BBAIN OF MAN. 
369 
from each other by the division of the broad fibrous band/ 
termed the corpus callosum ,, which unites them. Each hemi¬ 
sphere is considered as made up of three lobes or divisions, 
the anterior a, the middle 6 , and the posterior c; but these 
are not by any means distinctly marked-out, either on the 
external surface or in the internal structure of the organ. The 
vesicular or ganglionic nerve-substance is disposed for the 
most part upon the exterior, forming a continuous layer, whose 
extent is greatly increased by the convoluted folds in which it 
lies 3 and it is very copiously supplied with blood from the 
pia mater , a membrane which consists almost entirely of blood¬ 
vessels and of the areolar tissue that holds them together, and 
which so closely enfolds the hemispheres as to dip down into all 
the furrows of their surface. The principal part of the internal 
substance of each hemisphere is composed of nerve-fibres, of 
which some pass between its convolutions and the chain of 
ganglionic masses on which the cerebrum is superposed, others 
B B 
