214 D. DAVIDSON BLACK 



It is to be noted that the posterior poles of the so-called basal 

 lobes of the cerebral vesicle project backward for a short distance 

 on either side of the thalamic mass and peduncular region. 



In connection with the foregoing description it must be borne 

 in mind that beside the malformations which are the direct out- 

 come of the cyclopic condition, others are present which are 

 consequent on the internal hydrocephalus. It is to this factor 

 that we must look for the cause of the marked dilatation of the 

 forebrain vesicle with the bulging of its thin walled roof, and 

 probably also for the lack of continuity in the ventricular sys- 

 tem. These points will be more fully dealt with later. 



Brain stem and cerebellum 



The midbrain, as may be seen in figures 48 and 49, is plainly 

 visible from above and on account of its thick pial covering shows 

 no external indications of division into corpora quadrigemina. 

 The marked flexure which obtains between the primary fore- 

 brain vesicle and the brain stem causes the peduncular region 

 to be completely hidden in a ventral view of the brain. No 

 cerebral peduncles are present. 



Both vermis and lateral lobes of the cerebellum are well devel- 

 oped. Their various subdivisions show but little departure from 

 the normal. The pons, which cannot be distinctly seen in the 

 figures on account of the marked flexure of the brain stem, is 

 also well developed. 



The absence of the pyramids and the great prominence of the 

 olives are the only external features in which the medulla appears 

 to vary from the normal. The attachments of the cranial nerves 

 could not be made out in the gross as they had been removed, 

 together with the vessels and pia, before the material came into 

 my hands but subsequent to the drawings shown in figures 48 

 to 51. 



It is to be noted that the marked abnormal flexure of the brain 

 stem in the region of the midbrain is apparently due to the non- 

 development of the cerebral peduncles. 



