584 OLFACTORY AND OPTIC NERVE. 



with each other, as, for example, in co-ordinated movements, and in extensive reflex 

 acts. 



THE CRANIAL NERVES. 



343. I- NERVUS OLFACTORIUS. Anatomical. The three-sided prismatic tractus olfac- 

 torius, lying in a groove on the under surface of the frontal lobe, arises by means of an inner, 

 outer, 'and middle root, from the tuber olfactorium (tig. 428, 1). The tractus^ swells out upon 

 the cribriform plate of the ethmoid bone, and becomes the bulbus olfactorius, which is the 

 analogue of the special portion of the brain, existing in different mammals with a well-developed 

 sense of smell (Gratiolet). From twelve to fifteen olfactory filaments pass through the foramina 

 in the cribriform plate of the ethmoid bone. At first they lie between the periosteum and the 

 mucous membrane, but in the lower third of their course they enter the mucous membrane of 

 th- regio olfactoria. The bulb consists of white matter below, and above of grey matter mixed 

 with small spindle-shaped ganglionic cells. Henle describes six, and Meynert eight layers, of 

 nervous matter seen on transverse section. [The centre for smell lies in the tip of the uncinate. 

 gyrus on the inner surface of the cerebral hemisphere {Fcrricr).~\ According to Gudden, removal 

 of the olfactory bulb is followed by atrophy of the gyrus uncinatus on the same side. Accord- 

 ing to Hill, the three roots of the olfactory bulb stream backwards, the inner one is small, the 

 middle one is a thick bundle, which grooves the head of the caudate nucleus, curves inwards 

 to the anterior commissure, and crosses rid this commissure where it decussates, and passes to 

 the extremity of the temporo-sphenoidal lobe. The outer roots pass transversely into the 

 pyriform lobe, thence rid the fornix, corpora albicantia, the bundle of Vicq d'Azyr into the 

 anterior end of the optic thalamus. Hill also points out that the elements contained in the 

 olfactory bulb are identical with those contained in the four outer layers of the retina. Flechsig 

 traces its origin (1) to the gyrus fornicatus, (2) through the lamia perforata anterior to the 

 internal capsule (sensory part), and to the gyrus uncinatus (sensory area of the cerebrum) ( 378, 

 IV. ). Probably the fibres at their origin cross to the cerebrum. There is a connection between 

 the olfactory bulbs in the anterior commissure. [Each nerve is related to both hemispheres.] 



Function. It is the only nerve of smell. Physiologically, it is excited only by 

 gaseous odorous bodies (Sense of Smell, 420). Stimulation of the nerve, by any 

 other form of stimulus, in any part of its course, causes a sensation of smell. [It 

 also conveys those impressions which we call flavours, but in this case the sensation 

 is combined with impressions from the organs of taste. In this case also the stimulus 

 reaches the nerve by the posterior nares.] Congenital absence or section of both 

 olfactory nerves abolishes the sense of smell (easily performed on young animals 

 Biffi). 



Pathological. The term hyperosmia is applied to cases where the sense of smell is 

 excessively and abnormally acute, as in some hysterical persons, and in cases where there is a 

 purely subjective sense of smell, as in some insane persons. The latter is perhaps due to an 

 abnormal stimulation of the cortical centre ( 378, IV.). Hyposmia and anosmia (i.e., 

 diminution and abolition of the sense of smell) may be due to mechanical causes, or to over- 

 stimulation. Strychnin sometimes increases, while morphia diminishes, the sense of smell. 

 [Method of Testing, 421.] 



344. II. NERVUS OPTICUS. Anatomical. The tractus opticus (fig. 428, II) arises from 

 the anterior corpora quadrigemina, the corpus geniculatum externum, and the thalamus opticus 

 (fig. 428), as well as from the grey matter which lines the third ventricle (Tartuferi). A 

 broad bundle of fibres passes from the origin of the optic tract to the cortical visual centre, 

 at the apex of the occipital lobe on the same side (Wernicke 379, IV.). Fibres pass from 

 the cerebellum through the crura. 



The optic tract bends round the pedunculus cerebri, where it unites with its fellow of the 

 opposite side to form the chiasma, and from the opposite side of this the two optic nerves 

 spring. 



[Connections of Optic Tract. There is very considerable difficulty in ascertaining the exact 

 origin of all the fibres of the optic tract. Although as yet the statement of Gratiolet is not 

 proved that the optic tract is directly connected with every part of the cerebral hemisphere in 

 man, from the frontal to the occipital lobe, still the researches of D. J. Hamilton have shown 

 that its connections are very extensive. It is certain that some of them are ganglionic, i.e., 

 connected with the ganglia at the base of the brain, while others are cortical, and form connec- 

 tions with the cortex cerebri. . The ganglionic fibres arise from the corpora geniculate, pulvinar, 

 and anterior corpora quadrigemina, and probably also from the substance of the thalamus. The 

 cortical fibres join the ganglionic to form the optic tract. According to D. J. Hamilton, the 

 connection with the cortex in the frontal region is brought about by "Meynert's commissure." 

 The latter arises directly from the lenticular-nucleus-loop, decussates in the lamina cinerea, and 



