CHAP. ii. EYE OF THE HORSE. 409 



parts of the body. The former are highly important, some of them 

 constituting nerves of special sense, such as the olfactory, the optic, 

 and the auditory. 



THE EYEBALL consists of a tough fibrous coat, transparent in front, 

 and enclosing two liquid or semi-liquid masses, the vitreous humour 

 and the aqueous humour. At the back, the optic nerve enters it. In 

 front may be seen the transparent cornea, in the centre of which is the 

 pupil, fringed by a delicate muscular curtain of varying diameter, the 

 iris, which gives the " colour " to the eye. The iris, though variously 

 coloured, is usually brownish-yellow in the horse ; if it is nearly white, 

 or bright grey, the horse is " wall-eyed." The eyeball is lodged in the 

 orbital cavity, which is a recess, formed by the orbital process of the 

 frontal bone, and the margins of the frontal, lachrymal, and malar 

 bones. Posteriorly there is a fibrous membrane, the ocular sheath. 

 The orbital cavity lodges, besides the ball of the eye, its muscles, also 

 the membrana nictitans, and the lachrymal gland. 



The movements of the eye of the horse are controlled by seven 

 muscles, five rectus muscles, and two oblique. The posterior rectus 

 muscle, or retractor oculi, forms a sheath round the optic nerve, and 

 is of use in drawing the eye backward into the orbit in case of danger. 

 The superior, inferior, external, and internal rectus muscles are parallel 

 with the posterior rectus, and repeat on a larger scale the disposition 

 of the four bundles of the latter. Their function is, by their respec- 

 tive contractions, to turn the eye upward, downward, to the right, or 

 to the left, as the case may be. The great oblique and small oblique 

 muscles move the eyeball obliquely. 



The protective organs of the eye are seen externally. The eyelids 

 have two commissures, the superior one at the temporal angle, and the 

 inferior and rounder one at the nasal angle. The eye-lashes divert 

 dust and other matters that might otherwise enter the eye. The 

 Meibomian glands on the margins of the eyelids pour out an unctuous 

 secretion, offensive to insects. The conjunctiva is an extremely 

 delicate transparent membrane covering the front of the eyeball, and 

 keeping it moist. The membrana nictitans (winking eyelid) is a third 

 eyelid, which maintains the healthy condition of the surface of the eye 

 by removing any matters that may have escaped the eyelids. It sweeps 

 across the eye transversely from the nasal angle. 



HEREDITAKY DISEASE IN THE HORSE. The propagation of hereditary 

 disease amongst live stock is as undeniable a fact as the propagation of 

 other characters from parent to offspring. It is, therefore, incumbent 

 upon all breeders who are worthy of the name to take as great trouble 

 to prevent the transmission of disease, as they do to promote the 

 perpetuation of useful and desirable peculiarities. Accordingly, certain 

 hereditary diseases have been scheduled by the Royal Commission on 

 Horse Breeding as rendering stallions unfit for stud purposes. These 

 diseases are : Roaring, whistling, ringbone, unsound feet, navicular 

 disease, spavin, cataract. A horse suffering from any of these dis- 

 orders should not be allowed to become a parent, the probability being 

 that the disease would reappear in the offspring. Without entering at 



