BASE OF BKAIN, OPTIC NERVE, AND COEPORA QUADBIGEMINA. 385 



pora, like the thalami, receiye fibers from the territory of the occipital lobe, 

 which run to the internal capsule in the optic radiation, and from there 

 ascend to the corpora as the hrachia of the anterior corpora quadrigemina. 

 Fibers to the optic tract itself also run downward in this very brachium, or 

 arm. 



The brachium of the anterior corpus quadrigeminum, which is com- 

 posed, therefore, of fibers from the cortex and fibers passing to the optic 

 tract, sends its cortical fibers alone into the coipus quadrigeminum; its 

 optic-nerve fibers spread out over the gray surface of the quadrigeminal 

 body, thus forming the stratum zonale, and there sink below. 



The posterior corpus quadrigeminum appears, at first sight, it is true, 

 til stand in some relation with the optic tract, but it is very improbable that 

 it contains fibers which are used in the Adsiial act. Its brachium arises from 

 the corpus geniculatum mediale and also from the commissura inferior (Gud- 

 den's commissure), not previously mentioned, which passes along with the 

 •optic tract to the posterior angle of the ehiasma. 



The posterior quadrigeminal body receives its coronal fibers [Monalcow) from 

 the lobus temporalis. The extraordinary development of the posterior corpus quad- 

 rigeminum in whaleSj and the huge tracts which, in these vertebrates, pass from 

 there to the acusticus nucleus of the opposite side make it probable (Spitzlca) that 

 this ganglion stands in some relation with the auditory nerve. The results of ex- 

 perimental investigations, directed thereto, are in accord with this view. After de- 

 struction of the auditory-nerve nuclei, the secondary fiber-systems of the same 

 atrophy as. far as the posterior corpora quadrigemina (Baginsky, Bumm). 



When seen from the side, the relations of the quadrigeminal brachia 

 to the ganglia and the tractus opticus are very clear; likewise the rela- 

 tions of the corpora geniculata: the corpus geniculatum mediale, lying ad- 

 jacent to the posterior brachium; the corpus geniculatum laterale, appearing 

 to be thrust in between the pulvinar and the tractus opticus, of which 

 mention was made when the thalamus was described. 



The tractus opticus receives fibers from the latter ganglion, in addition 

 to fibers from the pulvinar thalami and its stratum zonale. 



The opticus fibers from the anterior corpora quadrigemina were men- 

 tioned previously. They probably run, for the greater part, in the brachium 

 of the anterior corpus quadrigeminum. 



In the lower vertebrates the optic nerve arises mostly- from the anterior corpora 

 quadrigemina; the other centers of origin, on the other hand, are very insignificant. 

 The greater the development of the occipital cortex, ho\vever, — the cortex which 

 sends its fiber-systems principally into the other centers, or terminals, and only 

 supplies the anterior corpora quadrigemina with a relatively insignificant afferent 

 tract, — the more do the fibers of the optic nerves arise from these terminals, and the 

 less from the anterior quadrigeminal bodies. This still continues in the mammalian 



