OLFACTORY LOBE. 



79 



mass as furnishes the ganglia of the auditory nerve, but its cells are early distin- 

 guishable by their larger size and clearer appearance. From these cells fibres, which 

 are probably afferent (His), grow centrally into the hind-brain and peripherally along' 



Fig. 90, C. SECTION FROM THE SAME EMBRYO AT THE EXIT OF THE FACIAL NERVE. (His.) 

 (Several sections have been combined to form this figure.) 



VI., fibres of sixth nerve taking origin from group of neuroblasts in basal lamina ; VII. G. g, gang- 

 lion geniculi of the facial ; \IlI.Gf.i.c, intracranial ganglion of auditory ; VIII. G.v, ganglion vestibuli ; 

 VIII. G.c, ganglion cochleae. 



the nerve, mingling with its efferent fibres. Some of these afferent fibres may form 

 the chorda tympani, but there are many more than are found in that nerve. 



The optic nerves take origin as hollow outgrowths of the brain, which afterwards 

 become solid, while nerve-fibres become developed in their walls. Their mode of 

 origin will be further treated of in connection with the development of the eye. 



The olfactory lobe, which consists of the olfactory bulb and tract (often 

 spoken of as the first or olfactory nerve), and the part of the base of the brain 

 from which the tract arises, makes its appearance as a protrusion of the antero- 

 ventral part of each cerebral hemisphere, extending towards the thickened olfactory 

 area of the epiblast (see fig. 70, B, Rl ; and fig. 74, olf ). This primitive olfactory 

 lobe is early seen to be divided into anterior and posterior parts by a broad sulcns 

 (fig. 95). Of the two parts, the anterior becomes considerably elongated, and 

 ultimately forms not only the tract and bulb but also the trigonum olfactorium, 

 and a small area on the mesial side of this (termed by His BroccCs area) ; whilst 

 from the posterior, the larger part of the anterior perforated space (mesial to 

 the lateral olfactory root) and the peduncle of the corpus callosum (gjrus 



