644 P110B\ R. A. DART ON THE 



as an explanation because of its slight development and the 

 doubtful presence of a pons Varolii, and it is equally unlikely 

 that the spino-cerebellar contribution could have been so great 

 as to account for the whole of the cerebellar expansion. But 

 we do know that so diminutive and primitive a mammal as 

 Ornithorhynclms, with a well-developed trigeminal apparatus can 

 and does possess not only an unexpectedly large neopallium, but 

 also a relatively large convoluted cerebellum. The absolute and the 

 relative size of the Gasserian ganglia in Zenylodon sensiiivus, and 

 indeed in all ZeuglodontidiB here represented, are far more 

 strikin" than they are in Ornithorhynchus. There can be no 

 doubt that the atierent impulses reaching the cerebellum from 

 the anterior end of the body in Zenglodontidio were of particular 

 value in supplying these animals with information concerning 

 disturbances of equilibration. The distribution of the trigeminal 

 tract within the cerebellum is not fully known, but it is believed 

 by many investigators that in Mammalia there is a tract for 

 conveying trigeminal impulses to the cerebellum. The huge 

 cerebellum in the Zeuglodontidse may well be due to the fact 

 that, by a rapid and hypertrophic development, the trigeminus 

 provided it with very precise information concerning its position 

 in space and hence afforded to this primitive creature a ready 

 solution of the problem of equilibration in a fluid medium. 



I have said that the distribution of a direct trigeminal tract 

 within the cerebellum is not fully known ; it would be nearer the 

 truth to state that it is frequently affirmed but is sometimes 

 denied. Since such denial exists it is valuable to put forward 

 other supporting evidence. It seems to me that the denial of 

 the existence of a direct trigeminal connection with the cere- 

 bellum can only come from an imperfect appreciation of the 

 developmental history of the cerebellum and a consequent failure 

 to recognise its extent in the brain-axis — or else fi'oni a tendency, 

 very manifest during the last two decades, to regard the cere- 

 bellum as an overgrown part of the vestibular apparatus. 



As regards the development of the cerebellum it must be 

 remembered that the cerebellar ridge first appears in early 

 embryos miich further forward in the hind brain (metencephalon) 

 than would be anticipated on the "vestibular" hypothesis. 

 Even in human developmental history, the nerve which is more 

 obviously associated with the cerebellum is the trigeminal nnd 

 not the vestibular. Ingvar(1918) finds that "Die Basis (cere- 

 belli) ist frontalwarts gerichtet. Von dem ventralen Rande 

 dieser Basis laufen die kriiftig entwickelten N. N. Trigemini 

 aus"; or (on p. 343) " Ventralwiirts grenzen die Cerebellar- 

 wiilste an die Insertionstelle des Trigeminus." 



It is not absolutely certain what actual neuromere of the hind 

 brain gives rise to the cerebellum, nor is that question pertinent 

 to the present discussion. The embryological facts significant 

 here are: (1) that the cerebellum arises in the most antoiior 

 portion of the hind'brain roof, and (2) that the territory of the 



