RRAIN OF THE ZEUGLODONTID^. 645 



neural tube in this vicinity is associated in the vertebrate embryo 

 with the trigeminus (c/. Wilson and Hill, 1902; Streeter, 1908), 

 while the aconstico-facialis mass lies entirely posterior to the 

 trigeminus and is related to a different territory which lies medial 

 to it. Hence, any encroachment of vestibular fibres from its 

 territory of the neural tube into the territoiy which is related 

 to the trigeminus is a secondary phenomenon whether con- 

 sidered phylogenetically or ontogenetically, even though such 

 fibres be direct fibres. In other words, the appearance of vesti- 

 bvdar fibres in the cerebellar region is an intrusion just as 

 certainly as is the appearance of the mesencephalic fibres of the 

 trio-eminal in the mid-brain, or of thalamic fibres in the fore 

 brain. 



The simplest cerebella reproduce more or less faithfully this 

 condition characteristic of the embryos of all Vertebrates. They 

 are mere ridges roofing the most anterior portion of the fourth 

 ventricle and are consequently a link between trigeminal terri- 

 tories and contain decussations of the trochlear and trigeminal 

 nerves (Herrick, 1914; Larsell, 1920; Palmgren, 1921). 



In the simplest cerebella intrusions of alien fibres are already 

 found, even as we find alien fibres iu the simplest-known fore 

 brain and tectum opticum. These intrusions occur in the cere- 

 bellum in such a way that just as we find the olfactory apparatus 

 is relegated to the periphery of the fore brain and the optic fibres 

 to the periphery of the mid-brain, so we find that the trigeminal 

 territory forms the true fringe of the cerebellum. Consequently, 

 Beccari, Ingvar, and others (Kappers, 1921) have found a direct 

 distribution of the trigeminus to the cerebellum {e. r/., in 

 Reptilia). 



Topographically, the contribution of the vestibular nerve to the 

 cerebelhim is always surrounded by the trigeminal territory and 

 the fibres proceeding to the cerebellum therefrom. Ingvar (1918) 

 has shown (in Mammalia) a distribution of the vestibular com- 

 ponent mainly in the flocculus, lingula, and nodulus, and thus 

 peripherally to tlie still more recent intrusions of spino-cerebellar 

 contributions. On the other hand, spino- and olivocerebellar 

 fibres have not been demonstrated in such peripherally lying 

 parts such as the lobus floccularis, and it is to be noted {vide 

 Kappers, 1921) that this region is not dependent upon a cerebro- 

 pontine contribution ; whereas the clinical researches of Winkler 

 have corroborated the conception that the latest portions of 

 the cerebellum in the phylogenetic sense are more centrally 



situated. 



This "laminar" arrangement of the cerebellar constitution, as 

 it might be termed, although not so clearly defined as the some- 

 what similar arrangement of successive fibre-intrusions into the 

 fore brain, is nevertheless present. It is to be expected also 

 from the fact that those tracts, which later in phylogeny become 

 incorporated with any region, attain this incorporation by infil- 

 tration and a spreading apart of the pre-existing mechanism. 



42* 



