GENERAL SURVEY OF THE STRUCTURE OF THE BRAIN. 377 



to the projection system, is, on the contrary, the centre for a 

 vast increase of them, which in ordinary parlance is expressed 

 by saying that the number of fibres belonging to the nerve 

 roots springing from the spinal cord considerably exceeds the 

 number of fibres of the columns of the spinal cord. This 

 grey substance (fig. 231) commences in the region of the 

 deutencephalon (Zwischenhirn, optic thalami) as an invest- 

 ment lining the third ventricle (m), which is rendered annular 

 by the middle commissure (Cm), and which is continued into 

 the infundibulum (7). In the region of the mesencephalon it 

 surrounds the aqueduct of Sylvius (Ag), expands itself in the 

 cavity of the fourth ventricle (Fr), and finally encloses the 

 central canal (Cc) in the lower half of the medulla oblongata, 

 and throughout the spinal cord (Gr). Just as in the spinal 

 cord, the roots of the cranial nerves (P 3 ), representing in figs. 

 230 and 232 the third segment of the projection system, origi- 

 nate from the grey substance of the central cavities more 

 numerous fibres than those which represent this segment in the 

 crus cerebri. From the superficial impression obtained from 

 fig. 230, the progress of the reduction of the projection system 

 in the substance of the pons and upper half of the medulla 

 oblongata is seen to be replaced by a variable amount of in- 

 crease, since the circumference of this segment of the brain 

 exceeds that of the crus cerebri. This increase of bulk depends 

 in great measure on the interposition of grey matter that essen- 

 tially accounts for the enlargement, whilst the portions of the 

 cerebellar medullary substance which enter into the composition 

 of the projection system diverge from this as it passes onwards, 

 and, after forming an interweaving plexus with it, run to the 

 cerebellum. Below the point where this occurs, the rapid dimi- 

 nution of the size of the medulla oblongata to that of the spinal 

 cord corresponds also to the transition of the cerebral structure 

 into the type of structure presented by that of the cord. In 

 accordance with this general view, the description of the cere- 

 bral structure may be divided into the following natural sec- 

 tions : 1, The lobes of the cerebrum ; 2, The crus fa or base of 

 the crus cerebri, with its ganglia ; 3, The tegmentum of the 

 crus cerebri, with its ganglia ; 4, The region of interlacement 

 of the arms of the cerebellum ; 5, The cerebellum ; 6, The form- 



