THE NERVOUS SYSTEM. 437 



The Mid-brain Roof. 



This expansion of the dorsal part of the neural tube constitutes a higher 

 coordinating center for impulses received by various somatic nerves spinal, 

 cochlear and optic. Owing to its being, in all forms below Mammals, the 

 principal visual center, the optic part (optic lobes) varies in proportion to the 

 development of the eye, animals with poorly developed eyes having small optic 

 lobes. In Mammals, the optic part (anterior corpora quadrigemina or col- 

 liculi) is relatively less important, owing to a taking over of a portion of its 

 coordinating functions by the neopallium (pp. 440, 442) , but the cochlear part 

 (posterior corpora quadrigemina or colliculi) has increased in importance, 

 owing to the rise of the cochlear organ (organ of Corti). The centripetal and 

 centrifugal connections of the mid-brain roof are not so massive or extensive 

 and consequently do not modify the other parts of the brain and cord as pro- 

 foundly as do those of the cerebellum. It sends descending tracts to after- 

 brain and cord segments. 



The Prosencephalon. 



The division of this part of the brain into the telencephalon and diencephalon 

 has already been indicated (p. 425). In the diencephalon may be noted (i) the 

 absence of the notochord ventral to the brain, thereby permitting a ventral ex- 

 pansion of the brain walls, the hypothalamus, associated with an organ not 

 well understood, the hypophysis; (2) certain more or less vestigial structures, 

 such as the pineal eyes (epiphyses), and other primitive structures, such as 

 the ganglia habenulae, in the dorsal part, this dorsal portion being collectively 

 termed the epithalamus; (3) nuclei in (i) and (2) connected with olfactory 

 and gustatory tracts; (4) receptive nuclei for the optic tract and the cochlear 

 path from the posterior colliculus; (5) receptive nuclei for secondary tracts from 

 the end stations of more caudal somatic ganglia (nuclei of Go 11 and Burdach 

 and medial lemniscus). The last two (4 and 5) constitute the thalamus and 

 increase in importance in the higher Vertebrates (see p. 440, Fig. 371). 



In the telencephalon there may be roughly distinguished an anterior and basal 

 part, the rhinencephalon, in especially intimate relations with the olfactory nerve; 

 a thickening of the basal wall, the corpus striatum ; and a thinner- walled dorsal 

 part, the pallium. The latter may be regarded in a sense as a dorsal develop- 

 ment of the corpus striatum and first appears as a distinct structure in the 

 Amphibia. 



The peripheral or segmental apparatus which are connected with the pros- 

 encephalon are the highly modified optic and olfactory organs. While the optic 

 apparatus primarily originates from the prechordal brain, in the lower Verte- 

 brates its highest coordinating center, as mentioned above, lies partly in the 



