The Nervous System. 107 



therefore may be considered as simple when compared 

 with that of man, consequently the known list of nervous 

 troubles in farm stock is comparatively small. Roaring 

 in horses is probably the most frequently observed in the 

 list, latterly stringhalt, milk fever, tetanus have been 

 removed from the list of pure nervous diseases. 



THE SPECIAL SENSES. 



The organs of the special senses are the eye, ear, nose, 

 and tongue; there remains for description, which must 

 necessarily be brief, the two former. 



The eye is the organ of vision, and owing to the ease 

 with which it may be destroyed or affected, becomes at 

 once an object of importance to the stockman, especially 

 so when engaged in the raising and marketing of horses. 

 It is protected by the eyelids, eyelashes, and the haw 

 (membrana nictitans) or winking eyelid and the tear 

 apparatus. The eye may be compared to a ball having 

 a watch glass (cornea) fitted in at the front, its work 

 being similar to that of the lens of a camera. 



The ball (eye) consists of three layers, outer (scler- 

 otic and cornea), middle (choroid and iris), inner (re- 

 tina), enclosed in these layers are substances termed 

 humors, which have various functions. The fluid in the 

 front part of the eye is termed the aqueous humor and is 

 for the purpose of giving shape to that part of the eye, 

 it is quite watery and is being constantly secreted during 

 life; as a result of operations some of it may be drained 

 away, but is replaced by more fluid later on; behind this 

 fluid is the crystalline lens made up of layers resembling 

 an onion, the eye is focussed by movements of the lens. 



