OPTIC AND VISUAL-MOTOR SYSTEMS 223 



that are clearly evident are the deep gray and the superficial white, 

 though there are obscure indications of the incipience of further 

 stratification, more evident in the alba than in the grisea. For de- 

 scriptive purposes I have divided the optic tectum, somewhat ar- 

 bitrarily, into eight concentric layers, as shown in figures 36 and 93; 

 for their characteristics see page 244 of 1942. 



Two features of tectal structure merit special emphasis. The first is 

 that the convergence of most of the afferent fibers into a common 

 pool of neuropil of almost homogeneous structure suggests a totaliz- 

 ing or integrative function for the tectum as a whole. The second 

 point of major interest is that the efferent fibers from the tectum are 

 extremely diversified in structure and distribution. There are all 

 gradations from diffusely spread fibers in the deep neuropil to well- 

 defined long myelinated tracts like the tecto-bulbar and tecto-spinal 

 systems. This suggests that such localization of function as exists in 

 this tectum is determined more by what is going on in the efferent 

 side of the arc than in the afferent side, a point which has been em- 

 phasized by Crosby and Woodburne ('38). 



Five strong systems of myelinated fibers leave the tectum. These 

 take widely divergent courses. They are as follows: 



1. Commissura posterior. — These are the first to appear in the 

 embryo. The crossed fibers are joined by uncrossed fibers from the 

 eminence underlying the commissure, and they spread widely in the 

 peduncle and adjoining parts of the thalamus and dorsal tegmentum. 

 The primary connection is with the nucleus of Darkschewitsch 

 (p. 217 and figs. 6, 18, 22) and thence to the f . longitudinalis medialis, 

 providing innervation of the trunk musculature in response to visual 

 stimulation. 



2. Tractus tecto-peduncularis rectus et cruciatus (p. 303 and figs. 18, 

 22, tr.t.p.c; '42, p. 267). — These thick fibers arise from the anterior 

 part of the tectum, descend parallel with fibers from the posterior 

 commissure, decussate (in part) in the commissure of the tuberculum 

 posterius, and arborize in the alba of the peduncle of both sides more 

 ventrally than those of the posterior commissure. Their course as 

 seen in horizontal sections is shown in figures 30-36, tr.t.p.c.l. (see 

 also '42, p. 267, and the figures there cited). They activate the in- 

 terstitial nucleus of the f. longitudinalis medialis and probably also 

 the oculomotor nucleus and appear to be adapted to regulate mass 

 movements of trunk, limbs, and eyes in the orientation and move- 

 ments of the body with reference to objects in the visual field. These 



