Thus far we have described the diaphragm as found in the horse, ox, and sheep. There is some difference with regard to the dog. The muscular part of the diaphragm is thick and strong in every species of dog, while the aponeurotic expansion is comparatively smaller. From the smaller expanse of the thorax of the dog, and the consequent little expansion of the diaphragm, the action, although occasionally rapid and violent--for he is an animal of speed--is not so extensive, and more muscle and less tendon may be given to him, not only without detriment, but with evident advantage. Therefore, although we have occasional rupture of the heart of the dog, oftener perhaps than in the horse, there is no case of rupture of the diaphragm on record. The cavity of the thorax is lined by a membrane, termed pleura, which covers the surface of the lungs. The lungs on either side are enclosed in a separate and perfect bag, anil each lung has a distinct pleura. The heart lies under the left lung; and, more perfectly to cut off all injurious connexion or communication of disease between the lungs and the heart, the heart is enclosed in a distinct pleura or bag, termed the 'pericardium'. This membrane closely invests the heart, supports it in its situation, prevents too great dilatation when it is gorged with blood, and too violent action when it is sometimes unduly stimulated. Notwithstanding the confinement of the pericardium, the heart, when under circumstances of unusual excitation, beats violently against the ribs, and, were it not thus tied down, would often bruise and injure itself, and cause inflammation in the neighbouring parts. The 'heart' is composed of four cavities; two above, called 'auricles', from their shape, and two below, termed 'ventricles', occupying the bulk of the heart. In point of fact, there are two hearts--the one on the left side propelling the blood through the frame, and the other on the right side conveying it through the pulmonary system; but, united in the manner in which they are, their junction contributes to their mutual strength, and both circulations are carried on at the same time. The beating of the heart in the dog is best examined behind the elbow on the left side. The hand, applied flat against the ribs, will give the number and character of the pulsations. The pericardium, or outer investing membrane of the heart, is frequently liable to inflammation, milked by a quickened and irregular respiration, and an action of the heart, bounding at an early period of the disease, but becoming scarcely recognisable as the fluid increases. The patient is then beginning gradually to sink. A thickening of the substance of the heart is occasionally suspected, and, on the other hand, an increased capacity of the cavities of the heart; the parietes being considerably thinner, and the frame of the animal emaciated.