The dimension and weight of a healthy heart vary so much that it is not possible to give them accurately, but the average is stated as follows by Mr. Walsh: “It is about ten and a quarter inches from the base to the apex, seven inches in its antero-posterior diameter, and five and a quarter from side to side. In weight, it varies from six and a half to seven pounds. The auricles have much thinner walls than the ventricles, and the muscular substance or the left ventricle, occupying the apex of the heart, is very much thicker than that of the right.” The nerves which supply the heart are the pneumogastric and sympathetic. Idiopathic disease of the heart in the equine race is a very rare occurrence; but, as a consequence or accompaniment of influenza and fevers of a low type, is by no means uncommon; it is therefore of importance that we should know how to diagnose both the organic and functional derangement of so important an organ, which can only be done by making ourselves familiar with. THE ACTION OF THE HEART IN HEALTH.—There are three means by which this knowledge may be attained: first, by _Percussion_, which, in the human subject, affords an index to the size and position of the heart; but in consequence of the less exposed position of that organ in the horse, is of little value, except in diagnosing Hydrops Pericardii. Second, by _Impulse_, or the force of the beat imparted to the hand placed flat on the near side of the chest, just behind the elbow. Third, by _Auscultation_, which is the only reliable way of ascertaining the condition of the heart, and consists in placing one ear, or a stethoscope, in the same position as that already described. Two distinct sounds can be heard; the first, termed the “systolic,” is louder and more prolonged than the second (which immediately follows), and seems to depend upon the contraction of the ventricle; during this contraction the organ elongates and rotates on its axis from right to left, thus producing the impulse and sound on the left side of the thorax. The second, “diastolic,” or “flapping,” sound, cannot be heard in a fat, circular-chested horse; it is supposed to arise from the regurgitation of the blood, which is in the aorta and pulmonary arteries, forcibly closing the valves placed at their origin in the ventricles, so as to prevent the blood coming back into the heart. These sounds together form what is called the “beat,” and are succeeded by a brief interval of repose. In the healthy animal these beats are uniform and regular, corresponding with the pulse; but in disease there may be, first, increase or diminution in the frequency and strength of the beats; second, irregular or intermittent action of the heart. In the latter case, we hear two or three beats in quick succession, and then a longer interval of repose than is natural, followed by the consecutive, quick beats and long repose, the number of the former and duration of the latter being singularly uniform.