brAIn of the ZEUGLoioNTiBii. 647 



nerve ; that subsequently tlie auditory nerve entered into con- 

 nection with it, and that, incieasing in dimension with its in- 

 creasing neural connections, it attained the high development 

 found m the human cerebellum through the spinal and cerebral 

 tracts that are detached into its medullary substance," There- 

 fore the i^resent statement of this relationship, although more 

 detailed, cannot be regarded as a novelty. 



If we consider the nature of the iniiuences at work in the 

 production of the early chordate brain, it is evident that the more 

 anterior segments were concerned in the production of the 

 olfactory and visual specialisations. These specialisations were 

 bought at the price ' of successive anterior segments of the 

 primitive vertebrate skin and neural tube. When these anterior 

 segments lost by their specialisation the capacity for the appre- 

 ciation of pressure, the succeeding segment wiiicii retained this 

 power grew forward invading the territory of the more specialised 

 segments lying anterior to it. Such an invasion meant an in- 

 crease or hypertrophy of this (trigeminal) segment and a special- 

 isation within it of the tactile function. How exuberant has 

 been its response to this demand is demonstrated by the fact that — 

 amongst Mammalia, for instance — it subserves the tactile sense 

 . for the entire region anterior to the second cervical nerve, with the 

 exception of the vestigial somatic elements of the vagus (Arnold's 

 nerve). 



The information conveyed by the trigeminal system proved 

 extremely valuable, because it told the animal so much about 

 what moved in space around it and also about its own position in 

 sjKice. Hence the central processes from the trigeminal ganglion 

 assumed a sudden reflex ascendency over an increasing number 

 of the succeeding segmental nerves through the radix descendens 

 and the substantia gelatinosa Ilolandi in most Vertebrata. Even 

 in Man the descending root of the trigeminal represents one of 

 the most impressive features of the medulla oblongata. Always 

 a striking feature in Mammalia, this reflex dominance is shown 

 to its best advantage (in living forms) in Ornithorhynchus ; in 

 this creature the trigeminal seems to exert a reflex control over 

 practically the whole of the spinal cord. 



Thus there has been a downward encroachment of the trige- 

 minus upon lower segments of the medulla oblongata in a fashion 

 comparable with the upward encroachment of the vestibular 

 nerve into the trigeminal territory or of the mesencephalic root 

 of the trigeminal upon optic territory. It is a further example 

 of the Law of Infiltration. Seemingly this encroachment afforded 

 the bridge whereby the spino-cerebellar systems were conducted 

 to the originally trigeminal territory of the cerebellum. For the 

 spinal fibres naturally lay peripheral to the infiltrating descending 

 root of the trigeminus, and as the spinal fibres coursed towards 

 the cerebellum tiiey came to embrace the trigeminus trunk before 

 piercing its territory to enter the cerebellum. The differentiation 

 into anterior and posterior spino-cerebellar tracts emerged from 



