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suprapyramidalis (commonly called postpyramidalis or praegracilis) 

 develops in the mesial region (vermis) ; and at the same time or some- 

 what later a fissure makes its appearance in each lateral area (ala) 

 opposite the suprapyramidal fissure. This pair of lateral (alar) 

 fissures I shall distinguish by the name "parapyramidal", because each 

 ot them constitutes the dorsal boundary of that part of the "para- 

 vermis" (the vertical worm-like band found in most Mammalia along- 

 side and parallel to the vermis) which lies opposite to and alongside 

 the pyramis. In other words the parapyramis is that part of the 

 paravermis which (in the more primitive types) is opposite to and 

 linked to the pyramis of the vermis (see fig. 24). 



In most cases the suprapyramidal and parapyramidal fissures 

 develop almost or exactly synchronously and become confluent. In 

 some brains the parapyramidal fissures thus seem to be merely the 

 lateral extensions of the single (vermian) suprapyramidal. In other 

 cases the suprapyramidal fissure appears to be formed by the con- 

 fluence of the two lateral (alar or paravermian) parapyramidal fissures 

 on the vermis. But the true state of affairs is demonstrated in those 

 brains where the three fissural elements exist independently the one 

 of the other. The failure to recognise the three constituent parts in 

 the fissure commonly called "postpyramidalis" or "praegracilis" explains 

 much of the misunderstanding of this region of the cerebellum. Thus 

 it often happens that the suprapyramidal fissure does not join the 

 parapyramidal but becomes confluent with some other lateral fissure, 

 usually the parafloccular (Fig. 1 c, also Fig. 23). 



In a similar manner the fissura secunda, which usually bends 

 downward (Fig. 1 h) to cut off the uvula (and the lobus posticus) from 

 the rest of the cerebellum, may be prolonged in a more lateral 

 direction and become confluent with the floccular or parafloccular 

 fissures or even (as usually happens in the human brain) with a fissure 

 in the paravermis (Fig. 1 d). The failure to recognise these merely 

 mechanical phenomena incidental to the process of fissure-formation 

 has been responsible for most of the confusion which has hitherto 

 characterised the attempts at comparing the human and the common 

 mammalian types of cerebellum. 



At about the same period in development as the appearance of 

 the suprapyramidal fissure a pair of fissures develop in the middle 

 lobe behind and parallel to the lateral extensions of the fissura prima. 

 They map out a semilunar area (which represents the lobus lunatus 

 posterior of Koelliker): this may therefore be not-inappropriately 

 called the "area lunata" and each of its caudal limiting furrows the 



24* 



