Two perfectly distinct conditions are included in the term fatty disease of the heart. One consists in mere infiltration of the fatty material among the muscular fibres of the heart, which may be observed in animals when overfed (fatty infiltration), and the other consists in the actual conversion of the muscular structure into fat (fatty degeneration). In the former, the cells in the connective tissue lietween the muscular fibres become filled with fat, and there is an excessive deposit of fatty material outside the heart and round the base, and in the grooves in the walls of the organ, along which the blood-vessels pass. In connection with this deposit of fat the muscular structure becomes pale and flabby.
Fatty infiltration is found to exist in the case of animals which spend an idle life or do very little work, and are supplied with an undue quantity of food. Such animals are usually referred to by stablemen as being in soft condition, and it is recognized in reference to them that they are incapable of active work, rapidly becoming exhausted and suffering from shortness of breath and palpitation of the heart on slight exertion. The circulation is necessarily weak and languid, the extremities are cold, and an examination of the heart would reveal the characteristic symptoms of feeble impulse and much-diminished intensity in the normal sounds; when the deposit of fat is excessive, it may happen that no sound can be detected at all. The condition is modified by the circumstances under which the fatty infiltration takes place. In horses which have been fed to be brought into what is known as dealer's condition, a process which has probably only occupied a few weeks, regular exercise and change in the character of the food will, in the majority of cases, restore the animal to a healthy condition. It is only after the excessive feeding, with insufficient exertion, have been continued for a long period that the diseased state is likely to become permanent, and even in such cases considerable improvement in the animal's condition may be effected by persistent employment of the ordinary measures, which would come under the head of physical training, including carefully-regulated exercise, the avoidance of food containing a large proportion of fattening material, and the careful adjustment of the food given to the quantity of work performed.