THE OPTIC NERVES 335 



These nerves have each a superficial and a deep origin. 

 The former corresponds to its point of attachment at the 

 surface of the brain; the latter to certain nuclei or collections 

 of nerve cells in the structure of the brain. 



The Olfactory Nerves 



The olfactory nerves, or fila, are the special nerves of the 

 sense of smell. Twenty in number on each side. They are 

 distributed to the olfactory region in the upper part of the 

 superior turbinated process of the ethmoid and corresponding 

 portion of the nasal septum. These filaments represent the 

 axones of the olfactory cells and pass through the cribriform 

 plate of the ethmoid to join the under surface of the olfactory 

 bulb, which rests on the cribriform plate, and is the oval 

 mass of a grayish color that forms the anterior extremity of 

 a slender process of brain substance called the olfactory tract. 

 The olfactory nerves differ in structure from the other nerves, 

 containing only amyelinic fibers. 



The Optic Nerves 



The fibers of the optic nerves, the special nerves of the sense 

 of sight, are situated in the retina; they start as the central 

 processes of the ganglion cells, which converge and pierce the 

 choroid and sclera as a cylindric cord. The point of emergence 

 is situated a little internal to the posterior pole of the globe. 

 Passing through the orbital fat, in an inward and backward 

 direction, it passes through the optic foramen to end in the 

 optic chiasm. The optic chiasm is somewhat quadrilateral 

 in shape, rests on the olivary eminence and the diaphragma 

 sellse, being bounded above by the lamina terminalis; behind, 

 by the tuber cinereum; on either side by the anterior perforated 

 substance. Within the chiasm the fibers decussate as follows: 

 Those from the nasal side of the left and right halves of the 

 retina cross in the centre, to the opposite optic tract; those 

 from the temporal side of the right and left eyes pass back- 

 ward without decussating, to end in the optic tract. In the 

 posterior portion of the chiasm is Gudden's commissure, which 

 contains fibers, completing an arc with the medial geniculate 

 bodies on either side. The fibers from the temporal sides of 



