Those large spongy membranous and vascular parts which we call lungs, consist chiefly of two divisions, to which anatomists give the name of lobes. Each of these lobes is again subdivided into smaller portions also called lobes; the right lung being generally divided into three, and the left lung into two lobes. These small lobes ale also slightly subdivided into still smaller portions, called the lobules of the lungs. The lungs in their general form are very irregular. They arc convex next the sides of the thorax, concave next the diaphragm, une flattened next the heart, terminate in a broad round part behind, and by edges more or less acute on their sternal and sacral sides. Indeed, from their tex ture, their form must in a great degree depend on that of the adjoining parts. Their colour varies at different ages. In children and young people they are usually of a fine red or pink colour ; they assume a light blue or grayish tinge in middle age, and in old people they be come more or less (lark and livid. To the touch they arc soft and spongy, and extremely elastic.
specific gravity they are the lightest of all the animal organs, even when completely exhausted of air. On their peripheral surface, the lungs are smooth and glossy. They are inveloped in a very fine and delicate transparent membrane, derived from the pleura, and through this the peripheral substance of the lungs has the appearance of net-work. They are connected at their dorsal side to the spine by the piciira; to the lower part of the neck by the wind-pipe ; and to the heart by the roots of the pulmonary artery and veins ; but towards the ribs, the mediastinum, and the dia phragm, they are in their natural state unconnected, so as readily to yield to the motions of the ribs and dia phragm.
The lungs in their internal structure, are composed of a great number of membranous cells, of numerous ramifications, of blood-vessels, with nerves and lympha tics, all connected by cellular substance. The cells of the lungs constitute the greatest part of their bulk. These are very- small, of an irregular figure, with very thin membranous sides. They are closely connected and compressed at their sides, and they freely commu nicate with each other, but have no communication with the cellular substance by which they are connected. From the cells there arise small hollow tubes, which gradually form other larger tulles, till, at the upper part of the thorax, all the tubes oil each side unite in one, and these two branches at length join to form the \chid pipe. At their commencement, these tubes are mem branous, but as they unite together to form the two branches of the w iod-pipe, they become cartilaginous.
They are generally called br9nchi or brogchi,c. Tile ramifications of the blood-vessels Corm also a large part of the substance of the lungs, and chiefly consist of the divisions 01 the pulmonary artery and veins, with the capillaries between their extremities. These ramifica tions are spread over every part of the cellular structure of the lungs, running throughout the cellular substance that connects the air-cells. There are however, other blood-vessels, called the bronchial vessels, intended to convey nourishment to the lungs; while the ramifica tions of what are called the pulmonary vessels, seem destined to distribute the circulating fluids through every part of those spongy bodies, for the purpose of being !Feely subjected to the action of the air.
B-sides the common coat that surrounds the lobes of the tut.gs, and is derived from the pleura, there is a delicate membrane closely connected with the sub stance of the lungs, and surrounding each of the compo nent lobules. This appears to be derived from the cel lular substance.
Tile windpipe, called by modern anatomists the tra chea, and by the older medical writers, as/itra arteria, is situated in the sternal part of the neck, extending from that remarkable protuberance a little below the chin, into the thorax, where it enters the posterior or dorsal mudiastinum, and about the third vertebra of the back, di\ ides into the two branches which Lorin the union of the bronchi. The \\ indpipe is a tube of con siderable size, of nearly a cylindrical form at its sternal side, but llatt,ned at its dorsal part. Next the head it joins what is called the larynx, which may properly be considered as all appendage to the windpipe. Behind it is closely connected with the gullet. It is chiefly composed of caltilaginous rings that are complete on the dorsal side. Each of these rings is about inch broad, and A- line in thicken ss. They are generally 16 or 18 in number, and arc placed horizontally, with their edges next each other. There is a small space between them, and this is filled up by a ligamentous elastic sub stance. In the atlantal part of the windpipe, the carti lages are sometimes joined to each other, but in the middle and sacral parts they arc perfectly distinct. The cartilages are continued from the trunk of the windpipe to the bronchi, Lib they begin to enter the cells of the lungs; but in that part of the bronchi that is next the two principal branches, they are not composed each of one piece, as in the windpipe, but consist of two or three pieces, so connected as to surround the bronchi, and preserve their cylindrical form.