OPTIC AND VISUAL-MOTOR SYSTEMS 221 



intrinsic musculature of the eyeball and have other functions still 

 unknown. 



3. The basal optic tract is present in all classes of vertebrates from 

 cyclostomes to man. It is evidently of major importance, though 

 little attention has been given to it in current studies of the physiol- 

 ogy of vision. Two possible functions have been suggested for it in 

 Amblystoma. The more primitive of these is to act as a general ac- 

 tivator of the ocular and somatic muscles in response to visual 

 stimuli, manifested in such behavior as the "regarding reaction" de- 

 scribed by Coghill (pp. 38, 78). In the second place, the structure 

 and connections of the area ventrolateralis pedunculi, within which 

 the basal optic fibers terminate, suggest that this neuropil may be 

 part of the apparatus of conditioning of visual reflexes (p. 38). 



4. Optic terminals are widely spread in the thalamus and are espe- 

 cially concentrated in its posterior part. This ill-defined area of neu- 

 ropil receives the brachia of the superior and inferior colliculi and is 

 regarded as the primordium of the lateral and medial geniculate 

 bodies (p. 239; '42, p. 280). Large numbers of efferent fibers go 

 out from this field to the peduncle and the dorsal and isthmic teg- 

 mentum. The fibers to the tegmentum take four courses. Two are 

 superficial, the uncrossed tractus thalamo-tegmentalis rectus and a 

 crossed path ('42, p. 224, tr.thieg.d.c.A.) which decussates in the 

 postoptic commissure. Two other tracts take deep courses close to 

 the gray, one of which decussates ('42, p. 224, tr.th.teg.d.c.B.) and the 

 other descends uncrossed in dorsal tegmental fascicles of group (7). 

 For a detailed description see the analysis of the postoptic com- 

 missures in chapter xxi. These four tracts end chiefly in the dorsal 

 and isthmic tegmentum, two of them in the superficial neuropil and 

 two at deeper levels. They provide pathways for visual control of 

 local reflexes, particularly those employed in capturing food. There 

 is no separate optic projection tract from the thalamus to the cerebral 

 hemisphere, though visual influence may reach the hemisphere 

 through the thalamo-frontal tract, which is a common pathway of 

 sensory projection from the dorsal thalamus (pp. 95, 238, and fig. 

 19, tr.th.f.). 



5. There is a small number of optic terminals and collaterals in the 

 hypothalamus near the chiasma ('42, pp. 219, 233, 237) . They connect 

 with the large cells of the preoptic nucleus, axons of which form the 

 hypophysial tract (p. 245). This is doubtless the pathway for visual 

 influence upon hypophysial endocrine activity. This connection is 



