remark, first, the length a reference to the human organ) are calculat- of the intestines, which, in the human subject, ed for detaining the food long enough for the is six times that of the body. Simply for a action of its digestive juice. It has the shape passage, these voluminous bowels, this prolixiof the pouch of a bagpipe; lies across the bo-ty of gut, seems in no wise necessary: but in dy; and the pylorus, or passage by which the order to allow time and space for the succes food leaves it, is somewhat higher in the body sive extraction of the chyle from the digested than the cardia, or orifice by which it enters; aliment, namely, that the chyle which escapes so that it is by the contraction of the muscu- the lacteals of one part of the guts may be talar coat of the stomach, that the contents, af-ken up by those of some other part, the length ter having undergone the application of the of the canal is of evident use and conducivegastric menstruum, are gradually pressed out. ness. Secondly, we must also remark their In dogs and cats, this action of the coats of the peristaltic motion; which is made up of constomach has been displayed to the eye. It is tractions, following one another like waves up. a slow and gentle undulation, propagated from on the surface of a fluid, and not unlike what one orifice of the stomach to the other. For we observe in the body of an earth-worm crawlthe same reason that I omitted, for the pre-ing along the ground; and which is effected by sent, offering any observation upon the diges- the joint action of longitudinal and of spiral, tive fluid, I shall say nothing concerning the or rather perhaps of a great number of separbile or the pancreatic juice, farther than to ob- ate semicircular fibres. This curious action serve upon the mechanism, viz. that from the pushes forward the grosser part of the aliment, glands in which these secretions are elaborat- at the same time that the more subtle parts, ed, pipes are laid into the first of the intestines, which we call chyle, are, by a series of gentle through which pipes the product of each gland compressions, squeezed into the narrow orififlows into that bowel, and is there mixed with ces of the lacteal veins. Thirdly, it was necesthe aliment, as soon almost as it passes the sary that these tubes, which we denominate stomach; adding also as a remark, how grie- lacteals, or their mouths at least, should be vously this same bile offends the stomach it- as narrow as possible, in order to deny adself, yet cherishes the vessel that lies next to it. mission into the blood to any particle which is of size enough to make a lodgement afterSecondly, We have now the aliment in the wards in the small arteries, and thereby to obintestines, converted into pulp; and though struct the circulation: and it was also necesrately consisting of ten different viands, redu- sary that this extreme tenuity should be comced to nearly a uniform substance, and to a pensated by multitude; for a large quantity of state fitted for yielding its essence, which is chyle (in ordinary constitutions, not less, it has called chyle, but which is milk, or more near-been computed, than two or three quarts in a day) is, by some means or other, to be passed ciously; nay, not only checks its farther prothrough them. Accordingly, we find the num- gress, but restores putrid substances. ber of the lacteals exceeding all powers of com- 4. It is not a fermentative process: for the putation; and their pipes so fine and slender, solution begins at the surface, and proceeds as not to be visible, unless filled, to the naked towards the centre, contrary to the order in eye; and their orifices, which open into the which fermentation acts and spreads. intestines, so small as not to be discernable 5. It is not the digestion of heat: for the even by the best microscope. Fourthly, the cold maw of a cod or sturgeon will dissolve the main pipe which carries the chyle from the re- shells of crabs or lobsters, harder than the servoir to the blood, viz. the thoracic duct, be-sides of the stomach which contains them. ing fixed in an almost upright position, and In a word, animal digestion carries about it wanting that advantage of propulsion which the marks of being a power and a process comthe arteries possess, is furnished with a suc-pletely sui generis; distinct from every other, cession of valves to check the ascending fluid, at least from every chemical process with which when once it has passed them, from falling we are acquainted. And the most wonderful back. These valves look upward, so as to leave thing about it is its appropriation; its subthe ascent free, but to prevent the return of serviency to the particular economy of each the chyle, if, for want of sufficient force to push animal. The gastric juice of an owl, falcon, it on, its weight should at any time