OPTIC NERVE AND TRACT. 679 



cate gyrus, median root (Fig. 262) . (2) The lateral root passes through the anterior 

 perforated plate to the internal capsule (sensory path of the cerebrum) , and further 

 through the uncinate gyrus (sensory cortical center) , where its fibers enter into 

 contact with the ganglion-cells by means of telodendrites. Possibly the fibers of 

 origin decussate within the cerebrum. (3) Fibers may be traced also in the head 

 of the caudate nucleus and thence into the anterior commissure, in which there 

 is a communication between the two olfactory bulbs. 



The olfactory nerve is the nerve of smell, the physiological excita- 

 tion of which takes place only through volatile odorous substances. 

 Congenital deficiency or division of both nerves destroys the sense of 

 smell. 



Pathological. The designation hyperosmia is applied to a condition in which 

 the acuity of the sense of smell is abnormally exaggerated, for example in hysterical 

 individuals. Purely subjective impressions of the sense of smell, olfactory hal- 

 lucinations, for example in the insane, probably depend upon abnormal excitation 

 of the cortical center. In some persons the ingestion of antifebrin, which is 

 odorless and tasteless, excites a subjective sense of smell even when the existence 

 of marked coryza renders the nose incapable of smelling. Hyposmia and anosmia, 

 diminution and abolition of the sense of smell, occur as the result of catarrhal 

 conditions of adjacent cavities, through the action of injurious gases or fluids, 

 as one of the phenomena of general intoxication or disease, and in consequence of 

 absence of the pigment in the olfactory region. Strychnin increases and morphin 

 occasionally diminishes the sense of smell. 



II. OPTIC NERVE AND TRACT. 



The optic tract arises from the anterior quadrigeminal body, from the lateral 

 geniculate body, from the pulvinar and the zonal stratum of the optic thalamus 

 (Fig. 242) and from the tuber cinereum. By means of a broad bundle of fibers 

 (visual fibers of Gratiolet) , which pass directly 

 outward from the posterior horn, the origin 

 in the parts named is connected with the 

 cortical psycho-visual center in the occipital 

 lobe of the same side. Fibers pass from the 

 cerebellum through the crura. 



The two tracts unite to form the chiasm, 

 from which on each side arises the optic nerve, 

 the fibers of which are medullated but without 

 neurilemma. 



In the chiasm a semidecussation of the 

 fibers takes place as a rule (Fig. 240), so that 

 the left tract sends fibers into the left half 



of each retina and the right tract fibers into 



.1 Q .jrrU*. i, if FIG. 240. Diagrammatic Representation of 



the right half. the | emide cuisation of the Optic Nerves. 



It can thus be understood that in man 

 destruction of one tract causes so-called 



homonymous hemiopia, that is blindness of the two corresponding retinal halves 

 in the sense described. As the left half of each retina receives impressions from 

 the right half of the visual field, and conversely, all fibers intended for seeing 

 objects in the right half of the visual field are situated in the left tract, and vice 

 versa. The same effect is produced by destruction of its origins, as by de- 

 struction of the optic tract, according to' Bechterew also by that of the external 

 geniculate body and the anterior brachium alone. Sagittal division of the chiasm 

 has caused in man in one case blindness of the nasal half of each retina. In ex- 

 ceedingly rare cases the decussation has been wholly wanting in man. 



Among animals partial decussation occurs in the following : Ape, cat, dog; 

 complete decussation in the rabbit, mouse, guinea-pig, pigeon, owl. In the bony 

 fish both optic nerves cross separately; in the cyclostomata there is no decussation 

 at all. Two commissures lying upon the optic chiasm, that of Meynert and that 

 of Gudden, have really nothing whatever to do with the optic nerve. 



After extirpation of an eye in man there degenerate centripetally the fibers 

 that enter the optic nerve from the organ, therefore in man half of the fibers 

 in each tract. The degeneration extends to the origins in the quadrigeminal 



