94 VETERINAEY STATE BOARD 



three layers. The anterior layer is epithelial in character and is a 

 continuation of the conjunctiva. The middle is composed of con- 

 nective tissue and the internal layer, elastic in structure, is called 

 Descemet's membrane. 



Name the tunics of the eyeball. Describe one. 



See answer to preceding questions. 



Describe the membrana nictitans and give its use. 



The membrana nictitans, also called the third eyelid, is placed at 

 the greater angle of the eye, whence it extends over the eyeball to 

 remove foreign bodies from the cornea. It is made up of fibro- 

 cartilage, triangular in shape, thick anteriorly and thick at its base, 

 where it is continued by an adipose cushion which is insinuated be- 

 tween the muscles of the eye. It moves, almost instantaneously and 

 in a mechanical manner, over the eye, whenever the eyeball is 

 retracted in the orbit. 



Describe the lachrymal apparatus of the horse and compare it with that 

 of the ox. 



The lachrymal apparatus consists of the lachrymal gland, puncta 

 lachrymalia, lachrymal sac and lachrymal canal. 



The lachrjonal gland is conglomerate, situated between the orbi- 

 tal process and the upper part of the eyeball. It secretes the tears 

 which empty on the inner surface of the upper lid. After lubri- 

 cating the cornea and conjunctiva, the excess of tears enter the 

 puncta lachrymalia, two small canals in the upper and lower lids, 

 which carry the tears into the lachrymal sac, a small reservoir, 

 the beginning of the lachrymal canal. The latter runs down in the 

 lachrymal bone as a membranous canal, ending on the floor of 

 the nostril. 



In the ox, the lachrymal gland is much more voluminous than 

 in the horse ; it has two large and several small duets, and the nasal 

 opening is situated higher in the nostril than in the hoKse. 



Describe the humors of the eye. 



The vitreous humor occupies all the cavity of the eye, behind the 

 lens, surrounded by the retina. It is a colorless, transparent, jelly- 

 like tissue which concurs in refracting the rays of light and at the 

 same time acts as a supporting substance by maintaining the 

 rotundity of the eyeball. 



The aqueous humor is more fluid than the vitreous, containing 

 only 2 per cent, solids. It occupies the anterior and posterior cham- 

 ber of the eye in front of the lens and is secreted by Descemet's 



