CHAP. X 



THE FOEE-BRAIN 



527 



place in the lateral and frontal walls ; in amphibia and reptiles 

 the pallium is entirely composed of nerve substance; in birds and 

 especially in mammals it reaches a much higher development than 

 all the other brain segments together; and finally, in man it 

 attains the enormous development represented by the cerebral 

 hemispheres. 



It is noticeable that while the development of the pallium of the 

 fore-brain proceeds pari 2^ssu with the higher psychical activity 

 of the animal, the olfactory apparatus and corpus striatum (which 

 develop from the basal lobe of the embryonic brain) present, like 



FIG. 260. Median section through brain of a human embryo in fifth week. (His.) 



all the other segments of the cerebrospinal axis, comparatively 

 little difference throughout the whole scale of vertebrates. 



The olfactory apparatus in the human foetus of two to four 

 months appears in the form of a hollow protuberance from the fore- 

 1 rain ; but during development its walls thicken till the cavity is 

 completely obliterated. In the adult it is possible to distinguish 

 (Fig. 261) :- 



(a) The olfactory bulb, which rests on the lamina cribrosa of the 

 ethmoid and receives through its pores the fibres of the olfactory 

 nerves that originate in the nasal mucosa ; 



(&) The olfactory tract, which divides into two divergent roots ; 



(c) The olfactory area, in which the median or grey roots of the 

 tract arise ; 



(d) The posterior olfactory lobule, formed from that part of the 



