HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 151 



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. HISTOLOGY OF THE MESENCEPHALON. (PI. I. figs. 9 and 10.) 



A section across the mid-brain in the region 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. 



Each 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 tegmentum 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 crustae ; 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 crustee, 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 quadrigemina 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. 



Each 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 ; 



(ROY. soc. EDJN. TRANS., VOL. XLVIII., 859.) 



