THE EYE AND ITS DISEASES. 155 



CHAPTER VIII. 



THE EYE AND ITS DISEASES. 



THE diseases that attack the same organ are essentially different, in different 

 jmimals, in their symptoms, intensity, progress, and mode of treatment. 

 ] n periodic ophthalmia that pest of the equine race and opprobrium of the 

 veterinary profession the cornea becomes suddenly opaque, the iris pale, 

 the aqueous humour turbid, the capsule of the lens cloudy, and blindness is 

 the result. After a time, however, the cornea clears up, and becomes as 

 bright as ever ; but the lens continues impervious to light, and vision is lost. 



Ophthalmia in the dog presents us with symptoms altogether different. 

 The conjunctiva is red ; that portion of it which spreads over the sclerotica 

 is highly injected, and the cornea is opaque. As the disease proceeds, and 

 even at a very early period of its progress, an ulcer appears on the centre ; 

 at first superficial, but enlarging and deepening until it has penetrated the 

 cornea, and the aqueous humour has escaped. Granulations then spring 

 from the edges of the ulcer, rapidly enlarge, and protrude through the lids. 

 Under proper treatment, however, or by a process of nature, these granu- 

 lations cease to sprout ; they begin to disappear ; the ulcer diminishes ; it 

 heals; scarcely a trace of it can be seen ; the cornea recovers its perfect 

 transparency, and vision is not in the slightest degree impaired. 



There is a state of the orbit which requires some consideration. It is 

 connected with the muscles employed in mastication. Generally speaking, 

 the food of the dog requires no extraordinary degree of mastication, nor is 

 there usually any great time employed in this operation. That muscle 

 which is most employed in the comminution of the food, namely, the 

 temporal muscle, has its action very much limited by the position of the 

 bony socket of the eye ; yet sufficient room is left for all the force that can 

 bo required. In some dogs, either for purposes of offence or defence, or 

 the more effectual grasping of the prey, a sudden violent exertion of mus- 

 cilar power, and a consequent contraction of the temporal muscle, are re- 

 qi isite, but for which the imperfect socket of the orbit does not seem to 

 af'ord sufficient scope and room. There is an admirable provision for 

 th ; s in the removal of a certain portion of the orbital process of the frontal 

 bcne on the outer and upper part of the external ridge, and the substitu- 

 tion of an elastic cartilage. This cartilage momentarily yields to the 

 sw elling of the muscles ; and then, by its inherent elasticity, the external 

 rk ge of the orbit resumes its pristine form. The orbit of the dog, the 

 pij.?, and the cat, exhibits this singular mechanism. 



The horse is, to a certain extent, also an illustration of this. He re- 

 qu res an extended field of vision to warn him of the approach of his 

 enemies in his wild state, and a direction of the orbits somewhat forward 

 to enable him to pursue with safety the headlong course to which we 

 BOI letimes urge him ; and for this purpose his eyes are placed more for- 

 ward than those of cattle, sheep, or swine. That which Mr. Percivall 

 states of the horse is true of our other domesticated animals: " The 



