PAETS DEEIVED FEOM THE DIENCEPHALON. 



609 



FORE -. 



The optic nerve is formed chiefly by the passage of fibres backwards from the retina 

 i t n the wall of the original optic stalk, whilst the chiasma takes form by the transit of 

 ^fibres across the median plane 



n front of the infundibulum 



ind behind the optic recess. 



To a large extent these fibres 



ire derived from the optic 



;ierve. The optic recess of 



:he third ventricle marks the 



spot where the hollow optic 



vesicle was originally attached 



to the inferior and lateral 



part of the fore-brain, and in 



the adult it therefore repre- 

 sents a portion of the primitive 



cavity of the tubular stalk of 

 ^he optic vesicle. In the 



course of development the 



optic nerve fibres, which ap- 

 pear in the stalk of the optic 



vesicle to form the optic nerve, 

 ' seek an attachment much 



further back, and through the 



optic tract they are even car- 



Tied as far as the mesen- 



cephalon. 



The roof of the fore- 

 brain remains thin, and does 



not proceed to the develop- 

 ; ment of nervous elements, 



although its posterior part 



becomes invaded by nervous 



tissue to form the pineal body 



and the posterior commissure. 



In front of these structures 

 the roof of the fore-brain is 

 epithelial, and remains so dur- 

 ing life. It constitutes the 

 : epithelial roof of the third 

 ventricle, and it becomes in- 

 volutedalongthe median plane 

 into the cavity to form the 



FIG. 538. Two DRAWINGS OP THE EMBRYONIC BRAIN (by His). 



A, Reconstruction of the fore-brain and mid-brain of His's embryo KO ; 



profile view. B, Same brain as A, divided along the median plane 



and viewed upon its inner aspect. 

 M, Mamillary eminence ; Tc, Tuber cinereum ; Hp, Hypophysis 



(hypophyseal diverticulum from buccal cavity) ; 'Opt, Optic stalk ; 



TH, Thalamus ; Tg, Tegmental part of mesencephalon ; Ps, Pars 



hypothalamica ; Cs, Corpus striatum ; FM, Foramen* interventricu- 



lare ; L, Lamina terminalis ; RO, Recessus opticus ; Ri, Recessus 



infundibuli. 



chorioid plexuses of the ven- 

 tricle (Fig. 549, p. 622). The posterior commissure appears as a transverse thickening 

 at the bottom of a transverse groove which appears in the roof of the early brain- tube, 

 behind the pineal diverticulum. 



PARTS DERIVED FROM THE DIENCEPHALON. 



Under this heading we have to consider : (1) the thalamus ; (2) the epithalamus, 

 which comprises the pineal body and the habenular region ; (3) the metathalamus, 

 or the corpora geniculata ; and (4) the hypothalamus. 



The hypothalamus consists of two portions, viz., the pars mamillaris hypothalami, 

 which comprises the corpus mamillare and the portion of the central gray matter 

 which forms the floor of the third ventricle in its immediate vicinity ; and the pars 

 optica hypothalami, which embraces the tuber cinereum, the infundibulum, the 

 hypophysis (O.T. pituitary body), and the lamina terminalis. Strictly speaking, the 

 optic part of the hypothalamus does not belong to the diencephalon, but it is 

 convenient to study the parts which it represents at this stage. 



The original cavity of that part of the brain-tube which forms the diencephalon 

 is represented by the greater part of the third ventricle of the brain. 



Thalamus. The thalamus is the principal object in this section of the brain 



40 



