448 



EMBRYOLOGY. 



usually less developed in other features possesses a smooth surface, and thus, 

 as it were, remains permanently in the foetal condition of the human brain. 

 On the other hand the brains of the Carnivores and Primates, owing to the 

 great number of their convolutions, approach more closely to the human brain. 



Finally, in treating of the development of the cerebrum there is 

 still to be considered an appendage to it, the olfactory nerve. This 

 part, as well as the optic nerve, is distinguished from the peripheral 



nerves by its entire development, 

 and must be considered as a 

 specially modified portion of the 

 cerebral vesicle. The older de- 

 signation of nerve is therefore 

 now more frequently replaced by 

 the more appropriate name of 

 olfactory lobe (lobus olfactorius, 

 rhinencephalon). Even at an 

 early stage in the Chick on the 

 seventh day of incubation, in 

 Man during the fifth week (His) 

 there is formed on the floor of 

 each frontal lobe at its anterior 

 end a small evagination, which 

 is directed forward (figs. 240, 

 241 rn). This gradually assumes 

 the form of a club, the enlarged 

 end of which, the part ]ying 

 on the cribriform plate of the 

 ethmoid bone, is designated as 

 the bulbus olfactorius. The bul- 

 bus encloses a cavity which is in 

 communication with the lateral 

 ventricle. 



During the first month of development the olfactory lobe, even in 

 Man, is relatively large and provided with a central cavity. Later 

 it begins to diminish somewhat, the sense of smell being only 

 slightly developed in Man ; its growth is arrested and at the 

 same time its cavity also disappears. In most Mammals, on the 

 contrary, whose sense of smell, as is well known, is more acute 

 than that of Man, the olfactory lobe attains a greater size in the 

 adult animal and exhibits more clearly the character of a part of 

 the brain, for it permanently encloses in its bulb a cavity, which 



Fig. 257. Brain of Galeus canis .in situ, 

 dorsal aspect, after ROHON. 



Lol, Lobus olfactorius ; Tro, tractus nervi 

 olfactorii ; VH, fore-brain, provided at 

 fn with a vascular foramen (foramen 

 nntritium); ZH, between -br ain ; Mff, 

 mid-brain ; Hff, hind-brain ; NH, after- 

 brain ; It, spinal cord ; //, n. opticus ; 

 ///, n. oculomotorius ; IF, 11. trochlearis ; 

 V, n. trigeminus ; L,Trig, lobus trigemini ; 

 C,rest, corpus restiforme ; IX, glosso- 

 pharyngeus; X, vagus; ,t, eminentise 

 teretes. 



