is attached to die vertebra' lumborum, where it may be said tobe divided into two portions or crura. The diaphragm not onlyserves to divide the thorax from the abdomen, but by it- muscu-lar structure is rendered .one of the chief agents in respiration.When its fibres contract, its convex side, which is turned towardsthe. thorax, becomes gradually flat, and by increasing the cavityor t]ie breast, affords room for a complete dilatation of thelungs, by means of the air which is then drawn into tnem by theact of respiration. The fibres of the diaphragm then relax ; andas it resumes its former state, the cavity of the thorax becomesg-adtislly diminished, and the air is driven out again from thelungs by,n motion contrary to the former one, called exspiratinn.The diaphragm likewise serves several other purposes in (he ani-mal ".’economy. It is in some measure, by means of it, that wevoid the fxces at the anus, and empty the urinary bladder. Thenets of coughing, sneezing, speaking, laughing, gaping, and sigh-ing, could not take place without its assistance-; anti the gentlepressure, which all the .abdominal viscera receive from its con-stant and regular motion, cannot fail to assist in the perform-ance of the several functions which were' ascribed to those viscera.
The Pleura is a line membrane which lines tin: whole cavityof the thorax, and envelopes the whole o! its contents, in thesame way as the peritoneum incloses the contents of the abdomen.An inllumination of this membrane constitutes the disease calledpleurisy.
The Mediastinum is formed by the union of the two lateralportions of the pleura, and divides Hie thorax lengthways intotwo cavities, one for each lobe of the lungs. At tiie lower partof the thorax, these lamina' afford a lodgment to the heart; andat the upper part of the c’avitv, they receive between them thethymus. The mediastinum does not, however, divide the thoraxexactly in the middle, but towards the leftside; and is so dispo-sed, that the two cavities, into which it divides the thorax, donot end toward this membrane in an angle, but a segment of acircle. Its use is to prevent one lobe of the lungs from pressingon the other, as in lying on one side the uppermost might do. Hutit is of still more importance, in preventing the disorders of onelobe of the lungs from affecting the other; for if the functions ofboth lobes were suspended by any disease at the same time, deathmust ensue. If, for instance, the point of a sword were to pene-trate between the ribs into the cavity of the thorax, the lungs onthat, side would cease to perform their office; because the air be-ing admitted t .rough the wound, would prevent the dilatation ofthat lobe; but the other lobe, which is separated from it hv themediastinum, would remain unhurt, and continue to perform itsfunctions as usual.
The Trachea , or Wixdtipe, is a cartilaginous and membra-nous canal, through which the air passes into the lungs. Its upperpart, which is called the larynx, is composed of five cartilages.The uppermost of these cartilages is placed over the glottis ormouth of the larynx, and is called epiglottis, which performs theoffice i f closing the passage to the lungs in the act ot swallowing.At the sides of the glo.tis are placed the two arvtenoide carti-lages, which are of a very complex figure. The anterior andlarger part of the larynx is made up of two cartilages, one ofwhich is called thyroides or scutiformis, from its being shaped likea buckler; and the other cricoides or annularis, from its resem-bling a ring. Both there cartilages may he felt immediately un-der the skm, at the fore-part of the throat; where the thyroides,by its convexity, forms an eminence called Poimim Adami,which is usually more considerable in the male than in the femalesubject. All tnese cart.luges are united to each other bv means ofvery elastic, ligamentous fibres ; and are enabled, by the assist-ance of their several muscles, to dilate or contract the passage oft.’ie larynx, and to pertorm that variety cf.motb.n, w hich seems topoint out the h ryux as'the principal organ of the voice; for whenthe car passes out through a wound in the trachea, it produces noround..
