38 THE BRAIN OF THE TIGER SALAMANDER 



changed to a state of excitation in both the peduncular gray and the peripheral 

 musculature with which it is connected. This is immediately followed by volleys 

 from the tectum, pretectal nucleus, and thalamus tlirough the myelinated tecto- 

 and thalamo-peduncular tracts; and this may contribute a spatial factor determined 

 by the position of the exciting object in the visual field and the sector of the tectum 

 upon which this local stimulus is projected. The first overt movement, accordingly, is 

 an orientation of the body and the eyeballs with reference to the source of stimulus. 

 After an appreciable time the smaller fibers from the retina deliver their volleys, and 

 the small fibers of the correlating tracts are activated. These deliver to the peduncle, 

 not unmixed or purely visual impulses but discharges, fired or inhibited, as the case 

 may be, by the existing excitatory state of the correlating apparatus: and this, in 

 its turn, is determined by numberless nonvisual features of the total situation, 

 present and past. If, for instance, one of the visual or nonvisual components is 

 fatigued, this will affect the pattern of tectal discharge. 



In salamanders the delay between the preliminary orientation of the body and the 

 consummation of the reaction may be, and commonly is, very long — a period of 

 tension, which in a man we would call "attention" and which Coghill called the 

 "regarding" reaction (p. 78; Coghill, '33, Paper XI, p. 334; '36). During this 

 period an inconceivably complicated resolution of forces is in process within the 

 central apparatus of adjustment. Inhibition plays an important role here, and it may 

 be that the chief function of the ventrolateral superficial neuropil of the peduncle is 

 control of the inhibitory phase of these activities comparable with tliat suggested in 

 chapter xiv for the specific interpeduncular neuropil. Of the details of these adjust- 

 ments our knowledge is scanty, but some hints may be gathered from urther exami- 

 nation of the structure involved; and here, to simplify the problem, we shall con- 

 fine tJie discussion to the superficial peduncular neuropil. 



This neuropil is interpolated "in series" in the visual-motor path of the basal 

 optic tract, and this short circuit is connected "in parallel" with the longer visual- 

 motor circuits by way of the tectum and thalamus. The latter are very much larger 

 and more complicated, and they evidently comprise the major part of the appa- 

 ratus by which behavior is regulated by visual experience. The basal optic system 

 seems to be ancillary to the major system and to be related with it in two quite 

 different ways, first, as a nonspecific primary activator, as already explained, and, 

 second (and subsequently), as a device for modifying or "inflecting" (to borrow 

 Arnold Gesell's expression) the standard patterns of behavior by intercurrent influ- 

 ence of present activity in other fields, including, perhaps, conditioning and the 

 individuation of hitherto unaccustomed types of response. This second feature in- 

 volves further consideration of the nonvisual components. These have already been 

 listed, and some of them are shown in figure 23. 



The largest components of this peculiar neuropil are the optic and tlie visceral- 

 gustatory systems, and their fibers end exclusively here. This is why these are re- 

 garded as the primary components; all other systems of entering fibers are spread 

 more or less widely also in the deeper layers of peduncular neuropil. In the visceral- 

 gustatory system this neuropil is inserted in series in the ascending pathway 

 toward the hypothalamus, and there is probably a parallel system of conduction 

 with no synapse in the peduncle (fig. 8). As seen in tliis figure, peduncular neurons 

 activated from this neuropil send axons around the tuberculum posterius into the 

 hypothalamus; and there is a return path from both dorsal (mamillary) and 

 ventral (infundibular) parts of the hypothalamus to this neuropil and contiguous 

 parts of the peduncle (figs. 18, 'TIS). All visceral-gustatory influences which reach 

 the hypothalamus are, accordingly, previously modified or "inflected" in the superior 



