164 University of California Pullications in Anatomy [Vol. 2 



parietal lobes (lower degenerated bundles tot in fig. 69 and especially 

 figs. 95, 96). These latter form in the parietal lobe the internal 

 sagittal layer of Gratiolet's radiation, that layer hardly existing in 

 the oecipital lobe (in the monkey). A number of efferent fibers, 

 however, descend also in the external sagittal layer itself (fig. 95). 

 The tapetum which is quite poorly developed in the occipital lobe 

 (better in the parietal lobe), is a callosal system (figs. 37, 38, 67, 68, 

 70, 95). 



As to the possibility of a partial decussation of the fibers of the 

 visual radiation through the splenium of the corpus callosum, a pos- 

 sibility accepted by some investigators (Heine, Lenz, Niessl von 

 Mayendorf, R. A. Pfeifer, 1925; Foerster, 1929, Foerster-Penfield) to 

 explain the preservation of macular or "central" vision in cases of 

 the hemianopsia of a central origin, no evidence whatsoever has been 

 found in the present experiments which would favor such an hypo- 

 thesis (see also Flechsig, 1927, pp. 93, 94). The present experiments 

 decidedly demonstrate the unilateral character of the central visual 

 path from its beginning in the external geniculate body to its termina- 

 tion in the striate area of the same hemisphere.- This observation is 

 the counterpart of the observation of a similar unilateral character 

 found in the present experiments for the other two main afferent paths 

 of the cerebral cortex (see: Somatic Sensory System, and Auditory 

 System). In so far as the transmission of peripheral impulses to the 

 cerebral cortex alone is concerned, all three afferent paths : the somatic 

 sensory, the auditory, and the visual, represent in their central por- 

 tion above their diencephalic origin strictly unilateral, non-decussating 

 fiber systems. (The same was found in other experiments as valid 

 for the intrahemisplieric portion of the pyramidal fibers since no 

 cortico-bulbar and cortico-spinal fibers decussating in the corpus cal- 

 losum, as claimed by some investigators, were seen.) Where there is 

 a partial or a total decussation of the three main afferent paths, this 

 is accomplished below the level of the diencephalon or in the latter. 

 In the optic system : the chiasm ; in the auditory system : the corpus 

 trapezoideum and perhaps the midbrain; in the somatic sensory 

 system : the spinal cord, the brain stem, and perhaps also the between- 

 brain. The sparing of macular or "central" vision is due to special 

 anatomical peculiarities of the visual radiation which will be dealt 

 with later. 



See remark on p. 180. 



