BRAIN 



419 



approximately vertical, the anterior end of the tube, from which the 

 brain develops, is bent as shown in Fig. 432, A. The first portion, begin- 

 ning at the anterior extremity where the neuropore is still open, passes 

 vertically upward. At the head-bend it turns backward and passes to the 

 neck-lend, where it curves downward, becoming continuous with the 

 part of the tube which forms the spinal cord. The anterior ascending 

 portion is the fore-brain (prosencephalon) ; the part where the head-bend 

 occurs is the mid-brain (mesencephalon) ; and the remainder is the hind- 

 brain (rhombencephalori) . These three fundamental parts have become 

 more distinct and exhibit 

 subdivisions in the 10 mm. 

 embryo shown in Fig. 432, B. 



Prosencephalon. The 

 fore-brain becomes subdi- 

 vided into the telencephalon 

 anteriorly, and the diencepK- 

 alon posteriorly; the latter 

 connects with the mid-brain. 

 In very early stages the fore- 

 brain produces two lateral 

 outpocketings, one on either 

 side, called the optic vesicles. 

 Each expands distally to 

 form the retina of an eye, 

 and its connection with the 

 fore-brain becomes reduced 

 to a slender stalk. In later 

 stages, the depression on the 

 inner wall of the brain which 

 marks the position of the 

 stalk is called the optic re- 

 cess. It is shown in the me- 

 dian sagittal sections of the bran of an embryo of three months and of 

 an adult, in Figs. 433 and 434 respectively. 



Telencephalon. The principal derivatives of the telencephalon are a 

 pair of lateral outpocketings which arise somewhat later than the optic 

 vesicles and are known as the cerebral hemispheres. Each contains a 

 cavity, or lateral "ventricle, which opens into the medullary tube through 

 the interventricular foramen (foramen of Monro). In later stages this 

 foramen is relatively small, and it appears in Figs. 433 and 434 as a darkly 

 shaded cleft in front of the thalamus (th.). As the hemispheres expand, 

 they approach one another in the median line above the brain, being sep- 

 arated by a thin plate of connective tissue. They grow backward, cover- 



sp.c. 



FIG. 432. A, THE BRAIN OF A 4.0 MM HUMAN EMBRYO 

 (after Bremer); B, THB BRAIN OF A 10.2 MM. EMBRYO 

 (after His). 



Except the isthmus, is. the principal subdivisions of the brain 

 are indicated by prefixes of the term encephalon; sp. c., 

 spinal cord; h., hemisphere; o. v., optic vesicle; r., rhinen- 

 cephalon; v., roof of the fourth ventricle. 



