438 TEXT-BOOK OF EMBRYOLOGY. 



epichordal portion (optic lobes). It is possible that this connection is secon- 

 dary and contingent upon two functional necessities, the importance of cor- 

 relation with stimuli coming via more caudal nerves (cochlear and spinal 

 nerves) , and the innervation of its motor apparatus by epichordal nerves, the 

 III, IV and VI. With the development of the neopallium in Mammals (see p. 

 447) and the consequent projection of visual stimuli upon it, the lower pre- 

 chordal (thalamic) centers form part of the newer pathway to the neopallium 

 and thus increase in importance, while the optic lobes recede, assuming the 

 position of a reflex center, especially for the visual motor apparatus. 



The olfactory nerves enter the anterior extremity of the brain and are con- 

 nected by secondary and tertiary tracts with regions lying more caudally, where 

 in some cases the olfactory stimuli are associated with gustatory and probably 

 with visual stimuli. One of these regions is the hypothalamus which receives 

 both olfactory and gustatory tracts (Herrick) . More dorsal olfactory pathways 

 pass to the epithalamus. Both epithalamus and hypothalamus give rise to de- 

 scending systems which doubtless ultimately reach efferent nuclei. In fact, this 

 part of the brain presents, apparently, a complicated primitive mechanism for 

 the correlation especially of olfactory and gustatory stimuli, also to some extent 

 of visual stimuli and stimuli via the trigeminal nerve, the whole forming a sort 

 of oral sense, probably controlling the feeding activities (Edinger). 



The next factor in the further development of this part of the brain is the 

 rise in importance of the pallium upon which at first are projected mainly 

 olfactory stimuli (Fig. 370). 



A further and still more extensive development of the pallium arises when 

 other kinds of stimuli are projected to a considerable extent upon it, thus giving 

 rise to a distinction between the older olfactory pallium (archipallium) and the 

 newer non-olfactory pallium (neopallium} . The latter appears first in the lateral 

 dorsal portion of the pallial wall and by its subsequent development the archi- 

 pallial wall is rolled inward upon the mesial surface of the hemispheres. 

 Further changes consist in the extension caudally of this portion pari passu with 

 the extension caudally of the neopallium and then the practical obliteration 

 of its middle portion by the great neopallial commissure, the corpus callosum 

 (Fig. 370, G and H). 



In addition to the increasing projection of stimuli from all parts of the body 

 upon the neopallium and the consequent increase in centripetal fiber termina- 

 tions and in centrifugal neurone bodies lying in its walls, a second factor in 

 the development of the neopallium is the enormous increase of its association 

 neurones. It is the latter feature which especially distinguishes the human 

 from other mammalian brains. 



The biological significance of these changes lies in the fact that there is thus 

 produced a mechanism not only for the association of all kinds of stimuli, but 



