154 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



sides of the central nervous system. In the spinal cord the cross 

 fibres are numerous (fig. 169, which shows them beneath the central 

 canal). In the brain there are several yet to be mentioned. From 

 its position this one at the junction of mid- and 'twixt-brains is called 

 the posterior commissure (figs. 160, 167, pc). 



Diencephalon. — The 'twixt-brain is Umited behind by the poste- 

 rior commissure, in front by the velum transversum above, and the 

 lamina terminalis below (fig. 160); its cavity is the third ventricle, 

 to the region of the optic chiasma. 



The side walls are thick while roof and floor plates, though much 

 modified, still retain to a very large extent their primitive non-nerv- 

 ous character. The fibre tracts connecting the cerebrum with the 



c^ sc 



Fig. 160. — Sagittal section of brain of Ichthyophis, after Burckhart. a, aqueduct; 

 ac, anterior commissure; c"-^, chorioid plexuses of lateral, third and fourth ventricles; 

 cb, cerebellum; ec, pallial commissure;/, interventricular foramen; e, epiphysis; Ag, hn, 

 glandular and nervous parts of hypophysis; i, infundibulum; It, lamina terminalis; m, 

 mid-brain; p, paraphysis; oc, optic chiasma; pc, posterior commissure; sc, superior 

 commissure; III, IV, third and fourth ventricles. 



parts farther back pass in the side walls; the dorsal ones go through 

 the thalamus where there is a large nerve centre, while the ventral 

 tracts are the cerebral peduncles already mentioned. As already 

 explained, the thalamus is developed from the dorsal zone, and above 

 and in front of it is the epithalamus, which contains a nerve centre, 

 the habenula (fig. 167, h). In the hypothalamus of the ventral zone 

 are two pairs of centres, the tuber cinereum in front and the mam- 

 millary bodies behind. Epithalamus and hypothalamus are con- 

 nected with the sense of smell and are relatively larger in the lower 

 than in the higher vertebrates. Just behind the velum transversum 

 is the superior (habenular) commissure (fig. 160, sc). 



Under the head of epiphysial structures are included several 



