C46 . PROF. R. A. DART ON TJJK 



Tlie arrangement may be expressed by stating that the vestibular, 

 olivary, spinal, and cerebro-pontine contributions successively 

 come to reach the ceiebellum by penetrating a territoiy which 

 was originally, and so ancestrally, trigeminal. 



As has been suggested already, this phenomenon of " pene- 

 tration " may be illustrated equally well by the injiltralions of the 

 mid-brain, thalamus, or fore brain, or by the infiltration of the 

 hippocampnl commissure by the corpus callosum in the fore brain 

 of Mammalia as shown by Elliot Smith. In brief, if we accept 

 the doctrine of the segmental arrangement of the neural tube 

 elements, it is to be anticipated that the principle finds illus- 

 tration in the development of the majority of the inter-segmental 

 and supra-segmental apparatuses. 



The language of neurology is devoid of any term which 

 describes conveniently this uniformity of behaviour in the laying- 

 down of subsequent formations in the neural tube. Jt should 

 prove of service therefore to descriptive neurology to recognise in 

 this uniformity the consistent working of a general principle 

 Avhich underlies the whole architecture of the biain and which we 

 may term for convenience the Laio of Infiltration. 



By an inverse reasoning, if this law is correctly conceived, a 

 peripheral or fringing arrangement of the trigeuiinal territory 

 itself, and of fibres known to proceed to the cerebellum from it, 

 corroborates the identification of the trigeminal apparatus with 

 the cerebellum and the function of equilibrium, more primitively 

 even than the vestibular apparatus. 



These matters have been neglected by those who assert that 

 the auricle of the cerebellum, or lobus floccularis (Kappers, 1921), 

 is the oldest part of the cerebellum. It is not the auricle, but the 

 anterior medullary velum which would better deserve this appella- 

 tion. The " vestibular" liypothesis has been attractive because 

 it has seemed to demonstrate the fact that the cerebellum was 

 ** equilibratory" from the beginning. But this hypothesis tends 

 to ignore the equilibratory potential of all tactile sense, it fails to 

 account for the existence of a cerebellum (Cyclostomes) which 

 has no auricle, and does not account in any way for the striking 

 relationship which the trigeminal nerve always has to the cere- 

 bellum in all Vertebrata. 



The fifth nerve displays a relationship to that segment of the 

 neural tube, in which the cerebellum becomes developed, Avhich is 

 paralleled only by the relationsliip of the olfactor}'- nerve to that 

 segment of the tube in which the prosencephalon, and of the 

 optic nerve to that segment in which the mesencephalon is deve- 

 loped. The oldest part of the organ is the trigeminal territory 

 itself just as the olfactory sensorium was the oldest representative 

 of a fore bi'ain and the visual sensorium of a mid-brain. 



I have discovered that Spitzka (1886) put forward a somewhat 

 similar view concerning the cerebellum nearly forty yeais ago : — 

 " I am inclined to consider it as a homologue of the gelatinous 

 nucleus of the fifth paii^, and as in a primitive relation with that 



