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mouth, in some of their other properties. It has , ed by the rest : each at least at liberty in a degree been said, and that by an eminent physiologist, that sufficient for the end to be attained. 'If we cannot whenever nature aitempts to work two or more eat and sing at the same moment, we can eat ons purposes by one instrument, she does both or all moment, and sing the next: the respiration proimperfectly. Is this true of the tongue, regarded ceeding freely all the while. as an instrument of speech, and of taste; or re There is one case however of this double office; garded as an instrument of speech, of taste, and and that of the earliest necessity, which the mouth of deglutition? So much otherwise, that many alone could not perform; and that is, carrying on persons, that is to say, nine hundred and ninety- together the two actions of sucking and breathing. nine persons out of a thousand, by the instru- Another rout therefore is opened for the air, namely mentality of this one organ, talk, and taste, and through the nose, which lets the breath pass backswallow, very well. In fact, the constant warmth ward and forward, whilst the lips, in the act of and moisture of the tongue, the thinness of the sucking, are necessarily shut close upon the body skin, the papillæ upon its surface, qualify this or- from which the nutriment is drawn. This is a gan for its office of tasting, as much as its inex. circumstance which always appeared to me worthy tricable multiplicity of fibres do for the rapid of notice. The nose would have been necessary, movements which are necessary to speech. Ani- although it had not been the organ of smelling. mals which feed upon grass, have their tongues The making it the seat of a sense was superadding covered with a perforated skin, so as to admit the a new use to a part already wanted; was taking dissolved food to the papillæ underneath, which, a wise advantage of an antecedent and a constituin the mean time, remain defended from the rough tional necessity. action of the unbruised spiculæ.
There are brought together within the cavity of the mouth more distinct uses, and parts executing But to return to that which is the proper subject more distinct offices, than I think can be found of the present section,--the celerity and precision lying so near to one another, or within the same of muscular motion. These qualities may be parcompass, in any other portion of the body: viz. ticularly observed in the execution of many species teeth of different shape, first for cutting, secondly of instrumental music, in which the changes profor grinding; muscles, most artificially disposed duced by the hand of the musician are exceedingly for carrying on the compound motion of the lower rapid ; are exactly measured, even when most mijaw, half lateral and half vertical, by which the nute; and display, on the part of the muscles, an mill is worked: fountains of saliva, springing up obedience of action, alike wonderful for its quickin different parts of the cavity for the moistening ness and its correctness. of the food, whilst the mastication is going on : Or let a person only observe his own hand glands, to feed the fountains; a muscular constric- whilst he is writing ; the number of muscles, tion of a very peculiar kind in the back part of the which are brought to bear upon the pen; how the cavity, for the guiding of the prepared aliment joint and adjusted operation of several tendons is into its passage towards the stomach, and in many concerned in every stroke, yet that five hundred cases for carrying it along that passage; for, al- such strokes are drawn in a minute. Not a letter though we may imagine this to be done simply by can be turned without more than one, or two, or the weight of the food itself, it in truth is not so, three tendinous contractions, definite, both as to even in the upright posture of the human neck; the choice of the tendon, and as to the space and most evidently is not the case with quadru- through which the contraction moves; yet how peds, with a horse for instance, in which, when currently does the work proceed! and when we pastúring, the food is thrust upward by muscular look at it, how faithful have the muscles been to strength, instead of descending of its own accord. their duty, how true to the order which endeavour
In the mean time, and within the same cavity, or habit hath inculcated! For let it be rememis going on another business, altogether different bered, that, whilst a man's handwriting is the from what is here described, -that of respiration same, an exactitude of order is preserved, whether and speech. In addition therefore to all that has he write well, or ill. These two instances, of mubeen mentioned, we have a passage opened, from sic and writing, show not only the quickness and this cavity to the lungs for the admission of air, precision of muscular action, but the docility. exclusively of every other substance; we have II. Regarding the particular configuration of muscles, some in the larynx, and without number muscles, sphincter or circular muscles appear to in the tongue, for the purpose of modulating that me admirable pieces of mechanism. li is the air in its passage, with a variety, a compass, and muscular power most happily applied; the same precision, of which no other musical instrument quality of the muscular substance, but under a is capable. And, lastly, which in my opinion new modification. The circular disposition of the crowns the whole as a piece of machinery, we have fibres is strictly mechanical; but, though the most a specific contrivance for dividing the pneumatic mechanical, is not the only thing in sphincters part from the mechanical, and for preventing one which deserves our notice. The regulated degree set of actions interfering with the other. Where of contractile force with