DISEASES OF THE NERVOUS SYSTEM 415 



ever, is not always successful, as in some cases the animal continues the 

 lateral movements of the head even when it is left altogether without 

 any means of restraint. 



TUMOURS IN THE BRAIN AND CRANIUM 



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 consider- 

 able 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. 



Cause. — 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 development7 

 and it is very likely this may be a predisposing or even an exciting 

 cause. 



Symptoms. — 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 



