CAUSES.—Excessive effort of the heart to overcome obstruction to its action; constriction of the vessels and of the openings between the different cavities of the heart; deposits on the semi lunar valves; Aneurism of the aorta or the pulmonary artery; excessive physical exertion and consequent exhaustion of the heart. Mr. Pritchard remarks that “horses, particularly those employed in quick draught, are commonly called on to perform arduous tasks with full stomachs, by which the free action of the lungs is considerably impeded; thus, obstruction being given to the circulation through the pulmonary vessels, corresponding increase of force in the action of the heart is the consequence.” Hypertrophy has been often observed in broken winded horses. SYMPTOMS.—Strong, impulsive movement of the heart, which remains constant; intensity of sound, with a loud, hollow thumping beat; a metallic _bruit_ or “clack”; irregularity of rhythm; dullness on percussion. The horse is easily distressed, palpitation comes on with ordinary quick work, and there is an anxious expression in his eye on these occasions, which leads us to suspect that there is something amiss with the animal. These symptoms are attended with languor, coldness of legs and ears, dyspnœa, giddiness or Megrims, loss of appetite, and, in a later stage of the disease, with œdematous swelling of the chest, abdomen, and extremities. PATHOLOGY.—The thickening of the walls of the heart may be regarded as a beautiful provision of nature to strengthen the organ and enable it to overcome the obstacle which exists to the free circulation of blood through it. The thickening of the muscle depends upon excess of nutrition consequent on increased action; similar to the change in the biceps muscle of the blacksmith’s arm from hard work. It may be an idiopathic disease not dependent on obstruction; but such cases are very rare. The obstruction will usually be found to depend on disease of some of the valves, or constriction of the aorta or pulmonary artery at its origin. The Hypertrophy may be general, or we may find only one auricle or ventricle (generally the latter) whose walls are thickened. DIAGNOSIS.—The increased impulse of the heart, especially after quick work; the irregularity of the pulse and thick wind, without any lung disease being present; and the dullness on percussion over the cardiac region, are the chief pathognomic symptoms. PROGNOSIS.—Although the disease is incurable, a horse may live for years with enlarged heart. TREATMENT.—Doses of A.A. will be of value in conjunction with only moderate work and good care. Atrophy of the Heart DEFINITION.—Emaciation or wasting away of the walls of the heart. CAUSES.—Similar to those of dilatation. Loss of nervous power through the degenerating influence of fevers induces Atrophy by weakening the muscular fibres of the heart.