Tumours in the brain are not of rare occurrence in the horse, although they are very limited in variety. Moreover, they are seldom found to exist save in the lateral ventricles or cavities within the hemispheres. They are almost invariably of that variety termed " psammoma", a structure comprising a quantity of fibrous tissue, in the mesh-work of which are found granules of earthy matter, fatty particles, and thin glistening plates of cholesterine.
These tumours are developed in the fringe of vascular membrane, termed the "choroid plexus", which is situated on the floor of the lateral ventricles.
They vary in size from a pin's head to a hen's egg, and frequently occur in both ventricular cavities. Being slow in their growth they seldom produce any obvious disturbance in the conduct of the animal until they have reached considerable dimensions, although in the course of their development the ventricles become much dilated, and a considerable amount of brain matter is caused to be absorbed by the pressure which they make upon it. They are usually ovoid in form, of a bluey-gray appearance, smooth on the surface, and firm to the touch.
Brain tumours in the horse are mostly found in the adult and later periods of life, although the writer has removed them from the ventricles of so young a horse as a four-year-old.
As to the origin of these formations, nothing definite can be said; inasmuch, however, as they are more prevalent in harness horses than others of the riding class, it has been suggested that the pressure of tight, ill-fitting collars on the jugular veins may, by interrupting the circulation from the brain, be the means of causing their development, and it is very likely this may be a predisposing or even an exciting cause.
The symptoms developed as the result of the continued growth of these formations may be of a chronic or an acute and fatal character.
In the former the animal suffers periodic attacks of loss of power and unconsciousness for some time, the intervals between each attack becoming shorter as time goes on, and the attacks more and more severe. They are specially excited when the horse is worked on a full stomach, or urged too freely uphill when the pressure of the collar is increased, or driven in face of a hot sun. The frequent occurrence of these symptoms in the early spring and during the summer is mainly on account of exposure to the last-named cause.
The attack comes on without warning. The animal stops and suddenly falls to the ground, the muscles quiver or are rigidly set, the eyes roll, there is loss of consciousness, and for a time also of feeling and muscular power. After a brief period the stricken beast regains his lost powers and is able to rise, but for several days he remains dull, feeble, and stupid, .and altogether unfitted for work. In other cases the animal hangs his head, and presents an expression of drowsiness and an indisposition to move. In less severe attacks he will suddenly stop while being driven, lay back his ears, shake his head violently, or throw it up and down without any obvious reason, and in a few minutes resume his journey as if nothing had happened.
In the more severe attacks the patient is stricken down paralysed and unconscious and quickly succumbs.
Treatment in these cases is of no avail, and although something may be done to ward off the attacks by a judicious system of general management and feeding, horses affected with brain tumours are dangerous beasts to possess, and should be destroyed.