366 



CHORDATE ANATOMY 



sulci which are such a characteristic feature of the surface of the human 

 brain. 



Changes in the striate body (basal gangUon) are summarized in Fig. 

 ^24. In mammals each striate body extends posteriorly and rests upon 

 the thalamus — Uke a sack of flour on a horse's back. 



Brain Coimnissures. 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- 



\^y/^\ MYELENCEPHALDN 



■pic,. 325. — 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 commis- 

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



out the vertebrate series, and serve as important topographic landmarks 

 to determine homologous regions. (Fig. 325) 



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 also from the hypothalamus 

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