which they are endowed, various functions are united, the difficulty is to sufficient for retention, yet vincible when requiguard against the inconveniences of a too great site, together with their ordinary state of actual complexity. In no apparatus put together by art, contraction, by means of which their dependence and for the purposes of art, do I know such multi- upon the will is not constant, but occasional, gives farious uses so aptly combined, as in the natural them a constitution, of which the conveniency is organization of the human mouth; or where the inestimable. This their semi-voluntary character, structure compared with the uses, is so sim.ple. is exactly such as suits with the wants and funcThe mouth, with all these intentions to serve, is a tions of the animal. single cavity; is one machine; with its parts nei III. We may also, upon the subject of muscles, ther crowded nor confused, and each unembarrass- observe, that many of our most important actions
are achieved by the combined help of different were proportionably more confined and slow. It muscles. Frequently, a diagonal motion is pro- is the same with a mechanic in the use of his tools. duced, by the contraction of tendons pulling in It is the same also with other animals in the use the direction of the sides of the parallelogram. of their limbs. In general, the vivacity of their This is the case, as hath been already noticed, motions would be ill exchanged for greater force with some of the oblique nutations of the head under a clumsier structure. Sometimes the number of co-operating muscles is We have offered our observations upon the very great. Dr. Nieuentyt, in the Leipsic Trans- structure of muscles in general; we have also noactions, reckons up a hundred muscles that are ticed certain species of muscles; but there are employed every time we breathe ; yet we take in, also single muscles which bear marks of meor let out, our breath, without reflecting what a chanical contrivance, appropriate
as well as parwork is thereby performed; what an apparatus is ticular. Out of many instances of this kind, we laid in, of instruments for the service, and how select the following. many such contribute their assistance to the effect ! I. Of muscular actions, even of those which are Breathing with ease, is a blessing of every moment; well understood, some of the most curious are inyet, of all others, it is that which we possess with capable of popular explanation; at least, without the least consciousness. A man in an asthma is the aid of plates and figures. This is in a great the only man who knows how to estimate it. measure the case, with a very familiar, but at the
IV. Mr. Home has observed,* that the most same time, a very complicated motion,--that of important and the most delicate actions are per- the lower jaw; and with the muscular structure formed in the body by the smallest muscles: and by which it is produced. One of the muscles he mentions, as his examples, the muscles which concerned may, however, be described in such a have been discovered in the iris of the eye, and the manner, as to be, I think, sufficiently compredrum of the ear. The tenuity of these muscles is hended for our present purpose. The problem is astonishing. They are microscopic hairs; must to pull the lower jaw down. The obvious method be magnified to be visible; yet are they real, effect should seem to be, to place a straight muscle, viz. ive muscles: and not only such, but the grandest to fix a string from the chin to the breast, the conand most precious of our faculties, sight and hear-traction of which would open the mouth and proing, depend upon their health and action. duce the motion required at once. But it is
V. The muscles act in the limbs with what is evident that the form and liberty of the neck called a mechanical disadvantage. The muscle forbid a muscle being laid in such a position; and at the shoulder, by which the arm is raised, is that, consistently with the preservation of this fixed nearly in the same manner as the load is form, the motion, which we want, must be effecfixed upon a steelyard, within a few decimals, we tuated by some muscular mechanism disposed will say, of an inch, from the centre upon which farther back in the jaw. The mechanism adoptthe steelyard turns. In this situation, we find ed is as follows. A certain muscle called the diathat a very heavy draught is no more than suffi- gastric, rises on the side of the face, considerably cient to countervail the force of a small lead plum- above the insertion of the lower jaw, and comes met, placed upon the long arm of the steelyard, at down, being converted in its progress into a round the distance of perhaps fifteen or twenty inches tendon. Now it is manifest that the tendon, from the centre, and on the other side of it. And whilst it pursues a direction descending towards this is the disadvantage which is meant. And an the jaw, must, by its contraction, pull the jaw up, absolute disadvantage, no doubt, it would be, if instead of down. What then was to be done the object were, to spare the force of muscular This, we find, is done: the descending tendon, contraction. But observe how conducive is this when it is got low enough, is passed through a constitution to animal conveniency. Mechanism loop, or ring, or pulley, in the os hyoides, and then has always in view one or other of these two pur- made to ascend; and having thus changed its line poses; either to move a great weight slowly, and of direction, is inserted into the inner part of the through a small space, or to move a light weight chin : by which device, viz. the turn at the loop, rapidly, through a considerable sweep. For the the action of the muscle (which in all muscles is former of these purposes, a different species of contraction) that before would have pulled the lever, and a different collocation of the muscles, jaw