356 MANUAL OF ZOOLOGY SECT. 



The cerebral or cranial nerves correspond pretty closely 

 in their general arrangement in the three examples. The 

 olfactory nerve-fibres, which originate from the olfactory 

 lobes, the optic nerves, which are derived from the thalamen- 

 cephalon, and the auditory nerves which originate from the 

 medulla oblongata, are the nerves of the special senses of 

 smell, sight, and hearing respectively, the first ending in the 

 epithelium of the nasal cavities, the second in the retina of 

 the eye, and the third in the epithelium of the interior of 

 the inner ear. Other cranial nerves supply the muscles 

 that move the eyeball, the skin of the head, the muscles of 

 the jaws, the tongue, pharynx, heart, stomach, etc. 



The structure of the eye is in all essential respects the 

 same in all the three examples ; such differences as there 

 are will be referred to later. The eye of a bullock or a 

 sheep, being larger, may with advantage be substituted. 

 The eyeball is globular, and is encased in a rough opaque 

 capsule, the sclerotic. It lies in the cavity of the orbit, and 

 is capable of being turned about in various directions by a 

 number of muscles inserted into it. On the side of the 

 eyeball directed towards the light, the opaque sclerotic is 

 replaced by a transparent membrane, the cornea, which 

 forms a window through which the rays of light enter the 

 eye. Within the sclerotic is a more delicate pigmented 

 layer, the choroid. Towards the cornea the choroid passes 

 into a circular pigmented diaphram, the iris, the opening 

 of which is known as the pupil. Through the pupil, the 

 size of which is capable of being increased or diminished, 

 the light is admitted into the interior of the eye. The sen- 

 sitive part of the eye, the part on which the image produced 

 by the rays of light proceeding from an object must fall in 

 order to produce the sensation of sight, is a soft gray layer 

 lining that part of the cavity of the eye which lies within the 



