576 



DEVELOPMENT OF THE CEREBRO-SPINAL AXIS. 



alter their position, in correspondence with the curving downwards of the cephalic 

 end of the embryo. Thus, at the seventh week there is an angular bend forwards 

 between the hindmost vesicle and the rudimentary spinal cord, the projecting angle 

 (backwards) being named the cervical tuberosity. Another bend, but in the opposite 

 direction, exists between that part of the third vesicle which forms the medulla 

 oblongata, and that which gives rise to the cerebellum. Lastly, a third angle is pro- 

 duced by a bend forwards and downwards in the region of the middle vesicle, from 

 which the corpora quadrigemina are developed, and which forms, at this period, the 

 highest part of the encephalon ; whilst the anterior, or first vesicle, is bent nearly at a 

 right angle downwards. 



Fig. 391. Fig. 391. SKETCHES OF THE EARLY 



FORM OF THE PARTS OF THE 



CEREBRO- SPINAL Axis IN THE 

 HUMAN EMBRYO (after Tiede- 

 mann). 



A, at the seventh week, lateral 

 view ; 1, spinal cord ; 2, medulla 

 oblongata ; 3, cerebellum ; 4, mid- 

 dle vesicle or corpora quadrigemina; 

 5, 6, 7, first vesicle. B, at the 

 ninth week, posterior view ; 1, 

 medulla oblongata ; 2, cerebellum ; 

 3, corpora quadrigemina ; 4, 5, tha- 

 lami optici and cerebral hemispheres. 

 C and D, lateral and posterior views 

 of the brain of the human embryo 

 at twelve weeks, a, cerebrum ; b, 

 corpora quadrigemina ; c, cere- 

 bellum ; 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, medulla 

 oblongata ; 2, cerebellum ; 3, cor- 

 pora quadrigemina ; 4, thalami 

 optici ; 5, the hemisphere turned 

 aside ; 6, the corpus striatum em- 

 bedded in the hemisphere ; 7, the 

 commencement of the corpus cal- 

 losum. F, the inner side of the 

 right half of the same brain sepa- 

 rated 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, thata- 

 mus, now solid ; 11, optic nerve ; 12, cleft leading into the lateral ventricle ; 13, com- 

 mencing corpus callosum. 



F 



At an early period of the development of the brain, the anterior primary vesicle 

 undergoes a peculiar change, by which two sets of parts are originated, the first of 

 which corresponds to the cerebral hemispheres, the second to the thalami optici ; the 

 middle vesicle, remaining single, gives rise to the corpora quadrigemina ; and the 

 posterior vesicle, like the first, but at a somewhat later period, serves as the basis of 

 the rudiments of two parts, viz., the cerebellum and the medulla oblongata. Thus, 

 out of the three primary vesicles five fundamental parts of the encephalon are 

 formed. 



The following tabular statement may serve as a synoptical view of the relation 



