260 



THE ANATOMY OF THE HORSE. 



twelve, and the other nine, pairs of nerves. This diversity of nomen- 

 clature is apt to lead to confusion, but fortunately this confusion does 

 not extend to veterinary anatomy, in which, both at home and abroad, 

 the first and most natural of these methods is exclusively employed. 

 This system is also that employed by human anatomists on the 

 Continent, but by British human anatomists the number of cranial 

 nerves is stated as nine pairs. The following table exhibits in the 

 central column the special names of the nerves, and in the side 

 columns their numerical designations under the two systems : — 



7th 



■ 8th 



Olfactory nerves 1st pair 



Optic nerves ...... 2nd ,, 



Oculo-motor nerves ..... 3rd ,, 



Pathetic or Trochlear nerves . . . 4th ,, 

 Trifacial or Trigeminal nerves . . . 5th ,, 

 Abducent nerves ..... 6th ,, 



Facial nerves (Portio dura) 



Auditory nerves (Portio mollis) 



Glosso-pharyngeal nerves 



Pneumogastric or Yagus nerves 



Spinal Accessory nerves . 



Hypoglossal nerves 9th , , 



The Olfactory or 1st nerve. The fibres of this nerve leave the 

 surface of the olfactory bulb, and pass through the foramina of the 

 cribriform plate to reach the summit of the nasal chamber. They are 

 there distributed in the olfactory division of the lining membrane of 

 that chamber. 



The Optic or 2nd nerve arises from the optic chiasma or commissure, 

 and reaches the back of the orbit by passing through the optic foramen. 

 Piercing the sclerotic and choroid tunics of the eyeball, its fibres radiate 

 outwards and form one of the layers of the retina. As already seen, the 

 optic chiasma is formed by the fusion of the optic tracts, each of which 

 derives its fibres from the optic thalamus, corpora geniculata, corpora 

 quadrigemina, and decussation of the pyramids. In the optic chiasma 

 some of the fibres of each tract cross and are continued in the optic 

 nerve of the opposite side. Some of the fibres of each optic tract, it is 

 stated, cross in the chiasma and return to the brain by the opposite 

 tract, while in the same way fibres pass from the one optic nerve to the 

 other optic nerve. 



The Oculo-motor or 3rd nerve arises from the inner side of the crus 

 cerebri by a number of bundles, the fibres of which are traceable to 

 nerve cells in the corpora quadrigemina. The nerve leaves the cranium 

 by the foramen lacerum orbitale, and reaches the orbit. 



The Pathetic, Trochlear, or 4th nerve, appears to rise in the valve 

 of Vieussens (Plate 35). Some of its fibres are decussate with those of 

 the opposite nerve, and the others are traceable to nerve cells of the 

 locus cteruleus, or of the corpora quadrigemina. Emerging from the 



