GENERAL CUTANEOUS SUBDIVISION. 117 



small cells form a thin layer of fine fibers covering the outer surface 

 of the acusticum and cerebellum. This represents the molecular 

 layer and the cell layer corresponds to the granular layer, which 

 are such conspicuous features of the cerebellum of higher forms. 

 The large cells are larger and more conspicuous and are arranged 

 with more regularity, their bodies being near the fourth ventricle 

 and their dendrites spreading in the fiber layer. Then neurites 

 of these cells go as internal arcuate fibers to join those from the 

 nucleus funiculi and run forward. The large cells of the acusticum 

 and of the cerebellum are shown in Fig. 58. The cerebellum 

 is very small and consists of a continuation of the acusticum for- 

 ward, upward and mesially so that the two halves of the cerebellum 

 meet over the fourth ventricle. Here there occurs a commissure 

 formed by the neurites of the small cells. By means of these 

 fibers the cutaneous impulses are carried from the centers of one 

 side to those of the other. In histology also the cerebellum 

 differs little from the acusticum. Its large cells are not special- 

 ized as are the Purkinje cells in the cerebellum of higher forms, 

 although by comparing them with the Purkinje cells in other 

 fishes one can see that they are the forerunners of such cells. 

 The neurites of the large cells go as internal arcuate fibers to 

 the opposite side of the brain, but whether they join those from the 

 nucleus funiculi and acusticum is uncertain. The latter fibers 

 pass forward and upward to enter the roof of the mesencephalon, 

 forming the tractus bulbo-tectalis. 



The roof of the mesencephalon is thus a secondary center for 

 general cutaneous impulses. Its structure and the differentia- 

 tion of centers in it will be described in the chapters on the visual 

 apparatus and on the higher correlation centers (Chaps. VIII 

 and XVI). Here it is necessary only to state the course of tracts 

 arising in the tectum mesencephali which are of importance for 

 the cutaneous apparatus. The first is a tract of fibers which 

 arise from the cells of the tectum and run in a direction opposite 

 to, but parallel with that of the tractus bulbo-tectalis and have 

 their endings in the medulla oblongata or spinal cord. The tract 

 is called the tractus tecto-bulbaris et spinalis. It descends from 

 the tectum over the outer surface of the mesencephalon and bends 



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