86 University of California PuMications in Anatomy [Vol. 2 



of the epithalamiis around a longitudinal axis. The first of the 

 peculiarities of the auditory radiation, as described, the consequence 

 of phylo- and ontogenetic factors, indicates an incomplete rotation of 

 that fiber system in the sense of a spiral around a transverse axis 

 which can be imagined as extending from the internal geniculate body 

 to the temporal cortex. The second rotation is evidenced by the 

 obliquely sectioned fibers of the "handle" of the auditory radiation 

 near the external geniculate body (see accompanying figures). We 

 can look upon the cause of this rotation of the radiation, as it appears 

 in the adult brain of primates, as an adaptation to the changed condi- 

 tions as compared with that in lower mammals. Its only interpretation, 

 however, must be the necessary preserv-ation of the original mutual 

 relations of the individual bundles and fibers of the radiation to one 

 another and above all to special segments of the internal geniculate 

 body and the auditory projection cortex. And this in turn renders 

 possible an exact projection of the receptor surface of the auditory 

 peripheral organ, the coehlea, upon the auditory projection cortex. 

 (Compare Chapters XII, XIX.) 



The caliber of most of the central auditory fibers is fairly large 

 and is hardly inferior to that of the coarse somatic sensory (thalamo- 

 cortical) elements. Yet there are also fibers of a medium size and 

 even a small number that are fairly thin. It would be of importance 

 to know if there are regional differences in caliber, whether for 

 instance, as seems to be the case, the fibers supplying the most anterior 

 (rostral) segments of the auditory projection cortex are of smaller size 

 than the fibers entering the most posterior (caudal) segments of that 

 cortex. This would be important since the "representation" of low 

 tones is claimed to be in the oral-lateral segments, and of the high 

 tones in the caudal-medial segments of the auditory projection cortex. 

 (Compare also different size of cells of the spiral ganglion correspond- 

 ing to different segments of the cochlea, my paper, 1926. ) 



The absolute number of fibers of the entire auditory radiation, 

 though considerable, is below that of the visual radiation and is con- 

 siderably smaller than the number of fibers forming the somatic 

 sensory (thalamo-cortical) radiation, which well harmonizes with the 

 difference in size of the peripheral receptive surfaces : of the cochlea 

 (papilla basilaris), of the retina, and of the surface of the body, 

 muscles, joints, etc. 



No evidence was found in support of a partial crossing or recross- 

 ing of the auditory radiation to the opposite hemisphere by way of 

 the corpus callosum. 



