[Footnote 4: 'Sporting Magazine', vol. xviii. p. 186.] [Footnote 5: Daniel's 'Rural Sports', vol. i. p. 220.] * * * * * CHAPTER VIII. THE EYE AND ITS DISEASES. The diseases that attack the same organ are essentially different, in different animals, in their symptoms, intensity, progress, and mode of treatment. In 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 granulations 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 be required. In some dogs, either for purposes of offence or defence, or the more effectual grasping of the prey, a sudden violent exertion of muscular power, and a consequent contraction of the temporal muscle, are requisite, but for which the imperfect socket of the orbit does not seem to afford sufficient scope and room. There is an admirable provision for this in the removal of a certain portion of the orbital process of the frontal bone on the outer and upper part of the external ridge, and the substitution of an elastic cartilage. This cartilage momentarily yields to the swelling of the muscles; and then, by its inherent elasticity, the external ridge of the orbit resumes its pristine form. The orbit of the dog, the pig, and the cat, exhibits this singular mechanism.