THE NERVOUS SYSTEM. 



547 



plate at the end of the tube is often taken to be at the recessus neuroporicus ; 

 and the lamina terminalis or end wall of the neural tube, more strictly speaking, 

 is limited to the median wall ventral to this point. Here it will be understood 

 as including the median wall to the point where the pallio-thalamic boundary 

 begins, marked later by the angulus prcethalamicus of His (see p. 554 and Fig. 

 480). 



Rhinencephalon. — The term rhinencephalon is a convenient one for 

 those basal structures of the fore-brain which are in most intimate connection 

 with the olfactory nerve. The term has been extended by some to include 

 the pallial olfactory structures. For descriptive purposes it is here used in 

 the more limited sense. 



At the fourth week, as already indicated (p. 546, Fig. 472), there is a slight longi- 

 tudinal furrow on the external surface, marking the ventral limit of the pallial 



Fig. 472. — Lateral view of outside of brain shown in Fig. 471. His. 



eminence. The part of the brain ventral to this furrow is the rhinencephalon, 

 Somewhat later the latter becomes better marked off, the fissure forming its 

 boundary on the lateral surface being the external rhinal fissure (Fig. 462). 

 Later the mesial side is also marked off by an extension of the fissure around 

 on the mesial side (medial rhinal fissure) and also by a notch, the incisura 

 prima, a continuation of which later ascends along the middle part of the 

 median surface of the hemispheres and is known as the anterior arcuate fissure 

 {fissura prima of His). (Fig. 480.) The existence of a fissura prima in early 

 stages, however, is doubtful. At about this time, the rhinencephalon shows a 

 beginning division into anterior and posterior portions, the anterior and posterior 

 olfactory lobes, the whole structure assuming a bean-shape (comp. p. 549) 

 (Fig. 465). On the lateral surface immediately above this constriction is the 

 beginning concavity in the lateral surface of the hemispheres which marks the 



