354 MANUAL OF ZOOLOGY SECT. 



parts to the nerve centres, to be translated in consciousness 

 into sensations. When, for example, the skin of some part 

 of the body is touched, the impulse by means of which we 

 become conscious of the contact passes from the surface 

 through branches of the spinal nerves, and enters the spinal 

 cord through the dorsal root, in order to be transmitted to 

 the brain. The ventral root, on the other hand, contains the 

 motor fibres the fibres through which impulses which lead 

 to the contraction of muscles pass outwards from the central 

 nervous system. 



More or less extensive intercommunications take place 

 between the spinal nerves that are situated opposite the 

 origins of the limbs : these spinal nerve-plexuses give off the 

 nerves to the limbs. 



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 dien- 

 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, &c. 



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 eye-ball (Fig. 206) is globular, and is enclosed in a 

 tough opaque capsule the sclerotic (Sc/.). It lies in the 

 cavity of the orbit, and is capable of being turned about in 



