7 i8 A MANUAL OF PHYSIOLOGY 



give off axons, which pass through the fourth layer, and then as fibres 

 of the olfactory tract to the grey matter at the base of the brain. The 

 course of the impulses from the olfactory mucous membrane to the 

 brain is shown in Fig. 257. The olfactory tract, as it runs back, 

 divides into portions called its 'roots.' Of these the lateral is the 

 most important, and it terminates in the uncinate gyrus of the same 

 side. The median root crosses to the opposite side of the brain, and 



FIG. 256. SCHEMATIC TRANSPARENT SAGITTAL SECTION OF MEDULLA 



OBLONGATA. 



The numerals V to XII refer to the nuclei of origin of the respective cranial nerves. 

 V is the motor nucleus of the fifth neive ; RV, the roots, and V, the sensory nucleus ; 

 V", sensory nucleus and ascending or spinal root of the fifth ; RVI, root of sixth 

 nerve; RVII, root of seventh nerve ; Py, pyramidal fibres. Py. kr. , decussation of 

 the pyramids ; O.s., superior olive ; O, olive ; G. genu of the facial nerve ; K, clavate 

 nucleus; VIII, inner auditory nucleus. 



some, though not all, of its fibres go to the opposite uncinate gyrus. 

 Excessive stimulation of the olfactory nerve by exposure to a strong 

 odour has been known to cause complete and permanent loss of smell. 

 The second or optic nerve contain-s mainly afferent fibres, which 

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

 synapses with nerve-cells in the lateral geniculate body, the pulvinar 

 (or posterior portion) of the optic thalamus, and the anterior corpus 

 quadrigeminum. This visual path is continued by the axons of 



