DISEASES OF THE NERVOUS SYSTEM. 285 



This affection is the commonest form of paralysis in the 

 horse. 



Causes. — The causes of paraplegia are : 



1. Central Nervous Lesions. — These are due to injury, 

 as fracture, or to diseases of the vertebrae, involving the 

 spinal cord, or to disease of the cord itself 



2. Reflex, or Functional Causes. — Functional paraplegia 

 depends upon alterations in the quality of the blood, as in 

 lead poisoning, azoturia, and after feeding upon rye-grass ; 

 or it may depend upon intestinal or other disturbances 

 of the system. 



Parapleo;ia resulting from the ingestion of rye-grass (en- 

 zootic paraplegia) will be considered fully below. 



4. Local Paralysis. — Local paralysis is very rare in the 

 horse, and is usually due to injury of a nerve from press- 

 ure by badly fitting harness or by some abnormal growth. 

 It may also be due to disease of the nerve at any part of its 

 course, from its connection with nerve-centre to the organ 

 supplied ; but paralysis resulting from this cause is very 

 uncommonly met with in the horse. The most common 

 form of local paralysis in the horse is that of the branches of 

 the facial nerve, by which the muscles of the face are chiefly 

 supplied tvith motor fibres. 



In paralysis of this nerve the lips are pendulous, and the 

 animal is unable to grasp his food and has difficulty in 

 mastication. 



HYPERiESTHESIA, DYSiESTHESIA, ANAESTHESIA. 

 — By hypersesthesia is meant exalted sensibility ; by dys- 

 sesthesia, persistent or painful sensibility. 



Practically, exalted sensibility is scarcely, if ever, distia- 

 guishable from painful sensibility. The hyper?esthetic eye 

 cannot bear light, and the hyperoesthetic skin is sensitive 

 to the slightest pressure. 



The term 'dyssesthesia' includes the sensations of itching, 



