80 Anatomy of the Nervous System 



run to the cortex, however, some passing to the region of the 

 optic chiasma, in the posterior part of which they decussate 

 as the commissure of v. Gudden or postoplic commissure {com- 

 missura supraoptica ventralis), and then running back to the 

 medial geniculate body or posterior colliculus of the opposite 

 side. The fibres of this commissure are rather fine and lie in 

 the ventro-medial edge of the optic tract. 



Covering the antero-dorsal aspect of the medial geniculate 

 body is the large, curved lateral or external geniculate body 

 {corpus geniculatum laterale), which is the terminal nucleus 

 for a large number of the fibres of the optic tract (Pis. III., 

 XML, X\^III.). The latter forms a covering of white matter 

 over the outer surface of the nuclear mass, into which a 

 large proportion of its fibres are seen to plunge, while the 

 remainder pass on to enter the anterior quadrigeminal body. 

 The percentage of the optic fibres ending in the geniculate 

 body is, as we have seen, smaller in the rat than in man, 

 corresponding with the relative functional importance of the 

 cortical connections in the two cases. The difference is not 

 so great as it might have been, however, on account of the 

 fact that a considerable part of the rodent geniculate body 

 does not serve, apparently, as a relay station to the cortex. 

 In the rat and other lower mammals, the lateral geniculate 

 body consists of distinct dorsal and ventral nuclei (PI. X\TII.), 

 of which only the dorsal one sends its axons to the cerebral 

 cortex. The ventral nucleus perhaps transmits its impulses 

 to the tectum of the midbrain. As one examines forms 

 higher in the mammalian series, the dorsal nucleus is found to 

 increase in importance and the ventral one to decrease, until 

 in man the latter cannot be recognized. The dorsal nucleus 

 in man, on the other hand, is greatly developed and shows a 

 lamellated structure with crossed and uncrossed fibres ending 

 in alternate layers. The fibres arising in the dorsal nucleus 

 form the radiatio optica or optic radiation, which joins the 



