328 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



cephalon, and into a caudal portion, the myelencephalon. At a third bend the 

 whole brain is flexed ventrally at an angle with the axis of the spinal cord. This 

 bend is the cervical flexure and is the hne of demarcation between the brain and 

 spinal cord (cf. Fig. 333 A). The telencephalon and diencephalon are more dis- 

 tinctly subdivided, and the evaginated optic vesicle forms the optic cup attached 

 to the brain wall by a hollow stalk, in which later grows the optic nerve. The 

 walls of the brain show a distinct differentiation in certain regions. This is 

 especially marked in the myelencephalon, which has a thicker ventro-lateral wall 

 and thinner dorsal wall. 



Embryos of 10.2 mm. show the structure of the brain at the beginning of the 

 second month (Figs. 341 and 344). The five brain regions are now sharply dif- 



Diencephalon 



Pallium 



Mesencephalon 



Cephalic 

 flexure 



B 



Thalamus 



Opti 

 cup 

 Pontine flexure' 



Myelencephalon — 



Mclen- 

 'cephalon 

 Corpus striatum 

 Optic raiis 



Hypolhahuims 



Mesencephalon 



I^llnnus 



Cerebellum 



Medulla oblongata 



Fig. 331.^Reconstructions of the brain of a 7 mm. human embryo (His). A, Lateral view; B, in median 



sagittal section. 



ferentiated externally, but the boundary line between the telencephalon and dien- 

 cephalon is still indistinct. The telencephalon consists of paired, lateral out- 

 growths, the anlages of the cerebral hemispheres and rhinencephalon (olfactory 

 brain). In Fig. 359 the external form of the brain is seen with the origins of the 

 cerebral nerves. It will be noted that, with the exception of the first four (the 

 olfactory, optic, oculomotor, and trochlear), the cerebral nerves take their super- 

 ficial origin from the myelencephalon. 



The cephalic flexure forms a very acute angle, and, as a result, the long axis 

 of the fore-brain is nearly parallel to that of the hind-brain (Fig. 359). The ocu- 

 lomotor nerve takes its origin from the ventral wall of the mesencephalon. Dor- 

 sally there is a constriction, the isthmus, between the mesencephalon and meten- 



