220 THE BRAIN OF THE TIGER SALAMANDER 



though the time of the appearance of these fibers has not been 

 recorded. Tecto-bulbar and spinal fibers mature very soon after 

 those of the posterior commissure, the uncrossed fibers preceding the 

 crossed fibers. These and the tecto-peduncular fibers described below 

 are apparently the pathways employed in the activation of the 

 skeletal musculature of the trunk and head when they are employed 

 in the capture of food. Early feeders (stage 46) have attained full 

 larval status, and in subsequent stages the visual system is not 

 radically changed except for an increase in the number of thin retinal 

 fibers and an enormous increment of these at metamorphosis, the 

 total number of fibers in adult Amblystoma tigrinum probably reach- 

 ing about 8,000 ('41, p. 506). 



Correlation of this developmental history with experimental work 

 by others has led me to the hypothesis that in Amblystoma the thick 

 retinal fibers, which appear first in ontogeny, activate generalized 

 total behavior patterns and that the thin fibers are concerned with 

 more refined analysis of visual experience. (It does not follow that 

 this is true in mammals, in which the visual responses are under 

 thalamo-cortical control.) What kind of mechanism is employed in 

 this analysis is still obscure. My opinion ('41, p. 528) that endings 

 of thin and thick fibers have separate locahzation in the tectum 

 proved on further study to be unfounded ('42, pp. 284, 293), and 

 there is little anatomical evidence of any other kind of visual localiza- 

 tion here, though experimental evidence cited below indicates that 

 retinal loci are projected locally upon the tectum. 



The more important known connections of the visual system are 

 shown in the simplified diagrams (figs. 6, 11-16, 18, 20-24, 93, 101; 

 '36, figs. 2, 6, 7; '42, figs. 4, 5, 79). Fibers of the optic nerve are dis- 

 tributed to five quite separate fields in the brain stem: (1) tectum, 

 (2) pretectal nucleus, (3) cerebral peduncle, (4) thalamus, and (5) 

 hypothalamus, as shown schematically in figure 14. This wide dis- 

 tribution is in marked contrast with the tendency of the other func- 

 tional systems of peripheral nerves to converge into a single primary 

 central field. This dispersal appears to have been determined by the 

 types of motor response to be evoked rather than by the sensory 

 qualities of the visual excitations. These five receptive fields will now 

 be examined. 



1. The tectum opticum evidently is the most extensive of these 

 fields and physiologically the dominant member (see below). 



. The pretectal nucleus may participate in the regulation of the 



