202 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



THE MID-BRAIN. 



The central canal of the mid-brain forms the aqueduct of 

 Sylvius (Fig. 167). In its roof are developed the corpora quadri- 

 gemina. The dorsal and ventral laminae of its medullary plates 

 form the tegmentum and crusta of the crura cerebri. 



The Three Neural Flexures (see Fig. 163). — The pontine 

 flexure, a convexity forwards of the pons, has already been men- 

 tioned ; the nuchal flexure is concave forwards and occurs between 

 the medulla and cord. Both of these are of small import, but 

 the anterior flexure, whereby, in the third week of foetal life, the 

 fore-brain is bent downwards and forwards until it comes to lie 

 on the ventral aspect of the cephalic end of the notochord, leads 

 to a great alteration in the form and relationships of the fore and 

 mid brains and is of great importance. The mid-brain, by this 

 flexure, is brought to be, for a short time, the most anterior part 

 of the neural canal; the fore-brain is doubled back under the 

 notochord. Eound the projecting end of the notochord — project- 

 ing between the mid and fore brains — is developed the posterior 

 clinoid processes and dorsum sellae (Fig. 163). The tentorium 

 cerebelli is developed between the mid-brain and fore-brain, and 

 lies at first at right angles to the axis of the mid-brain, but the 

 great subsequent growth of the cerebrum forces it backwards and 

 downwards until it becomes a horizontal partition between the 

 cerebral and cerebellar chambers of the skull. 



THE FORE-BRAIN. 



The Third Ventricle and Structures derived from its 

 Walls. — The Third Ventricle (see Fig. 167) is the cavity of the fore- 

 brain and represents the anterior dilated end of the neural canal. 

 From its walls many structures are derived. We have already seen 

 that the optic vesicles are produced from its ventro-lateral walls; 

 from its floor is produced the hypopophysis cerebri — the ancient 

 mouth of the alimentary canal if G-askell's views are right. 

 From the posterior part of its roof plate is produced the pineal 

 body — an ancient median eye ; from its an tero- superior part is 

 produced a bifid hollow outgrowth — the cerebral vesicles — which 

 come in time to dominate the whole nervous system. 