cause it or kite, will not touch grain; no, not even to to descend. Fifthly, the chyle enters the blood finish the macerated and half-digested pulse in an odd place, but perhaps the most commo-which is left in the crops of the sparrows that dious place possible, viz. at a large vein in the the bird devours. In poultry, the trituration neck, so situated with respect to the circula- of the gizzard, and the gastric juice, conspire tion, as speedily to bring the mixture to the in the work of digestion. The gastric juice heart. And this seems to be a circumstance will not dissolve the grain whilst it is whole. of great moment; for had the chyle entered Entire grains of barley, enclosed in tubes or the blood at an artery, or at a distant vein, spherules, are not affected by it. But if the the fluid, composed of the old and the new same grain be by any means broken or ground, materials, must have performed a considerable the gastric juice immediately lays hold of it. part of the circulation, before it received that Here then is wanted, and here we find, a churning in the lungs, which is, probably, ne- combination of mechanism and chemistry. cessary for the intimate and perfect union of For the preparatory grinding, the gizzard lends the old blood with the recent chyle. Who its mill. And as all mill-work should be strong, could have dreamt of a communication be- its structure is so, beyond that of any other tween the cavity of the intestines and the left muscle belonging to the animal. The intergreat vein of the neck? Who could have sus-nal coat also, or lining of the gizzard, is, for pected that this communication should be the the same purpose, hard and cartilaginous. But, medium through which all nourishment is de-forasmuch as this is not the sort of animal subrived to the body; or this the place, where, by a side inlet, the important junction is formed between the blood and the material which feeds it ? We postpone the consideration of digestion, lest it should interrupt us in tracing the course of the food to the blood; but in treating of the alimentary system, so principal a part of the process cannot be omitted. stance suited for the reception of glands or for secretion, the gastric juice, in this family, is not supplied, as in membranous stomachs, by the stomach itself, but by the gullet, in which the feeding glands are placed, and from which it trickles down into the stomach. In sheep, the gastric fluid has no effect in digesting plants, unless they have been previously masticated. It only produces a slight maceOf the gastric juice, the immediate agent ration; nearly such as common water would by which that change which food undergoes produce, in a degree of heat somewhat exceedin our stomachs is effected, we shall take our ing the medium temperature of the atmo account from the numerous, careful, and va-sphere. But provided that the plant has been ried experiments of the Abbé Spallanzani. reduced to pieces by chewing, the gastric juice 1. It is not a simple diluent, but a real solvent. A quarter of an ounce of beef had scarcely touched the stomach of a crow, when the solution began. then proceeds with it, first, by softening its substance; next, by destroying its natural consistency; and lastly, by dissolving it so completely, as not even to spare the toughest and most stringy parts, such as the nerves of the leaves. 2. It has not the nature of saliva; it has not the nature of bile; but is distinct from both. By experiments out of the body it ap- So far our accurate and indefatigable Abbé. pears, that neither of these secretions acts up--Dr. Stevens, of Edinburgh, in 1777, found, on alimentary substances, in the same manner by experiments tried with perforated balls, that as the gastric juice acts. the gastric juice of the sheep and the ox speedi 3. Digestion is not putrefaction: for the di-ly dissolved vegetables, but made no impres gesting fluid resists putrefaction most pertina- sion upon beef, mutton, and other animal bo dies. Dr. Hunter discovered a property of thus described: The gall-duct enters the duo this fluid, of a most curious kind; viz. that in denum obliquely; after it has pierced the first the stomachs of animals which feed upon flesh, coat, it runs near two fingers' breadth between irresistibly as this fluid acts upon animal sub- the coats, before it open into the cavity of the stances, it is only upon the dead substance that intestine. The same contrivance is used in it operates at all. The living fibre suffers no another part, where there is exactly the same injury from lying in conta with it. Worms occasion for it, viz. in the insertion of the ureand insects are found alive in the stomachs of ters in the bladder. These enter the bladder such animals. The coats of the human sto- near its neck, running obliquely for the space mach, in a healthy state, are insensible