OLFACTORY NERVES. 227 



partially in view. The occipital bone has been divided by an incision passing down from the 

 occipital protuberance and through the condyle to the left of the foramen magnum. The 

 cervical vertebras have been divided to the left of the middle, and the sheath of the spinal 

 cord opened so as to expose the roots of the cervical nerves. 



I. OLFACTORY NERVES. 



The olfactory nerves are 8lender filaments, about twenty in number, which spring 

 from the under surface of the olfactory bulb, and descend to the nose through the 

 foramina in the cribriform plate of the ethmoid bone, each being invested by tubular 

 prolongations of the membranes of the brain. They form an inner group, which are 

 lodged for a short distance in grooves on the surface of the vertical plate of the 

 ethmoid, and then break up into tufts of branches before being distributed to the 

 mucous membrane over the upper part (1 to 1*5 cm.) of that bone, and an outer 



Fig. 144. DISTRIBUTION OF THE OLFACTORY NERVFS ON THE SEPTUM OF THE NOSE. 



(From Sappey, after Hirschfeld and LeveiHe". ) f 



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

 olfactory bulb, and the remaining Roman numbers to the several cranial nerves ; 1, the olfactory nerves 

 as they pass through the cribriform plate ; 2, internal twig of the nasal branch of the ophthalmic 

 nerve ; 3, naso-palatine nerve. The area of distribution of the olfactory nerves is represented too large. 



group which ramify over the mesial surface of the lateral mass of the ethmoid, for 

 a similar extent. The olfactory fibres do not reach the lower edge of the superior, 

 or any part of the middle and lower turbinate bones. 1 The olfactory nerves are 

 composed exclusively of non-medullated fibres, and their branches communicate 

 freely with each other as they descend, thus giving rise to a narrow meshed plexus 

 beneath the mucous membrane, especially on the outer wall of the fossa. (See also 

 the anatomy of the nose.) 



II.-OPTIC NERVE. 



The second pair or optic nerves of the two sides meet each other at the optic 

 commissure where they partially decussate. From this point they may be traced 

 backwards round the crura cerebri, under the name of the optic tracts. 



The optic tract arises from the pulvinar of the optic thalamus, the geniculate 

 bodies, and the superior quadrigeminal body. As it leaves the under part of the 

 thalamus it makes a sudden bend forwards, and then runs, in the form of a 

 flattened band, obliquely inwards across the upper part of the anterior surface of 



1 A. v. Brunn. "Beitriigezur mikroskopischen Anatomic der menschlichen Nasenhohle." Arch. 

 f. mikroskop. Anat. xxxix, 1892. 



