106 ZOOLOGY. 



central nervous system. It is convenient, however, to treat 

 them as true nerves; and we may then say that the 

 olfactory nerve (i.) has its origin in the olfactory lobe, 

 runs through numerous foramina in the ethmoid bone, and 

 is distributed to the olfactory membrane. 



We may mention two interesting points : first, that the 

 olfactory fibres are the only ones in the system which do 

 not decussate at all ; and secondly, that they are non- 

 medullated. 



The optic nerve (n.) has its superficial origin in the 

 floor of the thalamencephalon, just in front of the infundi- 

 bulum : this region is called the optic chiasma (i.e. crossing), 

 because here the fibres decussate. The deep origin is, how- 

 ever, in the optic lobes (corpora quadrigemina), whence the 

 fibres may be traced through the side-walls of the thalam- 

 encephalon (hence called optic tracts) to the chiasma. 

 The course of the optic nerve is forwards and slightly 

 outwards, through an opening in the orbitosphenoid called 

 the optic foramen (fig. 46), straight to the eye. 



The auditory nerve (viii.), although we have grouped it 

 with these two as a purely sensory nerve, does not agree 

 with them in the peculiarities above-mentioned. We shall 

 consider it in its proper order later on. 



10. Oculomotor Series. Nerves in., iv., and vi. are 

 solely concerned with the innervation of (i.e. the supply 

 of nerve-fibres to) the muscles of the eye, and we shall 

 mention their exact distribution when describing that 

 organ. They are peculiar in their superficial origin, in. 

 and vi. arising from the floor of mid- and hind-brain 

 (crura and pons) respectively, while iv. arises from the 

 dorsal surface, in the depression between the optic lobes 

 and the cerebellum. Yet their deep origin is practically 

 the same, being the tract of grey matter around the iter. 

 If these three nerves are to be compared with spinal nerves 

 at all, they must be regarded as ventral roots only: the 

 ventral emergence of in. and vi. agrees with this, but the 

 dorsal appearance of iv. is a puzzle. All three nerves 

 leave the skull through a foramen in the alisphenoid called 

 the foramen lacerum anterius (tig. 46). 



