THE COMMISSUKES. 245 



radiation" is reproduced from a horizontal section through the brain of a 

 child 9 weeks old. 



In man its destruction leads to homonymous hemianopsia (see below). In ani- 

 mals it does not appear to be of so great importance, for the occipital cortex may be 

 destroyed on both sides without producing permanent actual blindness. The real 

 centers for the sense of sight, consequently, lie deeper. Sight may be retained if 

 these alone are preserved; it is impaired, however, when the connection between 

 these centers and the cortex is destroyed. This connection, probably serving for 

 psychical processes, is most important in man; apparently of lesser importance in 

 the other mammals, it is wanting entirely in lower animals, — e.g., the fish. These, 

 at least the Teleostei, see without possessing, in general, anything more than a thin 

 epithelial vesicle in place of a cerebrum. 



2. Cortical tracts extend into the midbrain, partly from the occipital 

 lobe by way of the above-mentioned optic radiation, partly from the tem- 

 poral lobe to the terminations of the secondary radiation of the auditory 

 nerve. The fibers arising from the end-nuclei of the acusticus extend up- 

 ward as far as into the ganglia of the midbrain. There they soon end, but 

 the cortical tracts begin at the place where they terminate. 



3. In the region ventral to the thalamus a tract is soon lost to further 

 observation which Flechsig has named the tegmental radiation. From the 

 cortex of the superior parietal lobule (and from the posterior central gyrus?), 

 perhaps from cortical regions, also, lying still farther posterior, its fibers 

 extend into the internal capsule and pass, in part, below the thalamus, 

 toward the spinal cord; in part, they sink into the lenticular nucleus. 

 They pass through the two inner divisions of the lenticular nucleus, then 

 unite again close to the base of the brain to form a more compact bundle, 

 the course of which we shall become familiar with later. These fibers are 

 the first to become medullated in the cerebrum. They alone are to be 

 recognized in fetuses of the eighth to ninth month as thin, white tracts 

 in the internal capsule, which at this time appears gray (Fig. 2). 



But this is not the end of the cortical radiation. Its most caudal por- 

 tions pass into the pons, the medulla oblongata, and the spinal cord. 



4. The cortical tracts to the pons, tractus corticis ad pontem, are divided, 

 after Flechsig, into the frontal system from the frontal lobe, and the pos- 

 terior, from the occipital and temporal lobes. The fibers end in the pons 

 in large ganglia, from which originate the pedimcles of the cerebellum. 



5. The speech-tract, the tractus cortico-bulbaris, runs to the nuclei, in 

 the oblongata, of the nerves that are necessary for speech. Its origin in the 

 cortex of the inferior frontal gyrus, its course throxigh the medullary layer 

 external to the tail of the nucleus eaudatus and its termination in the above- 

 mentioned nuclei, all have been inferred from clinical cases that have been 

 carefully observed and verified by autopsies. It has not as yet been demon- 



