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 281 ANATOM Y. Part VI. They are lined on the infide by a very fine membrane, pleurae, through the upper fpace left between the dupliwhich continually difcharges a mucilaginous fluid; and cature of the mediaftinum, behind the thymus. in the fubftance of the membrane are a great number of Having reached as low as the curvature, of the aorta, it divides into two lateral parts, one toward the rightfinal! blood-veflels. The bronchia are divided in all directions into an infi- hand, the other toward the left, which enter the lungs, nite- number of ramifications, which diminilh gradually and are diftributed through them in the manner already in fize; and as they become capillary, change their car- faid. Thefe two branches are called bronchia, and that on the right fide is fhorter than that of the left. tilaginous ftruCture into that of a membrane. Each of thefe numerous bronchial tubes is widened at The trachea is made up of fegments of circles or carthe extremity, and thereby formed into, a fmall membra- tilaginous hoops, difpofed in fuch a manner, as to form nous cell, commonly called a velicle. Thefe cells or a canal open on the back part, the cartilages not going follieuli are clofely connected together in bundles; each quite round; but this opening is filled by a loft glandular fmall branch producing a bundle proportionable to its membrane, which completes the circumference of the canal. Each circle is about the twelfth part of an inch in extent and the number of its ramifications, Thefe fmall veficular or cellulous bundles are termed breadth, and about a quarter of that fpace in thicknefs. lobules; and as the great branches are divided into fmall Their extremities are round; and they are fituated horami, fb the great lobules a?§Mivided into feveral fmall ’ rizontally above each other, fmall interftices being left ones. The cells or veficles of each lobule have a free between them, and the lower edge of the fuperior fegcommunication with each other, but the feveral lobules ments being turned toward the upper edge of thofe next below them. do not communicate fo readily. The lobules appear diftinCtly to. be parted by another They are-all connected by a very ftrong elaftic memcellulous fubftance, which furrounds each of them in pro- branous ligament fixed to their edges. portion to their extent, and fills up the interftices between The canal of the afpera arteria is lined on the infide them. This fubance forpis likewife a kind a)f irregular by a particular membrane, which appears to be partly fiiembrHneus cells, which are thinner, loofer, and broad- fleftiy or mufcularj and partly ligamentary, perforated er than the bronchial vefifcles. by an infinite number of fmall holes, through which a All the bronchial cells are furrounded by a very fine mucilaginous fluid continually pafies, to defend the inner reticular texture of the fmall extremities of arteries ^nd furface of tile trachea againft the acrimony of the air. veins, which communicate every way with each other. This fluid comes from fmall glandular bodies difperfed The blood-veflels of the lungs are of two kinds; one through the fubftance of the membrane, but efpecially Common, called the pulmonary artery and veins ; the o- from the glands, fomething-larger than the former, which tker proper, called the bronchial arteries and veins. lie on the outer or pofterior furface of that ftrong memThe pulmonary artery goes out from the right ventri- brane, by which the circumference of the 'canal is comcle of the heart; and its trunk having run almoft direftly pleted. The fame ftrufture is obfervable in the ramifi-. upward as high as th£ curvature of the aorta, is divided OationS of the trachea from the greateft to the fmalleft. into two lateral branches, one going to the right-hand, At the angle'of the firft ramification of the trachea arcalled .the right pulmonary artery, the other to the left, teria, we find on both the fore and back fides, certain’ termed the left pulmonary artery. The right artery foft, roundiih, glandular bodies, of a bluifh or blackiftv pafles under the curvature of’ the aorta, and is confe- colour, and of a texture partly like that of the thymus quently longer than the left. They both rUn to the already deferibed, and partly like that of the glandula lungs, and are difperfed through their whole fubftance by thyroide-s. There are other glands of the fame kind, as ramifications neatly like thofe of the brohchia, and lying the Origin of each ramification of the bronchia, but they in the fame directions. decreafe proportionibly in number and Jize. They are The pulmonary veins having been diftributed through fixed immediately to the bronchia, and covered by th£ . the lungs in the fame manner, go out on each fide, hy interlobular fubftance; and they leem to communicate two great branches, which open laterally into the refer- by fmall openings with the cavity of the bronchia. voir or mufcular bag of the right auricle. Refpiration is performed by organs of two kinds, one ot Befides thefe capital blood-veffels, there are two o- which may be looked upon as active, the other as paflive. thers called the bronchial artery and The lungs are of the fecond kind, and the firft compreUnder the root of each lung, that is, under that part hends chiefly the diaphragm and intercoftal mufcles. formed by the .fubordiiiate trunk of the pulmonary arte- As foon as the intercoftal mufcles begin to contrail, ry, by the trunks of the pulmonary veins, and by the the arches, of the ribs are raifed, together with the fter trunk of-the bronchia, there is a pretty broad membra- num, and placed at a greater diftance from each other; nous ligament, which ties the pofterior edge of each lung by which means the cavity of the, thorax is inlarged oa to the lateral parts of the Vertebrse of the back, from- the two lateral and anterior fides. ihata-oot all the way to the diaphragm. At the fame .inftant the diaphragm is flatted ,01The bronchia already deferibed are branches or rami- brought toward a plane by two motions, which are apfications of a large canal, partly cartilaginous, and part- parently contrary; that is, by the contraction of the ly membranous, called trachea, or afpera arteria. It diaphragm, and the dilatation of the ribs in which it isi£ fituated-anteriorJy in thp lower part of the neck, from infefted. The external furface of the thorax b fir.g thus whence it runs down into the thorax betwixt the two in a manner iccreafed, and the cavity of the bronchia Von. I. Numb.' 12. 3 . 4B being