DISEASES OF THE OX AND SHEEP. 607 



fully well suited for guarding the eye from all ordinary chances 

 of injury. Attached to the sclerotic, by which name the outer 

 covering of the eye is denominated (that transparent circular 

 portion in front, called the cornea, being excepted), are various 

 muscles which, by their contraction, move the eye, and turn it 

 in various directions, and even draw it backwards. The capsule 

 or outer covering of the eyeball is composed of the above- 

 mentioned transparent and circular cornea in front, and this 

 is continuous with the dense white coat called the sclerotic 

 behind. 



Situated internally to this external membrane, comes the 

 choroid coat, to which is attached the circular muscle called 

 the iris in front. This structure, the iris, is not quite con- 

 tinuous ; it is, so to say, perforated, and the perforation is 

 called the pupil. The iris imparts the colour to the eye. It 

 may be said to act the part of a curtain, since, by means of con- 

 tracting and dilating, it regulates the amount of light which 

 enters the eye. 



In the eye itself are certain refracting media, transparent 

 solids and liquids (called the humours) of the eye, and these 

 transmit and refract the light. The aqueous humour may be 

 seen to gush out if the cornea be cut. Again, the crystalline 

 lens is formed like a magnifying glass to collect into a focus the 

 rays of light which fall from divers sources upon the eye. The 

 vitreous humour is jelly-like and highly transparent, and occu- 

 pies the posterior chamber of the eye. 



The retina is a complex layer of nerve tissue situated at the 

 back of the eye, and most internally. It is in reality an expan- 

 sion of the optic nerve, and it receives the images of external 

 objects, and transmits them to the brain. 



In front, the eyeball is covered with a continuation of the lining 

 of the eyelids called the conjunctiva. As for the eyelids them- 

 selves, they protect the eye from an excess of light, from dust, and 

 dirt. Moreover, the ox is, in common with several other of our 

 domesticated animals, possessed of a structure called the haw, 

 a cartilaginous membrane known scientifically as the membrana 

 nictiia?is. This structure is capable of being voluntarily passed 

 over the eyeball, whereby any foreign body which may have 

 .become lodged there by accident can be cleared away. The 

 surface of the eyeball is moistened by the tears secreted by the 



