The windpipe and bronchi are covered on their peri pheral surface with a strong membrane that is of con siderable thickness on the trunk of the windpipe, but becomes thinner on the bronchi. it is composed chiefly of longitudinal fibres running parallel to each other, and very obvious to the sight. On their outer surface these fibres are of a reddish colour, but appear whitish next 11 e cartilages. The membranous part that forms the side of the windpipe, is partly composed of this outer membrane, and par.ly of two layers of what ap pear to be muscular fibres ; one laver being arranged longitudinally, the other in a transverse direction. On its central surface the windpipe is lined with a delicate, irritable membrane, that is cont.nued from the larynx through the broneni, and probably to the air-ceils of the lungs. In the neck, the whole outer sin lace of the windpipe is enveloped with loose cellular substance, Irons witch it derives a general covering, and within the chest it takes a peripheral coat trout the medias The cavity of the windpipe and bronchi is moistened by a mucous liquor that exudes through their internal membrane, and is derived from numerous small glands situated on their peripheral surlace, and opening ccntrud by numerous small excretory ducts. These are called the tracheal glands. Other glandular bodies connected the absorbent system of the lungs, are situated within the cellular substance that surrounds the roots of the bronchi, and the sacral extremity of the wind pipe. These are of various sizes, some being no larger than a millet seed, while others equal the tip of the little finger. They are of a dark colour, and of a similar tex ture with the other lymphatic glands. They are called the bronchial glands.
The windpipe is supplied with arterial branches from the inferior laryngeal arteries, and its nerves conic chiefly from the great sympathetics.
At the atlantal extremity of the windpipe, on its sternal side, there is a larger glandular body of a dark red colour, called the thyroid gland, from its being partly' situated over the thyroid cartilage. This gland consists of two lobes that extend downwards over the side of the windpipe and gullet, and are united before by an intermediate portion. Examined centrad, this gland is found to be composed of numerous small grains connected by cellular substance, and it gener ally contains a viscid liquor. The use of this gland has not been ascertained. It is smaller in women than in men, and from this circumstance the neck is less prominent in females. It is well supplied by particu lar arteries, which have been enumerated in the table, by' the names of superior and inferior thyroid arteries, and has several nerves from those branches called laryngeal. Numerous lymphatics also pass through it.
The diaphragm is that fleshy partition that is situa ted between the chest and the belly ; by the Latin anatomists it was called septunz transversztnz, and in com mon language it is known by the name of midriff. This
partition is composed chiefly of muscular and tendinous fibres, which arc arranged in various directions. But the tendinous fibres commonly occupy its central and sternal parts, while the muscular fibres compose its lateral and posterior parts. See Plate XIV. Fig. 3. The diaphragm is attached at its fore part to the central surface of the ensiform cartilage that terminates the sacral extremity of the sternum ; laterally it is united to the cartilages of the two sacral true ribs, and to those of all the false ribs. These attachments are by means of muscular fibres, which run in a radiating direction towards the central tendon. By other fleshy fibres for ming four bundles or heads, called the claim of the diaphragm, it is attached to the four superior vertebra of the loins. Two of these heads are longer than the others and are called the long crura. These run diverg ing, from each other towards the central tendon, near which their fibres cross each other, and form an oval opening through which the gullet passes from the chest to the cardiac portion of the stomach. See Fig. 3. G. A little dorsad of this oval opening there is ano ther separation between the fleshy fibres of the crura, for the passage of the great trunk of the aorta ; and the thoracic duct, a b c d c. Near the sternal part of the oval opening for the gu.let, on the right side, there is a considerable triangular space between the tendinous fi bres. Through this passes the trunk of the sacral versa cava, F.
The diaphragm, considered as a muscle, is usually divided by anatomists into two portions, called the great er and less muscles of the diaphragm, the [Miner consisting of the large central tendon, which forms the principal part of the portion, and of these fleshy fibres that are attached to the sternum and the ribs, while the latter is formed chiefly by the crura attached to the vertebra: of the loins. As the greater muscle is more atlantal, or, in the vertical position of the body, higher than the lesser muscle ; the former is sometimes cal led the superior, and the latter the inferior diaphragm.
In the natural position of the diaphragm, when it is neither much contracted nor much dilated, its mus cular fibres arc arched towards the thorax. \\Then these arc thrown into strong contractions, as during a deep inspiration, they become much less arched, and the whole diaphragm is considerably flattened ; while on the contrary, when they are in a state of relaxa tion, and the abdominal viscera strongly compressed by the action of the abdominal muscles, the convexity of the diaphragm towards the cavity of the thorax is greatly increased. This happens during a violent ex piration.