up, now as necessarily draws it down. "The might be better than the present; but for the second, mouth," says Heister, " is opened by means of this the present structure is the true one. Now so it hap- trochlea in a most wonderful and elegant manpens, that the second, and not the first, is that ner.” which the occasions of animal life principally call II. What contrivance can be more mechanical for. In what concerns the human body, it is of than the following, viz. a slit in one tendon to let much more consequence to any man to be able to another tendon pass through it? This structure carry his hand to his head with due expedition, is found in the tendons which move the toes and than it would be to have the power of raising from fingers. The long tendon, as it is called, in the the ground a heavier load (of two or three more foot, which bends the first joint of the toe, passes hundred weight, we will suppose,) than he can through the short tendon which bends the second lift at present. This last is a faculty, which, on joint; which course allows to the sinew more some extraordinary occasions, he may desire to liberty, and a more commodious action than it possess; but the other is what he wants and uses would otherwise have been capable of exerting.* every hour or minute. In like manner, a husband. There is nothing, I believe, in a silk or cotton man or a gardener will do more execution, by mill, in the belts, or straps, or ropes, by which mobeing able to carry his scythe, his rake, or his fail, tion is communicated from one part of the machine with a sufficient despatch through a sufficient to another, that is more artificial, or more evidentspace, than if, with greater strength, his motions ly so, than this perforation.
* Phil. Trans. part. i. 1800.p. 8.
• Ches. Anat. p. 119,
III. The next circumstance which I shall men- reason for any distinction of the sort. Mechantion, under this head of muscular arrangement, is ism may be displayed in the one kind of substance, so decisive a mark of intention, that it always ap as well as in the other. peared to me to supersede, in some measure, the Although the few instances we have selected, necessity of seeking for any other observation even as they stand in our description, are nothing upon the subject; and that circumstance is, the short perhaps of logical proofs of design, yet it tendons, which pass from the leg to the foot, being must not be forgotten, that, in every part of anabound down by a ligament at the ankle. The tomy, description is a poor substitute for inspec, foot is placed at a considerable angle with the leg. tion. It is well said by an able anatomist,* and It is manifest, therefore, that flexible strings, pass- said in reference to the very part of the subject ing along the interior of the angle, if left to them- which we have been treating of:-“Imperfecta selves, would, when stretched, start from it. The hec musculorum descriptio, non minùs arida est obvious preventive is to tie them down. And this legentibus, quàm inspectantibus fuerit jucunda is done in fact. Across the instep, or rather just eorundem præparatio. Elegantissima enim meabove it, the anatomist finds a strong ligament, chanicês artificia, creberrimè in illis obvia, verbis under which the tendons pass to the foot. Thé nonnisi obscurè exprimuntur: carnium autem effect of the ligament as a bandage can be made ductu, tendinum colore, insertionum proportione, evident to the senses; for if it be cut, the tendons et trochlearium distributione, oculis exposita, omstart up. The simplicity, yet the clearness of this nem superant admirationem.” contrivance, its exact resemblance to established resources of art, place it amongst the most indubitable manifestations of design with which we are acquainted.
CHAPTER X. There is also a farther use to be made of the present example, and that is, as it precisely con. Of the Vessels of Animal Bodies. tradicts the opinion, that the parts of animals may have been all formed by what is called appetency, The circulation of the blood, through the bodies i. e. endeavour, perpetuated, and imperceptibly of men and quadrupeds, and the apparatus by working its effect, through an incalculable series which it is carried on, compose a system, and tesof generations. We have here no endeavour, but tify a contrivance, perhaps the best understood of the reverse of it; a constant renitency and reluct- any part of the animal frame. The lymphatic
The endeavour is all the other way. The system, or the nervous system, may be more subpressure of the ligament constrains the tendons; tile and intricate: nay, it is possible, that in their the tendons re-act upon the pressure of the liga- structure they may be even more artificial than ment. It is impossible that the ligament should the sanguiferous, but we do not know so much ever have been generated by the exercise of the about them. tendon, or in the course of that exercise, forasmuch The utility of the circulation of the blood I asas the force of the tendon perpendicularly resists sume as an acknowledged point. One grand purthe fibre which confines it, and is constantly en- pose is plainly answered by it; the distributing to deavouring, not to form, but to rupture and dis- every part, every extremity, every nook and corplace the threads of which the ligament is com ner of the body, the nourishment which is receivposed.
ed into it by one aperture. What enters at the mouth, finds its way to the fingers' ends. A more
difficult mechanical problem could hardly I think Keill has reckoned up, in the human body, be proposed, than to discover a method of confour hundred and forty-six muscles, dissectible and stantly repairing the waste, and of supplying an describable: and hath assigned a use to every one accession of substance to every part of a compliof the number. This cannot be all imagination. cated machine, at the same time.
