BASE OF THE BRAIN. 289 



tudinally. The transverse are the commissural fibres 

 between the two hemispheres of the cerebellum, inter- 

 woven by the longitudinal fibres forming the crura. 



16. Fifth Pair of Nerves. Arise by two roots from 

 each side of the pons, about three-quarters of an inch 

 external to the median groove. 



17. Sixth Pair of Nerves. Abducens, arise in the 

 groove between the anterior pyramids and olivary body, 

 just behind the pons. 



18. Medulla Oblongata. Cuboidal in form, is the 

 upper enlarged portion of the spinal cord. 



19. Twelfth Pair of Nerves. Arise in the same 

 groove as the sixth. 



20. Seventh, Eighth, Ninth, Tenth, and Eleventh 

 Pairs of Nerves. Arise in the groove between the olivary 

 body, lateral tract, and restiform body. 



21. Cerebellum, or After-Brain. Lies in the lower 

 occipital fossae. It fits like a saddle over the upper part 

 of the medulla. 



A horizontal section below the convolutions through 

 the two hemispheres is oval in outline, and has been 

 called the centrum ovale minus. This section shows the 

 exterior cortical portion of the brain and the white, 

 fibrous portion within, on which are seen numerous red 

 points, the puncta vasculosa, which are the minute 

 blood-vessels of the white matter cut across. A section 

 through the cerebral hemispheres on a level with the 

 corpus callosum is called the centrum ovale majus. This 

 section exposes the upper surface of the corpus callosum, 

 which is the great transverse commissure between the 

 two cerebral hemispheres. On its upper surface are 

 seen some longitudinal elevations of white fibres called 

 the nerves of Lancisi. The upper surface of this com- 

 missure is slightly wavy from front to back. The under 



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