48 THE BRAIN OF THE TIGER SALAMANDER 



of cerebral control of all somatic activities. But in the adult animal 

 the parts of this natural subdivision have so many distinctive con- 

 nections and physiological properties that it seems preferable to 

 treat them separately. 



In vertebrates below the mammals the tectum opticum is the chief 

 central end-station of the optic nerve; and, since the eyes are the 

 chief distance receptors in most of these animals, fibers of correlation 

 of all other sensory systems concerned with external adjustment 

 naturally converge to this station. The tectum, accordingly, becomes 

 the dominant adjustor of all exteroceptive systems. The tectum 

 mesencephali of Aml^lystoma has a larger optic part — the superior 

 colliculus — and a small, poorly differentiated nucleus posterior — the 

 primordial inferior colliculus. The latter is interpolated between the 

 tectum opticum and the cerebellum, and its connections suggest that 

 its most primitive functions are proprioceptive. It receives a small 

 primordial lateral lemniscus and evidently also serves such gen- 

 eralized auditory functions as this animal possesses (chap. xv). The 

 development of the optic nerve and adult tectal structure and con- 

 nections have been described in detail ('41, '42). Chapter xvi is de- 

 voted to the visual system ; for the arrangement of the mesencephalic 

 nucleus and root of the V nerve see page 140 and figure 13. 



Optic and lemniscus tracts and smaller numbers of fibers from 

 various other sources all terminate in a broad sheet of intermediate 

 neuropil, which is spread through the entire tectum and is nearly 

 homogeneous in texture (figs. 11, 93). The tectum is not definitely 

 laminated, though separation of the layers, which are conspicuous in 

 the frog, is incipient. Fibers diverge from it in all directions (figs. 12, 

 18, 21-24, 93). It is inferred from this structure that movements 

 activated directly from the tectum are of total-pattern type. Such 

 local visual reflexes as this animal possesses are probably patterned 

 elsewhere — in the thalamus and dorsal and isthmic tegmentum. Con- 

 ditioning of reflexes is probably effected in these areas and perhaps 

 also in the ventrolateral peduncular neuropil (p. 38). Experiments 

 upon Triturus and anurans (Stone and Zauer, '40; Sperry, '43, '44, 

 '45, '456) demonstrate anatomical projection of retinal loci upon the 

 tectum opticum. This is true also in Amblystoma (Stone, '44; Stone 

 and Ellison, '45), though the nervous apparatus employed has not 

 been described. 



The pretectal nucleus (figs. 2B, 35, 36, nuc.pt.) receives fibers di- 

 rectly from the retina and from the tectum, habenula, and cerebral 



