THE CRANIAL NERVES. 449 



the dog, cat, sheep, and most other quadrupeds, both the olfactory 

 bulbs and the olfactory tracts are increased in a similar ratio. There 

 is accordingly a direct correspondence between their development and 

 that of the special sense with which they are connected. 



III. A number of cases are quoted by Longet in which congenital 

 absence of the olfactory nerves, in man, was accompanied by congenital 

 incapacity to distinguish odors ; and others in which loss of smell was 

 observed after affections causing their compression or destruction. 



Finally, experimental division or destruction of these nerves in dogs 

 abolishes, so far as observation can show, the power of discriminating 

 odors ; although it leaves the nasal mucous membrane sensitive to 

 pungent or caustic vapors. In the experiments of Magendie,* a dog, 

 after destruction of both olfactory nerves, would disentangle a package 

 containing meat when openly presented to him; but he did not find it, 

 when placed near by without his knowledge. The same result was 

 obtained by Vulpian ( in experiments upon hunting dogs. These ani- 

 mals, after recovering from the immediate effects of the operation, 

 were kept fasting for two days, and then introduced into an apart- 

 ment where a piece of cooked meat was concealed ; but they were 

 never able to discover it, when the division of the nerves had been 

 complete. Notwithstanding, therefore, the difficulty of experimenting 

 upon so obscure a function as that of smell, there is no doubt that the 

 olfactory nerves and bulbs are the internal organs of the olfactory 

 sense, and that they are disconnected both with ordinary sensibility and 

 the power of motion. 



Second Pair. The Optic Nerves. 



The optic nerves are distinguished by their very prominent decussa- 

 tion at the base of the brain, where they present the appearance of 

 being consolidated with each other. By this decussation, which is 

 called the " chiasma, "J they are divided into two portions. The optic 

 nerves proper, situated in front of the chiasma, are nearly cylindrical 

 in form and consist of fibres coming directly from the retina on each 

 side. Behind the chiasma they are known as the " optic tracts," and 

 appear as flattened bands of nerve fibres, connecting the visual organs 

 with the central parts of the brain. The optic tract on each side, after 

 following the contour of the crus cerebri in a backward direction, divides 

 into two roots, an internal and an external. The internal root is con- 

 nected with the corpus geniculatum internum, through and over which 

 its fibres pass, continuing their course upward and backward until they 

 reach the anterior tubercula quadrigemina. The external root, which 

 is the larger of the two, is attached to the corpus geniculatum externum. 



* Journal de Physiologie Experimental et Pathologique. Paris, 1825, tome iv., 

 p. 170. 



f Lepons sur la Physiologie du Systeme Xerveux. Paris, 1866, p. 882. 



J This term is of Greek origin, and is derived from a verb which signifies to mark 

 with the letter x. 



2D 



