{ 27 ) 



fiients of (his kind, wliicli would be present in liie Riiin. I, oi' iit 

 least relro-orbitallj. 



I do not consider, therefore, cillhoiigh there seems to be fairly strong evi- 

 dence of it, that my observation may be taken as a proof of the sensitive 

 nature of at least a part of the mesencephalic nucleus. What restrained 

 me from doing so, l)esides the uncertainty of the peripheral course 

 of the degenerated fibres (inside or outside the stem of Trig. I) and 

 the slight possibility that there might also be motor fibres in Trig. I, 

 was the relationshij) of the mesencephalic root to its nucleus. Afferent 

 nerve fibres end in the central nervous system without a single 

 exception known to me, with (fine) ramifications to the periphery 

 of a cell, or they j)enetrate through it, if the theory of neurones be 

 not adhered to. In no case do they stand in direct relationship to 

 this cell in the manner of a commencing axis-cylinder, or in the 

 way we know of in the spinal ganglia. If indeed the mesencephalic 

 quintus cells are sensory by nature, they prove an exception and 

 a very phenomenal one, to the regular relationships to be met 

 with elsewhere. Johnston ^), in fact, now accepts this exception, and 

 he bases his belief upon the dorsal position of the cells (above the 

 sulcus limitans of His), upon their spinal, ganglion-like appearance, 

 upon a supposed analogy to dorsal, giant cells in amphioxus and 

 other vertebi'ates (which he considers as sensory), and finally upon 

 the way in which the mesenc. root leaves the oblongata, which he 

 thought he could show in some fishes and mammal-embryos as taking 

 place with the pars major (sensibilis). 



In mj^ oi)inion, these arguments which, as we have seen, have 

 not a convincing foi'ce, must l)e used with caution, and therefore 

 1 had recourse to comparative anatomy where pathology and experi- 

 ments were unable to help me furthei-. 



I exandned specimens of cyclostomes, selachii, teleostei, amphibia, 

 birds and mammals, and, of these last, specimens of the orders of 

 monotremata, marsupials, insectivora, chiroptera, edentata, rodentia, 

 carnivora (fissipedia and pinnipedia), cetacea, ungulata and primates. 



Among all these groups ihei-e is only one in which a mesenc. trige- 

 minus root is not present, viz. (he ci/clostomes {Petroini/zon). This 

 observation agrees with Tkkt.iak()Ff's '), w ho, however, suggests the 

 analogy of the most frontal part of (he (piinlus nucleus in (lie 



1) Johnston. The Radix mescncephalica Trigemini. Jouin. of compar. Neurol. 

 and Psychol. 1909. 



-) Tretjakokf. Das Ncrvensyslem vt)ü Ammocoetes. Arch. 1'. Mikrosk. Anal. 1909. 