Bishop Wilkins hath observed from Galen, that This system presents itself under two views: there are, at least, ten several qualifications to be first, the disposition of the blood vessels, i.e. the attended to in each particular muscle; viz. its laying of the pipes; and, secondly, the construcjust magnitude ; its fulcrum ; its point of action, tion of the engine at the centre, viz. the heart, supposing the figure to be fixed; its collocation, for driving the blood through them. with respect to its two ends, the upper and the I. The disposition of the blood-vessels, as far as lower; the place; the position of the whole mus- regards the supply of the body, is like that of the cle; the introduction into it of nerves, arteries, water-pipes in a city, viz. large and main trunks veins. How are things, including so many ad- branching off by smaller pipes (and these again justments, to be made; or, when made, how are by still narrower tubes) in every direction, and they to be put together without intelligence ? towards every part in which the fluid, which they
I have sometimes wondered why we are not convey, can be wanted. So far the water-pipes struck with mechanism in animal bodies, as readi- which serve a town may represent the vessels ly and as strongly as we are struck with it, at which carry the blood from the heart. But there first sight, in a watch or a mill. One reason of is another thing necessary to the blood, which is the difference may be, that animal bodies are, in not wanted for the water; and that is, the carrya great measure, made up of soft, flabby substances, ing of it back again to its source. For this office, such as muscles and membranes; whereas we a reversed system of vessels is prepared, which, have been accustomed to trace mechanism in sharp uniting at their extremities with the extremities lines, in the configuration of hard materials, in of the first system, collects the divided and subdithe moulding, chiselling, and filing into shapes, vided streamlets, first by capillary ramifications of such articles as metals or wood. There is into larger branches, secondly, by these branches something therefore of habit in the case; but it is sufficiently evident, that there can be no proper Steno, in Blas. Anat. Animal. p. 2. c. 4.
into trunks; and thus returns the blood (almost | For our purpose it is unnecessary to ascertain the exactly inverting the order in which it went out) principle upon which the heart acts. Whether it to the fountain whence its motion proceeded. All be irritation excited by the contact of the blood, which is evident mechanism.
by the influx of the nervous fluid, or whatever The body, therefore, contains two systems of else be the cause of its motion, it is something blood vessels, arteries, and veins. Between the which is capable of producing, in a living muscuconstitution of the systems there are also two dif fibre, reciprocal contraction and relaxation. ferences, suited to the functions which the sys- This is the power we have to work with: and the tems have to execute. The blood, in going out, inquiry is, how this power is applied in the inpassing always from wider into narrower tubes; stance before us? There is provided, in the cenand, in coming back, from narrower into wider; tral part of the body, a hollow muscle, invested it is evident, that the impulse and pressure upon with spiral fibres, running in both directions, the the sides of the blood vessel, will be much greater layers intersecting one another; in some animals, in one case than the other. Accordingly the ar- however, appearing to be semi-circular rather than teries which carry out the blood, are formed of spiral. By the contraction of these fibres, the much tougher and stronger coats, than the veins sides of the muscular cavities are necessarily which bring it back. That is one difference: the squeezed together, so as to force out from them other is still more artificial, or, if I may so speak, any fuid which they may at that time contain: indicates, still more clearly, the care and anxiety by the relaxation of the same fibres, the cavities of the artificer. Forasmuch as in the arteries, by are in their turn dilated, and, of course, prepared reason of the greater force with which the blood to admit every fluid which may be poured into is urged along them, a wound or rupture would them. Into these cavities are inserted the great be more dangerous than in the veins, ihese vessels trunks, both of the arteries which carry out the are defended from injury, not only