CRANIAL NERVES. 



309 



wide, one and a half inches long, and terminates in the 

 olfactory bulb, which rests on the cribriform plate of the 

 ethmoid. Here it sends off from eighteen to twenty 

 filaments, which pass through the foramina of the cribri- 

 form plate, and are distributed to the mucous membrane 

 of the nose. 



FIG. 139. THE BASE OF SKTTLL, WITH THE NERVES WHICH ESCAPE 



FROM ITS FORAMINA. 

 The cranial nerves are numbered in their customary order. 



2. OPTIC NERVE. Origin, from the optic chiasm, 

 which is formed by the interlacement of the optic tracts. 

 The chiasm consists of three sets of fibres : one set pass- 

 ing from the optic tract directly to the optic nerve of 

 the same side ; another set passing from the optic tract 

 to the opposite optic nerve ; the third set being commis- 

 sural between the optic nerves and between the optic 



