ORIGIN OF THE CRANIAL NERVES. 



727 



They are very soft, and break off close to their origin when the brain 

 is removed from the skull. 



The SECOND or OPTIC (fig. 260, 2 ) is the largest of the cranial Second 

 nerves except the fifth, and appears on the crus cerebri as a flat ptic: 

 band (the optic tract), which is directed inwards to join the one of 

 the opposite side in a commissure. The name optic nerve is confined part called 

 to the portion in front of the commissure which is round and firm. rac * 

 The destination of the nerve is the eyeball. 



The optic tract winds round the crus cerebri to end, in front in Optic tract: 



FIG. 260. BASE OF THE BRAIN, WITH ORIGIN OP THE CRANIAL NERVES. 



7. Facial and auditory, the former 

 smaller and internal. 



8. Glosso-pharyngeal, pneumo- 

 gastric, and spinal accessory nerves, 

 in order from above downwards. 



9. Roots of hypoglossal nerve. 



1. Olfactory lobe. 



2. Optic commissure. 



3. Oculomotor. 



4. Trochlear. 



5. Trigeminal, with small and 

 large root. 



6. Abducent. 



the commissure. Behind it divides into two pieces which will be 

 subsequently seen to take their origin from the optic thalamus, the origin now 

 corpus geniculatum externum, and the superior corpus quadrigemi- concealed > 

 num. As the tract passes forwards it is attached to the crus cerebri 

 by its outer or anterior edge ; and internal to the crus it is placed relations, 

 between the anterior perforated spot on the outer, and the tuber 

 cinereum on the inner side ; it is said to be joined here by 

 additional fibres springing from the latter body. 



The commissure (chiasma) of the nerves measures nearly half an Its commis- 