by their ter- blood, and of the veins which bring it back. This ture, but by their situation; and by every advan- is a general account of the apparatus; and the tage of situation which can be given to them. simplest idea of its action is, that, by each conThey are buried in sinuses, or they creep along traction, a portion of blood is forced by a syringe grooves, made for them in the bones: for instance, into the arteries; and, at each dilatation, an equal the under edge of the ribs is sloped and furrowed portion is received from the veins. This produces, solely for the passage of these vessels. Sometimes at each pulse, a motion, and change in the mass they proceed in channels, protected by stout para- of blood, to the amount of what the cavity conpets on each side; which last description is re- tains, which, in a full-grown human heart, I unmarkable in the bones of the fingers, these being derstand is about an ounce, or two table-spoons hollowed out on the under-side, like a scoop, and full. How quickly these changes succeed one with such a concavity, that the finger may be cut another, and by this succession how sufficient across to the bone, without hurting the artery they are to support a stream or circulation throughwhich runs along it. At other times, the arteries out the system, may be understood by the followpass in canals wrought in the substance, and in ing computation, abridged from Keill's Anatomy, the very middle of the substance, of the bone: p. 117. ed. 3; “Each ventricle will at least conthis takes place in the lower jaw; and is found tain one ounce of blood. The heart contracts four where there would otherwise be danger of com- thousand times in one hour; from which it folpression by sudden curvature. All this care is lows, that there pass through the heart, every wonderful, yet not more than what the import- hour, four thousand ounces, or three hundred and ance of the case required. To those who venture fifty pounds of blood. Now the whole mass of their lives in a ship, it has been often said, that blood is said to be about twenty-five pounds; so there is only an inch-board between them and that a quantity of blood, equal to the whole mass death; but in the body itself
, especially in the ar- of blood, passes through the heart fourteen times terial system, there is, in many parts, only a in one hour; which is about once every four mimembrane, a skin, a thread. For which reason, nutes.” Consider what an affair this is, when we this system lies deep under the integuments; come to very large animals
. The aorta of a whale whereas the veins, in which the mischief that en is larger in the bore than the main pipe of the sues from injuring the coats is much less, lie in water-works at London Bridge ; and the water general above the arteries ; come nearer to the roaring in its passage through that pipe is inferior, surface; are more exposed.
in impetus and velocity, to the blood gushing from It may be farther observed concerning the two the whale's heart. Hear Dr. Hunter's account systems taken together, that though the arterial, of the dissection of a whale:-“The aorta meawith its trunk and branches and small twigs, may sured a foot diameter. Ten or fifteen gallons of be imagined to issue or proceed, in other words, blood are thrown out of the heart at a stroke, with to grow from, the heart; like a plant from its an immense velocity, through a tube of a foot root, or the fibres of a leaf from its foot-stalk, diameter. The whole idea fills the mind with (which, however, were it so, would be only to re- wonder."* solve one mechanism into another,) yet the venal, The account which we have here stated, of the the returning system, can never be formed in this injection of blood into the arteries by the conmanner. The arteries might go on shooting out traction, and of the corresponding reception of it from their extremities, i. e. lengthening and sub- from the veins by the dilatation, of the cavities of dividing indefinitely; but an inverted system, con- the heart, and, of the circulation being thereby tinually uniting its streams, instead of dividing, maintained through the blood-vessels of the body, and thus carrying back what the other system is true, but imperfect. The heart performs this carried out, could not be referred to the same pro- office, but it is in conjunction with another of cess. II. The next thing to be considered is the en
• Dr Hunter's Account of the Dissection of a Whale. gine which works this machinery, viz. the heart. 1 - Phil. Trans.
