DISEASES OF THE NERVOUS SYSTEM 417 



circumstances might have caused it, or it may have resulted from some 

 change in the blood consequent on the disease. 



Where, as in this instance, the choroid plexuses were torn up, treat- 

 ment could be of little avail. 



EXOSTOSES OR BONE TUMOURS 



Hard ivory-like growths from the petrous temporal bone sometimes 

 extend into the cavity of the cranium and occasion pressure upon and 

 absorption of the brain substance. These tumours, developing very slowly, 

 afford the neighbouring parts an opportunity to accommodate themselves 

 for a time to the altered state, but sooner or later the pressure they impart 

 to nerves and vessels produces various forms of structural and functional 

 derangement, among which may be mentioned deafness, paralysis of the 

 muscles of the face, loss of motor power, unsteady movements, convulsive 

 fits, followed by apoplexy and death. 



Exostoses or bone tumours sometimes occur on the floor of the cranium 

 as the result of a blow on the poll or back of the head, such as would 

 be inflicted by a horse falling backward or striking the head violently 

 against some fixed object. In these cases blindness may follow as a con- 

 sequence, from pressure on the optic nerves at their bifurcation; or the 

 muscles of the eye may suffer paralysis, and disorders of some of the 

 other nerves issuing from the base of the brain may result. 



THICKENING OF THE MEMBRANES 



Professor Williams, in his Principles and Practice of Veterinary 

 Medicine, refers to a case in which the dura mater or outermost covering 

 of the brain attained a thickness varying from one inch at the base to 

 several inches at the anterior part of the cranium, causing absorption 

 of the frontal and ethmoid bones, and closing the frontal sinuses. The 

 horse in which this was discovered had presented signs of brain disease, 

 sleepiness, partial paralysis, blindness, and paralysis of the muscles of 

 mastication for a considerable period before its death. 



" It is very probable", Professor Williams remarks, "that this condition 

 was the result of an injury, such as a blow upon the head, causing chronic 

 inflammation." 



