RECONSTRUCTION OF H^ LOBATES HOOLOCK 455 



part of the reticular formation. It is fairly well dillerentiated from the sur- 

 rounding reticular matrix by the fiber capsule derived from the superior 

 cerebellar peduncle. It is spherical in shape, its cephahc extremity extending 

 up to, if not into, the diencephalon. 



The Central Gray Matter 



The central gray matter in gibbon in the lowest levels of the recon- 

 struction appears as a more or less rectangular mass of gray matter receiving 

 at its ventrolateral angles the reticular formation, dorsally the nucleus of 

 Goll and at its dorsolateral angles, the substantia gelatinosa Roland i. 

 Passing upward the central gray matter presents a flat condensation which 

 represents the beginning of the nucleus of Burdach. 



As the central gray matter is followed upward in the stem it passes 

 dorsad and at the same time expands laterally, until it is disposed as a narrow, 

 flat band lying upon the dorsal surface of the reticular formation. 



The fourth ventricle presents very little modelling in any part. Above 

 the mid-point there is the suggestion of the nucleus abducentis, but this is 

 not so well defined as it is in some of the other forms. 



As the gray matter is flattening out from side to side, a long ventral 

 prolongation stretches forward in the vicinity of the raphe. This prolongation 

 of the central gray matter extends almost to the base of the pyramid. Above, 

 it passes into the reticular formation which gradually approaches its fellow 

 of the opposite side. In the upper portion of the pontile region the floor of 

 the fourth ventricle begins to narrow towards the aqueduct of Sylvius. The 

 lateral walls contract and finally the cavity of the ventricle is reduced to 

 the small dimensions of the iter. At the same time, the gray matter sur- 

 rounding the aqueduct increases in thickness and gradually becomes changed 

 in diameter. In the ventral prolongation of the mesencephalic central gray 

 matter arise the condensations of nuclear material which give rise to the 



