HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 859 
peduncles are very distinct, and their decussation may be seen about the middle of 
the section, although this does not form so obvious a feature as it does in the human 
pons. A noteworthy feature is the absence of any sign of substantia nigra, as also is 
the fact that the pyramid bundles are still isolated masses lying well in the grey 
matter of the lateral regions.. Pontine fibres are also well marked. 
The anterior longitudinal bundle lies ventral to the posterior longitudinal bundle, 
but is not a marked feature, since a large proportion of grey matter is present in the 
mid-region. Below the decussation of the superior peduncles in the mid-line is some 
grey matter, which, although ill-defined, would represent the so-called central nucleus of 
the higher types. 
The root-bundle of the 4th nerve appears just external to the grey matter of the 
floor of the ventricle, and indications of the nucleus of the 4th nerve are to be made 
out at the outer limit of the grey matter in the floor. 
IV. Hisrotocy or THE MrsencepHaton, (PI. I. figs. 9 and 10.) 
A section across the mid-brain in the reqion of the posterior corpora quadrigemina 
shows a structure very like that seen in the human mid-brain ; the aqueduct of Sylvius 
is, however, much nearer the dorsal surface, and moreover is rhomboidal in shape. 
Kach corpus quadrigeminum possesses an outer coat of white fibres and inter- 
mediate mass of grey matter, and an inner thin band of white fibres separating it from 
the central grey matter round the aqueduct. 
The reticular formation of the tezmentum is very distinct, and the posterior longitu- 
dinal bundles show clearly just below the grey matter surrounding the aqueduct of Sylvius. 
The nuclei of the 3rd and 4th cranial nerves are very well defined, lying mesially 
just ventral to the lower angle of the aqueduct. 
The substantia nigra forms a layer, containing some very large nerve-cells, lying 
between the fillet tracts and the crustz; prolongations from the substantia nigra pass 
into the crusta and tend to subdivide it into two or more regions on either side. 
There is an ill-defined mass of grey matter between the two cruste, representing an 
inter-peduncular ganglion, and above this a tract of decussating fibres which form the 
crossing (upper part) of the superior cerebellar peduncles. ‘The decussating fibres of 
the tegmenta are well marked in the mid-region above the substantia nigra. 
A section across the anterior corpora quadrigemima shows some divergence from 
the human type; the Sylvian aqueduct is still rhomboidal in shape, and is some dis- 
tance from the dorsal surface. The fibres of the 3rd cranial nerve are seen issuing 
through the posterior longitudinal bundles, and a portion of the section which in the 
human is occupied by the red nucleus: the latter, however, cannot be markedly 
distinguished as such in the section in question. 
Kach anterior corpus quadrigeminum possesses a very thin covering of white fibres, 
grey matter in the intermediate region, and a narrow layer of white fibres internally ; 
