336 



DEVELOPMENT OF THE HUMAN BRAIN 



At a very early stage of development, believed by Tiede- 

 mann to be about the 7th week, the primitive nervous 

 axis or tube undergoes a series of bendings (fig. 123, A). 



FIG. 123. Sketches of the early form of the parts of the Cerebro-spinal Axis in 

 the Human Embryo. (Sharpey, after Tiedemann.) 



A, at the seventh week, lateral view ; 1, spinal cord ; 2, medulla oblongata ; 

 3, cerebellum ; 4, mesencephalon ; 5, 6, 7, cerebrum. 



B, at the ninth week, posterior view. 1, medulla oblongata ; 2, cerebellum ; 



3, mesencephalon ; 4, 5, thalami optici and cerebral hemispheres. _,, 



C and D, lateral and posterior views of the brain of the human embryo as it 

 appears at the twelfth week of intra-uterine life, a, cerebrum ; b, corpora quadri- 

 gemina ; c, cerebellum ; d, medulla oblongata : the thalami are now covered by the 

 enlarged hemispheres. 



E, posterior view of the same brain, dissected to show the deeper parts. 1, me- 

 dulla oblongata ; 2, cerebellum ; 3, corpora quadrigemina ; 4, thalami optici ; 5, the 

 hemisphere turned aside ; 6, the corpus striatum embedded in the hemisphere ; 

 7, the commencement of the corpus callosum. 



F, the inner side of the right half of the same brain separated by a vertical 

 median section, showing the central or ventricular cavity. 1, 2, the spinal cord 

 and medulla oblongata, still hollow ; 3, bend at which the pons Varolii is formed ; 



4, cerebellum ; 5, lamina (superior cerebellar peduncles) passing up to the corpora 

 quadrigemina ; 6, crura cerebri ; 7, corpora quadrigemina, still hollow ; 8, third 

 ventricle ; 9, infundibulum ; 10, thalamus, now solid ; 11, optic nerve ; 12, aperture 

 leading into*the lateral ventricle ; 13, commencing corpus callosum. 



The ' posterior swelling ' becomes bent upon itself so 

 that its two regions (2, 3) are nearly at a right angle ; 

 while thence onwards the parts form a curve (4 5, 6), which 

 is directed forwards and downwards. 



