THE NERVOUS SYSTEM 



245 



and 150 show median and lateral sagittal sections of the same 

 stage. 



The flexures of the brain at this stage are: (1) the cranial 

 flexure marked by the plica encephali ventralis on the ventral 

 surface, (2) the cervical flexure at the junction of myelencephalon 

 and cord, somewhat reduced in this stage, and (3) the pontine 

 flexure, a ventral projection of the floor of the myelencephalon. 



Compost. 



\ 



Fig. 147. — Dissection of the brain of an 8-day chick. For description see 

 text. The arrows shown in the figure lie near the dorsal and ventral boun- 

 daries of the foramen of Monro. 



ch. PI., Choroid plexus. Com. ant., Anterior commissure. Com. post., 

 Posterior commissure. C. str., Corpus striatum. Ep., Epiphysis. H., 

 Hemisphere. Hyp., Hypophysis. L. t.. Lamina terminalis. Myel., Myel- 

 encephalon. olf.. Olfactory nerve, op. N., Optic chiasma. op. L., Optic 

 lobe. Par., Paraphysis. Paren., Parencephalon. pi. enc. v., Phca en- 

 cephali ventralis. pont. Fl., Pontine flexure. Rec. op., Recessus opticus. 

 S. Inf., Saccus infundibuli. Tel. med.. Telencephalon medium. Th., Tha- 

 lamus. T. tr., Torus transversus. Tr., Commissura trochlearis. 



The Hnes a-a, b-b, c-c, d-d, e-e, f-f, represent the planes of section A, 

 B, C, D, E, and F of Fig. 151. 



Telencephalon. The telencephalon is bounded posteriorly, 

 as noted in Chapter VI, by the line drawn from the velum trans- 

 versum to the recessus opticus. The telencephalon medium has 

 grown but little since the fourth day, but the hemispheres 



