5o8 



COMPARATIVE ANATOMY 



form a pathway for fibers to and from the cerebral cortex. From the 

 posterior walls of the infundibulum are differentiated a tuber cinereum, 

 and also paired mammillary bodies, which have fiber connexion with 

 the olfactory organ. Fibers are also received from the cerebral cortex 

 and sent to the thalami. (Fig. 420) 



OLFACTORY 

 LOBE 



U=-J TELENCEPHALON 

 DIENCEPHALON 

 MESENCEPHALON 

 METENCEPHALON 



MYELENCEPHALDN 



Fig. 425. — A diagram of the brain of a four-months fetus as seen in median longi- 

 tudinal section. The figure shows the location of the more important brain commissures. 

 (Redrawn from Coming's "Human Embryology," after Burckhardt.) 



Brain Commissures. Commissures are fiber tracts which cross the 

 median plane of the body and bring lateral halves of the nervous system 

 into relation with one another. Some of those in the brain persist through- 

 out the vertebrate series, and serve as important topographic landmarks 

 to determine homologous regions. (Fig. 425) 



Commissures have not been demonstrated in Amphioxus. In the 

 terminal lamina of the telencephalon of cyclostomes are two, a more 

 ventral anterior and a more dorsal pallial. Both connect the olfactory 

 lobes with the hippocampi of the opposite side. Two habenular ganglia 

 in the roof of the diencephalon are connected by the habenular commis- 

 sure. Fibers from the hemispheres and from the hypothalamus also 

 are contained in this commissure. Another commissure in the roof of 



