THE BRAIN 



329 



the anlage of the cerebellum. Its thickened ventral wall becomes the pans 

 (Varolii). Its cavity constitutes the cranial portion of the fourth ventricle. 



The caudal border of the pons is taken as the ventral boundary line between 

 the metencephalon and myelencephalon. The myelencephalon forms the medulla 

 oblongata. Its dorsal wall is a thin non-nervous ependymal layer, which later 

 becomes the posterior medullary velum. From its thickened ventro-lateral 



ol 



FIG. 315. Brains of human embryos, from reconstructions by His: A, brain from fifteen-day em- 

 bryo; B, from three-and-a-half-week embryo; C, from seven-and-a-half-week fetus; Jb, ib, mb, hb, ab, 

 fore-, inter-, mid-, hind-, and after-brain vesicles; o, optic vesicle; ov, otic vesicle; in, infundibulum; 

 m, mammillary body; pf, pontine flexure; I Vv, fourth ventricle; nk, cervical flexure; ol, olfactory lobe; 

 b, basilar artery; p, pituitary recess (American Text-Book of Obstetrics). 



walls the last eight cerebral nerves take their origin. Its cavity forms the greater 

 part of the fourth ventricle which opens caudally into the central canal of the 

 spinal cord, cranially into the cerebral aqueduct. The increase in the flexures of 

 the brain and the relative growth of its different regions may be seen by comparing 

 the brains of embryos of the third, fourth, and eighth weeks (Fig. 315). 



In the following table are given the primitive subdivisions of the neural tube 

 and the parts derived from them: 



