■ 1889 - 90 .] Dr Alexander Bruce on Third Cranial Nerve. 173 
postero-internal nuclei enter the innermost part of the nerve opposite 
the upper half of the nucleus. I have searched in vain for any 
decussation of the roots as described by Yon Gudden in the rabbit. 
While it is certainly premature to express any positive opinion as 
to the functions of the various nuclei described, there would appear 
to be ground (see Westphal’s case, loe. eit.) for the belief that the 
inferior, anterior, and postero-lateral (including external) groups of 
nuclei are connected with the extrinsic muscles of the eyeball and 
the elevator of the eyelids, and that the median and the postero- 
internal nuclei are the centres for accommodation and contraction 
of the pupil. If the view of Kahler and Pick he correct, then the 
postero-lateral group will form the centres for the elevators, while 
the anterior nucleus will he connected with the internal rectus, and 
the inferior nucleus with inferior rectus. With regard to the two 
central groups, the median and postero-internal, the evidence seems 
to be insufficient for the view which would place the accommodation 
centre at a higher level than that for the contraction of the pupil. 
The circumstance that a nuclear lesion seems invariably to affect 
the accommodation of both eyes, while a similar lesion may un- 
doubtedly affect the sphincter iridis of one side alone, seems best 
explained by the view that the accommodative act is controlled 
from one centre, while there are two centres for the contraction of 
the pupil. If this view be correct, then the median nucleus may 
form the centre for accommodation and the postero-internal necleus 
that for pupil-contraction. In this respect the intimate relation of 
position of the median nucleus to the main portion of the anterior 
nucleus (the supposed centres for the associated acts of accommoda- 
tion and convergence) is of interest. 
Description of Plates. 
Fig. 1 shows the nuclei (IY. n.) of the two fourth nerves lying imbedded 
amongst the outermost fibres of the posterior longitudinal fasciculus ( pd.f , “.). 
Its root (IY. r.) is distinguished from that of the third nerve by passing from 
the postero-external aspect outwards and backwards, along the anterior 
margin of the grey matter surrounding the aqueduct of Sylvius. From the 
inner aspect of the nuclei, fibres {t.f . ) pass inwards and forwards to enter the 
bundles of the posterior longitudinal fasciculus. The anterior parts of the two 
posterior longitudinal fasciculi meet so as to form a process shaped like the 
capital letter U. Into the anterior surface of this U-shaped process there 
enter medullated fibres, which arise apparently in the decussating superior 