'1 he Glottis is interiorly covered by a very fine membrane,which is moiitened by a constant supply of a watery fluid, l-'iomthe osryux, the canal begins to take the name of trachea or asperaiirter.a, awl extends from thence as far dow n as the third or fourthveiteora ot the back, wh. re it divides into two branches, whichare an;' right aim i-.-tt bronchial tubes. Kach of.these bronchi ra-mifies tiui.ugn the substance of that lobe of the lungs, to. which itIs ditilr.buU.il, by an minute Humic; of branches, which are form-
ed of cartil.iges separated from each other like those of the trachea,by an intervening membranous and ligameutery substance’. Thetrachea is furnished with Bushy or muscular fibres; some of whichpassthrough its whole lengtli’longitiidinaUy, while the others arecarried round it in a circular direction; so that by the contractionor relaxation of these fibres, it is enabled to shorten or lengthenitself, and likewise to dilate or contract the diann-t t ot its pas-sage. The trachea and its branches, in all their raimti -utions, arefurnished with a great number of small glands w hich are Indited intheir cellular snb.tance, and discharge a mucous lluid on the in-ner surface of these tubes.
The Luxes are composed of two lobes, one seated on each sideof the mediastinum. Kach of these lobes are subdivided intotwo or three lobules, a kind of structure particularly distinguish-able in such animals as have much motion in their backs, and forthe same end that the liver is in the same animals. Their sub-stance is composed of very small cells, which are f he extremitiesof the trachea arteria or bronchos. The figure of the e cells isirregular ; yet they are fitted to each other so as to li. ve commonsides, and leave no void space. .Within these cells there is secreteda quantity of mucus, and also a large quantity of tin materiaperspirabilis, which at once keeps the lungs from being dried bythe air, and produces the necessary discharge from the blood,which also takes place by perspiration through the skin.Throughout the whole texture of these ceils'are distributed-thebranches of the pulmonary artery and vein, which ramify to aninfinite degree of minuteness, and supply the last mentioned se-cretions. The pulmonary artery and vein, how ever, an* not in-tended for the nourishment of‘the lungs; but the blood, in itspassage through them, acquires certain essential properties fromthe action of the air, which it had lost in circulating through therest of the body. The lungs are nourished from the arteria: bron-chiales, whose ramifications are seen creeping every where on thebranches of the bronchi. The blood is brought back from theminto the vena azygos by the bronchial vein.
The Pericardium is a strong membranous capsula , which in-closes the heart in a two-fold manner; for, after surrounding (heheart, it is itself surrounded by a retlexion of its own substance.It is made up of three lamina', the middle and chief of which iscomposed of very* fine tendinous ligaments, closely interw oven, andcrossing each other in different directions. The internal laminaseems to be a continuation of the .outer coat of the heart, auricles,and great vessels. The uses of the pericardium are, to keep theheart in its place without interrupting its office ; to keep it fromhaving any injurious friction with the lungs ; and to contain a li-quor to lubricate its own surface and that of the heart, so as to pre-vent the friction which would otherwise happen between both.
'Die Heart is a hollow muscle of a conical shape, situatedtransversely between the two lamina'' of the mediastinum, at thelower part of the thorax ; having its basis turned towards the rightside, and its point or apex towards the left. It., lower surface issomcwduit flattened towards the diaphragm. Its basis, from whichthe great vessels originate, is covered witli fat, and it has two hol-low- and Heshv appendages, called auricles. Round these several-openings, the heart seems to be of a firm ligamentous texture, fromwhich all its fibres originate ; and as they advance from thencetowards the apex, the substance of the heart seems to becomethinner. The heart includes two ventricles, or cavities which areseparated from each other by a fleshy septum ; one of these is call-ed the right, and the other the left ventricle. The heart is exter-nally covered by a very line membrane; and its structure is per-fectly mu.vulur or fleshy, being composed of fibres passing in dif-ferent directions ; some longitudinally from the basis to the apex ;others in a>i oblique or spiral course; and a third soit placed in ittransverse direction. Within the two ventricles we observe seve-ral furrows; and there are likewise tendinvuis strings, which arisefrom tle-liy columns; in the two cavities; and are attached to thevalves ot the auricles. That the use of these and the other valvesof the heart may be understood, it must he observed, that tourlarge vessels pass; out from the basis of the heart, viz. two arteriesand two veins; and tiiat eaeli of these vessels is furnished with athin membranous production, which is attached all round to theborders of their several orifices, from whence hanging looselydown they appear to l»e divided into two or three distinct portions.Hut as their uses in the arteries and veins are different,. so are theydifferently disposed. Those of the arteries are intended to give9