CENTERS OF CORRELATION. 265 



of man and that of a fish. Whereas the sensory impulses in a 

 fish are correlated by the relatively slightly organized centers of 

 the mesencephalon and call forth simple, direct and often very 

 quick responses; the center for correlation in man is removed 

 to the highly organized cerebral cortex and the responses lose 

 something in quickness but gain vastly in precision and in the 

 completeness with which they are adapted to the several factors 

 of the situation. 



There should be added to this account some mention of the 

 centrifugal fibers in the optic tracts which end in the retina. The 

 presence of such fibers is demonstrated by the Golgi method 

 which shows their origin, course and their endings in the internal 

 molecular layer of the retina (Figs. 71, 72.). Their presence 

 and course is also shown by the methods of primary and secondary 

 degeneration. In fishes in which one eye has long been lost the 

 optic tract of the opposite side degenerates with the exception of 

 these fibers, which persist and are stained by the Weigert method. 

 In mammals, following section of the optic tract there occurs 

 secondary degeneration of cells in the anterior quadrigeminum, 

 and in the dorsal part of the geniculatum laterale and pulvinar. 

 These findings in mammals agree with those in fishes by the 

 Golgi and degeneration methods, where the centrifugal fibers 

 arise from the tectum opticum and geniculatum (Catois). The 

 significance of these fibers is not understood but their presence 

 in all vertebrates seems to show that they have some constant 

 function. It has been suggested that they are examples of fibers 

 which pass forward from the cutaneous center of one segment 

 to that of a center farther forward, and hence homologous with the 

 fibers of the lemniscus system. 



3. CENTERS RELATED TO THE POSTERIOR COMMISSURE. 



The posterior commissure, although it has long been used as 

 one of the most prominent and constant landmarks in the brain, 

 is still very imperfectly understood. In Petromyzon (Fig. 132) 

 the fibers of the commissure arise from cells widely scattered 

 through the dorso-caudal part of the thalamus and the cephalic 

 part of the tectum opticum and the collicular region. The fibers 



