516 THE CRANIAL NERVES. 



into an apartment where a piece of cooked meat was concealed ; but 

 they were never able to discover it by its odor, when the division of 

 the nerves had been complete. Notwithstanding, therefore, the com- 

 parative difficulty of experimenting upon so obscure a function as that 

 of smell, there is no doubt that the olfactory nerves and bulbs are really, 

 the internal organs of the olfactory sense, and that they are disconnectedS 

 both with ordinary sensibility and the power of motion. 



Second Pair. The Optic Nerves. 



The optic nerves take their first origin from the anterior pair of the 

 tubercula quadrigemina, two small rounded prominences, on each side 

 of the median line, situated just behind the posterior extremity of the 

 optic thalami. They consist essentially of swellings of the gray sub- 

 stance which surrounds at this situation the aqueduct of Sylvius, and 

 which is consequently continuous with that extending forward from the 

 floor of the fourth ventricle. Their surface is covered by a layer of 

 white substance from 1.5 to 4 millimetres in thickness, consisting of 

 nerve fibres which have mainly a transverse direction. Their gray sub- 

 stance contains nerve cells, some of which are small and rounded, while 

 others, especially in the anterior pair, are of larger size and provided 

 with branched prolongations. According to Henle, the fibres of origin 

 of the optic nerve pass from these bodies outward and downward to the 

 corpus geniculatum internum, an ovoidal prominence of gray matter 

 attached to the posterior border of the optic thalamus. They cover the 

 surface of this body in a thin superficial layer, and continue their course, 

 winding round the lateral surface of the crus cerebri ; where they are 

 joined at an acute angle by a second bundle of fibres, coming from the 

 corpus geniculatum externum, a gray eminence similar to the last, 

 lying in contact with the under part of the optic thalamus, but isolated 

 from its gray matter by a thin investing layer of white substance. 

 These two bundles of fibres, coming, one from the anterior tubercula 

 quadrigemina and the corpus geniculatum internum, the other from the 

 corpus geniculatum externum, form in man the two roots of the optic 

 nerve ; and the collections of gray matter contained in these bodies are 

 regarded as its " nuclei," or the nervous centres with which it is in 

 anatomical communication. It also receives some fibres from the sub- 

 stance of the optic thalamus itself. 



The fibres derived from these sources form a flattened band which 

 continues its course in a spiral direction, winding round the crus cerebri 

 to the base of the brain ; it thence runs forward and inward until the 

 two, from the right and left sides, meet upon the median line in the 

 so-called "chiasma," or decussation. From this they again diverge out- 

 ward and forward, leave the cavity of the cranium by the optic fora- 

 mina, and, joining the eyeballs, terminate in the nervous expansion of 

 the retina. That portion of the optic nerves situated behind the decus- 

 sation is sometimes designated by the special name of the "optic 

 tract." 