equal curiosity and importance. It was necessary, the heart itself is after this manner. The receivthat the blood should be successively brought into ing cavities respectively communicate with the contact, or contiguity, or proximity, with the air. forcing cavities, and, by their contraction, unload I do not know that the chemical reason, upon the received blood into them. The forcing caviwhich this necessity is founded, has been yet suf- ties, when it is their turn to contract, compel the ficiently explored. It seems to be made appear, same blood into the mouths of the arteries. that the atmosphere which we breathe is a mix The account here given will not convey to a ture of two kinds of air; one pure and vital, the reader, ignorant of anatomy, any thing like an other, for the purposes of life, effete, foul, and accurate notion of the form, action, or use, of the noxious: that when we have drawn in our breath, parts, (nor can any short and popular account do the blood in the lungs imbibes from the air, thus this;) but it is abundantly sufficient to testify conbrought into contiguity with it, a portion of its trivance; and although imperfect, being true as pure ingredient, and, at the same time, gives out far as it goes, may be relied upon for the only purthe effete or corrupt air which it contained, and pose for which we offer it, the purpose of this conwhich is carried away, along with the halitus, clusion. every time we expire. At least; by comparing “The wisdom of the Creator," saith Hamburghthe air which is breathed from the lungs, with er,“ is in nothing seen more gloriously than in the the air which enters the lungs, it is found to have heart.” And how well doth it execute its office! lost some of its pure part, and to have brought An anatomist, who understood the structure of away with it an addition of its impure part. the heart, might say beforehand that it would Whether these experiments satisfy the question, play; but he would expect, I think, from the comas to the need which the blood stands in of being plexity of its mechanism, and the delicacy of many visited by continual accesses of air, is not for us of its parts, that it should always be liable to de to inquire into, nor material to our argument: it rangement, or that it would soon work itself out. is sufficient to know, that, in the constitution of Yet shall this wonderful machine go, night' and most animals, such a necessity exists, and that day, for eighty years together, at the rate of a the air, by some means, or other, must be intro- hundred thousand strokes every twenty-four hours, duced into a near communication with the blood. having, at every stroke, a great resistance to overThe lungs of animals are constructed for this pur-come; and shall continue this action for this length pose. They consist of blood vessels, and air-ves- of time, without disorder and without weariness! sels, lying close to each other; and wherever there But farther: From the account which has been is a branch of the trachea or windpipe, there is a given of the mechanism of the heart, it is evident branch accompanying it, of the vein and artery, that it must require the interposition of valves ; and the air-vessel is always in the middle between that the success indeed of its action must depend the blood vessels * The internal surface of these upon these; for when any one of its cavities convessels, upon which the application of the air to tracts, the necessary tendency of the force will be the blood depends, would, if collected and expand- to drive the enclosed blood, not only into the mouth ed, be, in a man, equal to superficies of fifteen of the artery where it ought to go, but also back feet square. Now, in order to give the blood in again into the mouth of the vein from which it its course the benefit of this organization, (and flowed. In like manner, when by the relaxation this is the part of the subject with which we are of the fibres the same cavity is dilated, the blood chiefly concerned,) the following operation takes would not only run into it from the vein, which place. As soon as the blood is received by the was the course intended, but back from the arteheart from the veins of the body, and before that ry, through which it ought to be moving forward. is sent out again into its arteries, it is carried, by The way of preventing a reflux of the fluid, in the force of the contraction of the heart, and by both these cases, is to fix valves, which, like floodmeans of a separate and supplementary artery, to gates, may open a way to the stream in one directhe lungs; and made to enter the vessels of the tion, and shut up the passage against it in another. lungs; from which, after it has undergone the ac- The heart, constituted as it is, can no more work tion, whatever it be, of that viscus, it is brought without valves, than a pump can. When the pis back by a large vein once more to the heart, in ton descends in a pump, it it were not for the order, when thus concocted and prepared, to be stoppage by the valve beneath, the motion would thence distributed anew into the system.' This only thrust down the water which it had before assigns to the heart a double office. The pulmo- drawn up. A similar consequence would frusnary circulation is a system within a system; trate the action of the heart.* Valves, therefore, and one action of the heart is the origin of both.' properly disposed, i. e. properly with respect to the
For this complicated function, four cavities be- course of the blood which it is necessary to pro come necessary; and four are accordingly pro- mote, are essential to the contrivance. And ralres vided : two, called ventricles, which send out the so disposed, are accordingly, provided. A valve blood, diz.one into the lungs, in the first instance; is placed in the communication between each authe other into the mass, after it has returned from ricle and its ventricle, lest, when the ventricle con the lungs : two others also, called auricles, which tracts, part of the blood should get back again in receive the blood from the veins; viz. one, as it to the auricle, instead of the whole entering, as it comes immediately from the body; the other as ought to do, the mouth of the artery. A valve is the same blood comes a second time after its circu- also fixed at the mouth of each of the great arte. lation through the lungs. So that there are two ries which take the blood from the heart; leaving receiving cavities, and two forcing cavities. The the passage free, so long as the blood holds its prostructure of the heart has reference to the lungs; per course forward; closing it, whenever the blood, for without the lungs, one of each would have in consequence of the relaxation of the ventricle, been sufficient. The translation of the blood in would attempt to flow back. There is some varie
ty in the construction of these valves, though all • Keill's Anatomy, p. 121.
the valves of the body act nearly upon the same