THE CORTEX OF THE FORE-BRAIN, ETC. - 75 



5. Bundles run from the cortex of the superior parietal 

 lohc (and the posterior central convolution'?) and perhaps, also, 

 from more posterior cortical regions, to the internal capsule, and 

 in part pass under the thalamus to the spinal cord, and partly 

 enter the lenticular nucleus. They pass through both inner 

 divisions of the latter, and become united near the base of the 

 brain to form a thick strand, whose course we shall have occasion 

 to study later on (tegmental radiation). These latter are the 

 first cerebral fibres to receive their investment of medullary 

 sheaths. They alone at the eighth to ninth foetal month may 

 be recognized as a thin, white bundle in the internal capsule, 

 which latter, at this period, appears gray. 



6. Fibres pass from the occipital lobe to the points of origin 

 of the optic nerve. They connect the real optic nucleus with 

 the cortex. In Fig. 44 this optic radiation is shown in a hori- 

 zontal section of the brain of a nine-week-old child. 



Its destruction in human beings leads to disturbances of 

 vision, which will be described later on. In animals it does not 

 appear to be so important, for in them the occipital cortex can 

 be destroyed tin both sides without producing blindness.. The 

 actual centres for the sense of sight lie deeper; sight continues 

 if only they are preserved intact, but it is diminished to a certain 

 extent if the connection between these lower centres and the 

 cortex is destroyed. This connection, which evidently subserves 

 some psychic process, is most important in human beings; it is 

 apparently less so in the other mammals. In the lower animals, 

 fishes, for instance, it is altogether wanting. -These latter see, 

 at least in the case of teleostians, without anything more than a 

 thin, epithelial vesicle in place of a cerebrum. 



Doubtless there are a great number of other systems in the 

 corona radiata. Observations bearing on this point must be 

 undertaken on the brains of very \oung children. The fibres 



* * cj 



receive their medullary investment at different periods, and, so 

 for as we know, the whole corona radiata is medullary at the 

 end of the second year. 



