SECOND PAIR OPTIC NEEVES. 



491 



FIRST PAIE — OLFACTORY NERVES. 



The olfactory, or nerve of the special sense of smell, arises by 

 i;hree roots — an inner, short root of -white fibres, derived from 

 the inner and posterior part of the anterior lobe ; an outer, long 

 root, also white, which crosses the Sylvian fissure into the 

 greater lobe, where it is continuous with fibres of the optic 

 thalamus ; and a middle or grey root, which also contains white 

 fibres from the corpus striatum, and arises from a papilla of grey 

 matter (caruncula) in the mammillary eminence. The union of 

 these roots forms the olfactory 

 lobe, which, as it passes forward, 

 swells into an oblong mass, the 

 olfactory bulb, which rests on the 

 cribriform plate of the ethmoid 

 bone. From the under part of 

 the bulb spring the olfactory 

 nerve filaments, which pass 

 through the foramina in the cri- 

 briform plate, and are distributed 

 in the mucous membrane of the 

 posterior part of the nasal fossa. 

 These filaments are divisible into 

 three groups, an outer, sent to 

 the turbinated bones and sur- 

 face of the ethmoid; a middle, fig. i85. 



, . , , . , , Base of the brain— showing the apparent 



which supplies the mucous mem- origins of the craniid nerves, a, Olfactory; 



brane of the roof of the fossa; ^' ^p"'': "> ^°*°"' '^°^- ^- bathetic; «, 



, ' Trifacial ; /, Abducens ; g, Portio-dura ; 



and an inner, spread over the a, PorUo-moiiis; t, oiosso-pharyngeai; *, 



septum nasi. Pneumogastric: I, Spinal accessory: m, 



t^ Hypo-glossaL 



SECOND PAIR — OPTIC NERVES. 



The optic, or nerve of the special sense of sight, is a large 

 nerve, which has its deep origin in the corpora geniculata, optic 

 thalamus, and corpora quadrigemina ; the fibres from these three 

 different sources uniting to form the optic tract, which winds 

 round the brus cerebri to join its fellow, and form the optic com- 

 missure. After leaving the commissure, the nerves become 

 rounded, and diverge to pass through the optic foramina, where 

 they receive a covering of dura mater, which immediately splits 



