The mechanism by which the respiratory acts are accomplished consists of the partly fixed and partly movable bony and cartilaginous framework of the chest and of the muscles, which form a large part of the walls of that cavity. The lungs themselves are passive agents, and only contribute to the movement of expiration by their elasticity. They accurately fill and fit the thoracic cavity, except in so far as room is afforded for the heart and great vessels. The principle on which tranquil respiration is accomplished is that the cavity of the chest becomes enlarged by muscular effort. As the diaphragm descends, and as the ribs are raised, air rushes in to equalize the pressure, and inspiration is effected. Then the muscular effort ceases, the diaphragm becomes more arched, the abdominal parietes, chest, and lungs retract by virtue of their elasticity, and expiration results.

The thorax, or chest, is a conical cavity, narrow and compressed from side to side in front, and expanded behind. It is formed of the strong and unyielding back-bone above, and the sternum, or breast-bone, below. Between these two parts extend the ribs, which number eighteen on each side, the first eight, or true ribs, being attached by their heads or upper ends to the spine, and by their lower ends, with the intervention of a piece of cartilage, to the sternum or breast-bone. The remaining ten, false or asternal ribs, whilst attached above like the true ribs to the vertebrae, end below in cartilages which are only indirectly connected with the sternum.

The first and last ribs are the shortest, the ninth is the longest. They form a series of highly elastic arches, the sharpness of the curvature being-greatest in the first and diminishing to the last. The articulations formed by the heads of the ribs with the vertebrae permit a slight degree of motion upon them. In complete expiration the ribs of the opposite sides of the thorax fall towards one another, and at the same time approximate to those of their own side by their edges, thus diminishing the capacity of the chest; whilst in full inspiration the ribs are raised, form wider curves, separate from those of the opposite side and from each other, and thrust the sternum forwards, in this way effecting enlargement of the chest both from side to side and from before backwards. The movements of the ribs are effected by muscles placed between them, named the intercostals. These consist of two layers of short muscular fibres which extend between the edges of adjoining ribs, the external running downwards and backwards, the internal running downwards and forwards, the former serving to raise the ribs, and thus to effect inspiration, which is aided by those fibres of the internal intercostals which extend between the cartilaginous portion of the ribs.

The front aperture of the thorax is occupied by the trachea, oesophagus, large vessels, and nerves, together with firm connective tissue. The posterior opening of the thorax is very wide, and is closed by the great muscular arch of the diaphragm or midriff. This important muscle divides the thorax from the abdomen. Its posterior concave surface is covered with the peritoneum or lining membrane of the belly, and is in contact with the liver, stomach, and organs of the abdomen; the anterior convex surface is covered with the pleura or lining membrane of the chest, and is in contact with the lungs.

The diaphragm is the most powerful muscle of inspiration, for in the act of contraction the arch it naturally forms falls back and tends to become a plane surface, and thus the cavity of the chest is enlarged from before backwards at the expense of the abdomen. In forced respiration the chest is acted on by many muscles which are attached to its outer surface, and which tend to raise the ribs and effect inspiration. On the other hand, the muscles of the abdominal wall tend to pull down the ribs, compress the abdominal organs, and force forward the diaphragm; they consequently cause expiration.

The enlargement of the chest during tranquil respiration is smaller than might be expected, for if a tape lie passed round the animal, just behind the shoulder, it will be found to enlarge only about 1/8 inch, at the twelfth intercostal space about 1 inch, and at the last intercostal space about 1/2 inch. It is estimated that the diaphragm moves backward towards the belly about 5 inches, lengthening the chest to that extent. The quantity of air inspired and expired at each respiration varies within wide limits. In tranquil respiration it may amount on the average to about 2 quarts with each respiration, or about 5 gallons per minute, when the number of respirations is ten, but after violent exercise the number of respirations may increase to one hundred or more per minute, with a corresponding increase in the volume of air breathed, amounting in one case to 97 gallons.

In the horse the act of inspiration is much more prolonged than that of expiration, the proportion being sometimes as much as two to one, and there appears to be a very short interval between the end of the one act and the beginning of the other.

The air that is used in respiration has been divided into four portions. First, there is that portion taken in during quiet breathing (tidal air), which may be estimated at from 200 to 300 cubic inches. But it is manifest that during exertion the animal can take in an additional amount over and above what it inspires in ordinary breathing. This is termed complemented air. Under these circumstances it also gives out with each expiration much more air than when an expiration has been made during rest; that extra quantity is named supplemental air. And these three portions - tidal, complemental, and supplemental air - are spoken of collectively as the " vital capacity". In young and strong animals, with free movement of the chest walls, the vital capacity is large; in old and weakly animals, when the cartilages of the ribs are ossified, and the lungs and thorax are less elastic, it is small. Finally, there is a portion of air termed the " residual air", which cannot be dislodged by the deepest expiration, but which is still subservient to the respiratory process in the air-cells of the lungs.

If in a healthy living animal the walls of the chest be cut through, as, for example, by a sabre cut, or be perforated, as by a stake or by a rifle bullet, so that the cavity of the chest is opened, or even if this be done in a dead animal, the lung immediately collapses and retracts from the wall of the chest, and the hissing of air entering through the wound can be distinctly heard. If one side of the lung only be punctured, respiration, though greatly impeded, is yet capable of being performed, but if both cavities are opened suffocation quickly ensues, for with each expansion of the chest the outside air enters the pleural cavity more easily through the wound than through the trachea, in consequence of which the lungs become compressed and cease to perform their office.