CHAPTER II. 



GENERAL CONSIDERATIONS OF THE BRAIN OR 

 ENCEPHALON. 



Before taking up the special study of the cerebrum the student 

 should notice certain prominent features of the entire brain. To 

 do this the arachnoid and pia mater must be removed, and great 

 care and patience should be exercised to preserve the integrity 

 of the brain substance and to guard against evulsion of the roots 

 of the cerebral nerves. 



The human brain forms the greatly expanded superior extrem- 

 ity of the cerebro-spinal axis. It is derived from three sack-like 

 dilatations of the epiblastic neural tube, called the anterior, the 

 middle and the posterior brain- vesicles (Fig. 16). 



Cavities. The cavities of these brain-vesicles constitute the 

 adult ventricles, which form a continuous median series extend- 

 ing from the canal of the spinal cord up to the level of the 

 cerebral hemispheres; at that level the central cavity bifurcates 

 into a branch for each hemisphere of the cerebrum (Figs. 17 and 

 1 8). Thus is formed the lateral ventricle in the cerebral hemis- 

 phere and, below the cerebral hemispheres, the median series of 

 cavities comprises the third ventricle in the inter-brain, the cerebral 

 aqueduct in the mid-brain, and the fourth ventricle in the hind- 

 brain and after-brain. The walls of these simple embryonic 

 cavities undergo wonderful development and specialization; 

 ultimately they produce all the multiform and complicated struc- 

 tures of the adult human brain. 



Superior View. The superior surface of the brain is markedly 

 convex (Figs. 19 and 22). It is elliptical in outline, the major 

 axis being contained in the median line; the greatest transverse 

 axis is situated a little behind the middle and runs between the 

 points which, when the brain is in the skull, underlie the tubera 

 parietalia. This surface is closely adapted to the interior of the 



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