THE BRAIN 



325 



The obliquely coursing spinal nerves with the filum terminale constitute the 

 cauda equina. 



THE BRAIN 



We have seen that in embryos of 2 to 2.5 mm. the neural tube is nearly 

 straight, but that its cranial end is enlarged to form the anlage of the brain. The 

 appearance of two constrictions in the wall of the anlage subdivides it into the 

 three primary brain vesicles, the fore-brain or prosencephalon, mid-brain or 

 mesencephalon, and hind-brain or rhombencephalon (Fig. 306) . 



.Anterior neuropore 



iPallium of telencephalon 



Palli, 



Diencephalon 



Anterior neuropore 



Mesencephalon 

 Isthmus 



Corpus striatum 



Mese n c ephalan 



Cephalic 

 flexure 



Optic recess 



Future pontine 

 Rhombencephalon flexure 



Rhombencephalon 



Fig. 312. — Reconstructions of the brain of a 3.2 mm. human embryo. A, lateral surface; B, sectioned 

 in the median sagittal plane (after His). 



In embryos of 3.2 mm., estimated age three weeks, three important changes 

 have taken place (Fig. 312 A, B) : (1) the end of the neural tube is bent sharply 

 in the mid-brain region so that the axis of the fore-brain now forms a right angle 

 with the axis of the hind-brain. This bend is the cephalic flexure; (2) the fore- 

 brain shows indication dorsally of a fold the margo thalamicus which subdivides 

 it into the telencepalon and the diencephalon; (3) the lateral wall of the fore-brain 

 shows a distinct evagination, the optic vesicle, which projects laterally and caudad. 

 A ventral bulging of the wall of the hind-brain indicates the position of the future 

 pontine flexure. 



In embryos of 7 mm. (four weeks) the neuropores have closed. The ceph- 



