THE CRANIAL XEHVES. 



OLFACTORY NERVE. 



The olfactory or first cranial nerve, the special nerve of the sense of smell, 

 is distributed exclusively to the nasal fossae. 



From the under surface of the olfactory bulb about twenty branches 

 proceed through the holes in the cribriform plate of the ethmoid bone, each 

 invested by tubular prolongations of the membranes of the brain. These 

 tubes of membrane vary in the extent to which they are continued on the 

 branches : the offsets of the dura mater sheathe the filaments, and join the 

 periosteum lining the nose ; those of the pia mater become blended with 

 the neurilemma of the nerves ; and those of the arachnoid re-ascend to the 

 serous lining of the skull. 



Fig. 401. 



XII 



Fig. 401. DISTRIBUTION OP THE OLFACTORY NKRVES ON THE SEPTUM OF THE NOSE (from 

 Sappey after Hirschfeld and Leveille). 



The septum is exposed and the anterior palatine canal opened on the right side. I, 

 placed above, points to the olfactory bulb, and the remaining roman numbers to the roots 

 of the several cranial nerves ; 1, the small olfactory nerves as they pass through the 

 cribriform plate ; 2, internal or septal twig of the nasal branch of the ophthalmic nerve ; 

 3, naso-palatine nerves. (See Fig. 408 for a view of the distribution of the olfactory 

 nerves on the outer wall of the nasal fossa.) 



The branches are arranged in three sets. Those of the inner set, lodged for 

 some distance in grooves on the surface of the bone, ramify in the pituitary 

 membrane of the septum ; the outer set extend to the upper two spongy bones 

 and the plane surface of the ethmoid bone in front of these ; and the middle 

 set, which are very short, are confined to the roof of the nose. The distri- 

 bution of the olfactory > nerve is confined to the upper part of the nasal 

 fossa ; none of the branches reach the lower spongy bone. (See Anatomy 

 of the Nose.) 



OPTIC NERVE. 



The optic or second cranial nerve, the nerve of vision, extending from 

 the optic commissure, becomes more cylindrical and firm as it diverges from 

 its fellow and enters the orbit by the optic foramen. Within the orbit it 

 forms a cylindrical trunk, thick and strong, with a uniform surface. On dis- 



