54

Mechanism, by which the Chest

ceeding ribs are not continuous, and by disposing all its parts in one plane. If an oblique plane be imagined to pass through each articular surface of any pair of ribs between the second and tenth inclusively, the greater part of the shaft and cartilage of these ribs is seen to fall below it. If the intermediate part of these ribs be raised towards the imaginary plane by the rotation of each upon its sternal and vertebral joints, it is obvious that the transverse diameter of the chest becomes increased.

By this provision the same muscles, which contribute to enlarge the depth of the thorax, are rendered capable of adding to its breadth, and the same which diminish its area in the first dimension, are fitted to diminish it in the second.

The chest is closed above by a fascia or layer of condensed cellular membrane, which extends across from the spine to the sternum, from the first rib of one side to the opposite, and is perforated by the windpipe, the gullet, and the great vessels of the head and upper extremities. The intervals between the ribs are closed by the oblique fibres of the intercostal muscles, which in their action draw towards each other adjoining ribs, and are capable of contributing either to the enlargement or to the diminution of the area of the thorax.

The floor of the chest is formed by the diaphragm, or muscular partition, which separates it from the abdomen.

The diaphragm consists of three parts. 1. Of a central thin tendon of the shape of a trefoil leaf, of greater breadth than depth, which, although in a degree concave downwards, yet may be regarded as spread out horizontally at the level of the ninth dorsal vertebra, or of the lowest part of the fifth rib. 2. Of muscular fibres

admits of being alternately enlarged and diminished. 55

derived from the anterior and lateral margins of the tendon, which slope downwards to be inserted into the ensiform cartilage and into the inner and lower part of the seven lowest ribs, and are called the greater muscle of the diaphragm. 3. Of muscular fibres, which descend from the posterior edge of the centrum tendinosum to the lumbar vertebræ, and are called the lesser muscle. The diaphragm gives height by its action to the cavity of the chest. Under ordinary circumstances the diaphragm is raised to that degree, that the margin of the greater muscle is held, except at the back part, against the six or seven lower ribs, which therefore protect the viscera of the abdomen, not those of the chest. The diaphragm gives passage to different tubes and nerves; and it is remarkable, that while the oesophagus, the aorta, and thoracic duct pass through muscular apertures, the pressure of which they are calculated to resist, the great ascending venous trunk passes through an opening in the central tendon, with the margin of which its substance is interwoven, so that the vein is held open by the whole tonic force of the greater and lesser muscle.

The abdominal muscles are the antagonists of the diaphragm, which upon becoming relaxed admits of being raised by their lateral pressure upon the bowels.

In the cavity of the chest, thus amply furnished with the means of alternate expansion and diminution, are placed the lungs, one on either side, with the heart be

tween.

A lung is an organ composed of a light cellular flesh, fitted to the varying form of the lateral part of the chest by its original figure and by its great elasticity, and adherent by a part of its inner surface, at which blood ves

56

Of the Pleura and Pericardium.

sels and nerves, lymphatics, and a branch of the windpipe, enter. Each lung is covered by a fine transparent membrane termed a pleura, which is reflected from the adherent part or root of the lung towards the sternum anteriorly, towards the spine behind; and afterwards lines the diaphragm, the ribs, and intercostal muscles.

The pleura is one of a class of parts termed serous membranes: these are for the most part closed sacs, one half or one portion of which forms the investing tunic of a viscus, while the other is attached to the parts adjoining. The outer surface of a serous membrane coheres with the cellular texture of the organs, which it covers: the inner surface is unattached, and kept moist with a fluid resembling the serum of the blood. Serous membranes are employed to facilitate the movements of viscera upon the neighbouring parts by the interposition of two lubricated surfaces, and to isolate adjacent organs from one another. Sometimes there is a difference in the character of the visceral and reflected portions of a serous membrane. In the present instance there is none: but the pleura covering the lung is termed pleura pulmonalis, the reflected portion pleura costalis, pleura diaphragmatica, or pleura pericardiaca, in reference to the surfaces it adheres to. That part of each pleura, which extends from the sternum to the spine, constitutes the septum or mediastinum of the chest, between the two layers of which the heart is contained.

The substance of the heart is covered with a serous membrane termed the pericardium, which is reflected from the great vessels to form the sac, in which the heart plays. The reflected portion coheres firmly with the centrum tendinosum of the diaphragm, upon which the

heart rests: it has great strength, and is divisible into a thin internal layer, the true continuation of the pericardium covering the heart, and a thick outer adventitious membrane. Dr. Baillie met with a case, in which the pericardium was deficient, and the heart invested by the pleuræ. The heart is fixed by its base, from whence the vena cava inferior descends through the diaphragm, -the vena cava superior and aorta ascend towards the neck, where their branches are distributed, and the pulmonary artery and veins extend transversely outwards into either lung.

The form and dimensions of each lung, while the chest is entire, are determined by the atmospheric pressure. A lung is laid out in cells, into which the windpipe opens. The windpipe or trachea, continuous through the larynx with the fauces, is a tube nearly cylindrical, and about ten lines in diameter, consisting of from fifteen to twenty incomplete rings of cartilage, the deficiency of which at the back part is made up by transverse muscular fibres, and by an elastic membrane, which serves at the same time to connect each ring with those adjoining it. The tube is lined by a vascular and sensible membrane, continued from the lining membrane of the fauces, and termed a mucous membrane from the nature of its secretion. The trachea descends from the throat into the chest, and opposite to the third dorsal vertebra divides into two smaller tubes termed bronchi: of these the right is the shortest and most capacious; for the right lung is larger than the left, the greater part of the heart being placed upon the left side. The left lung is divided into two lobes, the right lung into three, by fissures extending to the root of each. The bronchus divides into

58

Tendency of the Resilience of the Lungs

a branch for each lobe; and in the substance of the lung, these branches, after a few subdivisions, lose all trace of the imperfect cartilaginous rings which belong to the first parts of the respiratory tube, and become membranous. The branching air-tubes terminate in minute cells at every point in the lung, each lobe of which is subdivided into innumerable lobules.

The cells of the lungs, while the chest is entire, are always distended beyond their natural limits. The substance of the lung is elastic, but its resistance is of no effect against the disproportionate pressure of the atmosphere. If at an intercostal space the skin, muscles, and pleura reflexa be cut through, atmospheric air enters the chest through the aperture, the lung recedes from the ribs and shrinks to a smaller dimension.. By this wellknown experiment the atmospheric pressure is equalized upon either surface of the lung, and the organ takes `a volume determined by its elasticity and weight. Dr. Carson ingeniously contrived to measure the resistance of the lungs in the contracted state of the chest to the atmospheric pressure, by observing the height to which a column of water must be raised in order to force air into the lungs, after the opening of the intercostal spaces, in sufficient volume to fill the cavity of the chest as before. He employed a hollow glass globe, to one side of which a tube was let in, that admitted of being securely fastened into the trachea of a slaughtered animal; to the other was attached a vertical tube bent near its junction with the globe, into which water was poured, after openings had been made between the ribs. Through the means of this apparatus Dr. Carson ascertained that in calves, sheep, and large dogs, the resilience of the lungs is