8:8 A MANUAL OF PHYSIOLOGY 



be caused by lesions in any part of the rhinencephalon, and also by 

 changes in the olfactory mucous membrane and olfactory fibres ; 

 but the symptoms do not obtrude themselves, and are doubtless 

 often overlooked. Excessive stimulation of the olfactory nerve by 

 exposure to a strong odour has been said to cause complete and 

 permanent loss of smell. 



The second or optic nerve contains mainly afferent fibres, which 

 arise from the ganglion cells of the retina, and terminate by forming 

 synapses with nerve-cells in the lateral or external geniculate body, 

 the pulvinar (or posterior portion) of the optic thalamus, and the 

 anterior corpus quadrigeminum. In young animals all these 

 structures undergo atrophy after extirpation of the eyeball. The 

 visual path is continued from the pulvinar and the external corpus 

 geniculatum by the axons of these nerve-cells, which proceed in 



FIG. 341. SCHEME OF THE OLFACTORY NERVOUS APPARATUS (HALLI- 

 BURTON, AFTER CAJAL). 



A, olfactory cells ; B, glomeruli ; C, mitral cells ; D, olfactory granule cell ; 

 E, lateral root of olfactory tract ; F, cortex of brain in the region of the uncinate 

 gyrus ; a, small cell of mitral layer ; b, brush of dendrite of a mitral cell ending in 

 a glomerulus ; c, thorns or spines on the processes of an olfactory granule ; 

 e, collateral coming off from the axon of a mitral cell ; /, collaterals ending in the 

 molecular layer of the uncinate gyrus ; g, pyramidal cells of the cortex ; h, sup- 

 porting epithelial cells of the olfactory mucous membrane. 



the optic radiation (p. 785) to the occipital cortex. The fibres which 

 pass from the retina to the anterior corpus quadrigeminum are 

 distinguished by their small size, and probably constitute the path 

 of the impulses which cause contraction of the pupil when light 

 falls on the retina. The reflex arc is schematically shown in Fig. 342, 

 where optic nerve-fibres are represented as forming synapses with 

 cells in the anterior corpus quadrigeminum whose axons pass to 

 the nucleus of the third nerve and arborize around some of its cells 

 (Figs. 326, 331, and 355). At the chiasma the fibres of the optic 

 nerve decussate, partially in man and some mammals, as the rabbit, 

 dog, cat, and monkey, completely in animals whose visual field is 

 entirely independent for the two eyes, as in fishes and birds. In 

 man the fibres for the nasal halves of both retinae cross the middle 

 line at the chiasma, those for the temporal halves do not. This 

 does not mean, however, that exactly half of the optic nerve-fibres 