to its of an inch between its coatst. It is, in both presence; yet in cases of sudden death (where- cases, sufficiently evident, that this structure in the gastric juice, not having been weakened has a necessary mechanical tendency to resist by disease, retains its activity,) it has been regurgitation; for whatever force acts in such known to eat a hole through the bowel which a direction as to urge the fluid back into the contains it. How nice is this discrimination orifices of the tubes, must, at the same time, of action, yet how necessary! stretch the coats of the vessels, and thereby compress that part of the tube which is includ ed between them. But to return to our hydraulics. III. The gall-bladder is a very remarkable contrivance. It is the reservoir of a canal. It] IV. Amongst the vessels of the human body, does not form the channel itself, i. e. the di- tne pipe which conveys the saliva from the place rect communication between the liver and the where it is made, to the place where it is want. intestine, which is by another passage, viz. the ed, deserves to be reckoned amongst the most ductus hepaticus, continued under the name intelligible pieces of mechanism with which we of the ductus communis; but it lies adjacent are acquainted. The saliva, we all know, is to this channel, joining it by a duct of its own, used in the mouth; but much of it is produced the duct cysticus; by which structure it is on the outside of the cheek, by the parotid enabled, as occasion may require, to add its gland, which lies between the ear and the angle contents to, and increase the flow of bile into of the lower jaw. In order to carry the sethe duodenum. And the position of the gall-creted juice to its destination, there is laid from bladder is such as to apply this structure to the the gland on the outside, a pipe, about the best advantage. In its natural situation, it thickness of a wheat straw, and about three touches the exterior surface of the stomach, fingers' breadth in length; which, after ridand consequently is compressed by the disten- ing over the masseter muscle, bores for itself sion of that vessel: the effect of which com- a hole through the very middle of the cheek, pression is to force out from the bag, and send enters by that hole, which is a complete perinto the duodenum, an extraordinary quantity foration of the buccinator muscle, into the of bile, to meet the extraordinary demand mouth; and there discharges its fluid very cowhich the repletion of the stomach by food is piously. about to occasion+. Cheselden describes the V. Another exquisite structure, differing ingall-bladder as seated against the duodenum, deed from the four preceding instances, in that and thereby liable to have its fluid pressed out, it does not relate to the conveyance of fluids, by the passage of the aliment through that but still belonging, like these, to the class of cavity; which likewise will have the effect of pipes or conduits of the body, is seen in the causing it to be received into the intestine, at a right time, and in a due proportion. There may be other purposes answered by this contrivance; and it is probable that there are. The contents of the gall-bladder are not exactly of the same kind as what passes from the liver through the direct passage §. It is possible that the gall may be changed, and for some purposes meliorated, by keeping. larynx. We all know that there go down the throat two pipes, one leading to the stomach, the other to the lungs; the one being the pas sage for the food, the other for the breath and voice: we know also that both these passages open into the bottom of the mouth; the gullet, necessarily, for the conveyance of food; and the wind-pipe, for speech and the modulation of sound, not much less so: therefore the difThe entrance of the gall-duct into the duo-ficulty was, the passages being so contiguous, denum furnishes another observation. When- to prevent the food, especially the liquids, ever either smaller tubes are inserted into lar-which we swallow into the stomach, from enger tubes, or tubes into vessels and cavities, tering the wind-pipe, i. e. the road to the lungs; such receiving tubes, vessels, or cavities, being the consequence of which error, when it does subject to muscular constriction, we always happen, is perceived by the convulsive throes find a contrivance to prevent regurgitation. In that are instantly produced. This business, some cases, valves are used; in other cases, which is very nice, is managed in this manner. amongst which is that now before us, a dif- The gullet (the passage for food) opens into ferent expedient is resorted to, which may be the mouth like the cone or upper part of a funnel, the capacity of which forms indeed the Keill's Anat. p. 64. Phil Trans, vol lxii. p. 447. 1 Anat. p. 164. * Keill's Anat. p. 62 + Ches. Anat. p. 260. |