SPECIAL CUTANEOUS SUBDIVISION. I37 



These facts lead to the conclusion that all these cells have been 

 derived by modification from the simple large cells of the general 

 cutaneous nuclei. The modification is due directly to the influence 

 of the fine fibers of the molecular layer and in the cerebellum, 

 where the granule cells are collected in greatest numbers and the 

 fine fibers are most numerous, the large cells are nearly all special- 

 ized into Purkinje cells. The presence of great numbers of 

 granule cells, of cells of type II and of highly developed Purkinje 

 cells marks the cerebellum as the most highly speciahzed part 

 of the special cutaneous nuclei. The specialization of both cere- 

 bellum and acusticum is to be attributed to the influence of 

 the special cutaneous system of sense organs. In higher verte- 

 brates these centers no longer receive many general cutaneous 

 fibers, but undergo a still higher speciaHzation, in part as the 

 centers for the ear and in part as an apparatus for controlHng 

 bodily movements. 



It is necessary now to foUow the tractus bulbo-tectalis w^hich 

 receives the greater part of the secondary fibers from the special 

 cutaneous nuclei, and see its relations in the mesencephalon. 

 The roof of the mesencephalon in the fishes begins to show a 

 differentiation into two parts, a median somewhat dome-shaped, 

 bi-lobed tectum opticum and a lateral thicker mass forming a 

 semicircular border about the tectum on either side. These 

 lateral masses are known as the colliculi or the lateral mesen- 

 cephalic nuclei. The origin and significance of these parts are 

 more fully treated in Chapter XVI. A process of differentiation 

 is seen in this region analogous to that which has been described 

 in the cutaneous centers of the meduUa oblongata. In the simplest 

 condition in vertebrates the fibers which pass from the cutaneous 

 center in the hindbrain to the midbrain (tractus bulbo-tectalis) 

 end indiscriminately in aU parts of the roof of the midbrain. 

 When the coUiculi are weU developed it is noticed that the greater 

 part of the tract ends in them, not in the tectum opticum. From 

 these nuclei in bony fishes tertiary tracts go to the tectum opticum, 

 as wefl as to the inferior lobes and the motor centers of the medulla 

 oblongata. From the tectum opticum in fishes an important 

 tract goes to the cerebellum. The presence of this tract is one 



