OF RESPIRATION.
In the more perfect animals, or those which are fur nished with the greatest number of distinct organs, respi ration is almost as essential to their existence as the cir culation of the blood. This function consists in the alter nate reception and emission of air into and out of the lungs ; while, at the same time, the blood is transmitted through a set of vessels so formed as to enable the air to act upon it, and to produce that change in its nature and properties which fits it for the support of life.. We shall arrange our remarks upon this subject into three heads : 1st, The mechanism of respiration ; 2d, Its direct effects; and 3d, The remote effects of respiration upon the living system.
The organs of respiration arc the trachea, with its ra mifications, the lungs, and the diaphragm. The trachea, or wind-pipe, is a long tube, composed of Cartilaginous rings, and furnished with muscular fibres, which extend from the mouth into the chest. It there divides into two branches, called bronchia, which pass respectively into the two lungs; here they subdivide into smaller branches, and these again into branches still smaller, until at length they terminate in minute cavities, which are called the air vesicles. These vesicles, which are destined for the ultimate reception of the air that is taken in by the trachea, are composed of a delicate membranous substance, and lined with a fine mucous membrane, on which the pulmo nary arteries ramify that proceed from the right side of the heart; those which compose the pulmonic circulation. This distribution of the blood was first minutely described by Malpighi, and has been generally received as correct, although there has been considerable difference of opinion respecting the form of the vesicles, and their connexion with the branches of the trachea. Some anatomists, as Willis, conceived that each air-tube terminated in a sepa rate rounded cell, which had no connexion with any other cavity, whereas others supposed that the lungs consist of an irregular tissue of membranous matter, the different parts of which, like those of the common cellular texture, have a communication with each other.
The lungs are two spongy bodies, each of them of an irregular oval form, united at their upper part by the trunks of the bronchia : they are kept distinct from each other by the mediastinum, a membranous body, which di vides the thorax into two cavities, that do not communi cate with each other. Each lung is divided into three principal parts, called lobes, and these into a number of smaller parts, or lobules, each lobule consisting of a branch of the air-tubes, with the accompanying blood vessels, and the connecting cellular substance. The lungs entirely fill up the two cavities of the chest, so as to leave no vacant space between the pleurx, or the membrane which encloses the lungs and that which lines the thorax, although, in the healthy state of the chest, these mem branes have no connexion, except at their origin, and ad mit of free motion upon each other.
The thorax itself is composed partly of bone, and partly of cartilage; its sides are formed by the series of arched bones called the ribs ; the spaces b-tween which are filled up by the intercostal muscles. The lower part of the chest, which is contiguous to the abdomen, consists of the diaphragm, an expanded membrane, furnished with muscles, which give it the power of contraction, and constitute it the principal organ in the mechanism of re spiration.
The mechanical act of respiration consists essentially in increasing the cavity of the chest. The diaphragm, in its natural state, forms an arch, which is convex towards its upper surface, so that when its muscles contract it be comes flattened, and in this way increases the capacity of the thorax. The thorax is likewise increased in size by the contraction of the intercostals, although in a much less degree than by the flattening of the diaphragm ; and indeed it is generally conceived that the principal use of these muscles is, to fix the ribs, and prevent them from being drawn down by the contraction of the diaphragm, and thus counteracting the effect which is produced by its action.