210 CENTRAL NERVOUS SYSTKM 



nucleus of the thalamus. These neurons that end in the thala- 

 mus are continued by a third set of neurons which end in the 

 parietal lobe of the cerebrum. In the medulla and pons the 

 lemniscus receives accessions of sensory fibers from the sensory 

 nuclei of the cranial nerves of the opposite side. 



The Visual Area. Removal of or injury inflicted upon the 

 occipital lobes is followed by defects in vision. Goltz contended, 

 from the reactions shown by his decerebrate dog, that some 

 degree of vision remains after extirpation of the occipital lobes, 

 for in this case the dog closed his eyes when a strong light was 

 thrown into them. This, however, together with constriction 

 of the pupil, may be regarded as a reflex through the mid- 

 brain, and not accompanied by a visual sensation. Complete 

 removal of the occipital lobes is followed, apparently, by total 

 blindness. When the lesion is unilateral the blindness affects 

 symmetrical halves of the two eyes, a condition known as 

 hemiopia. A right-handed destruction causes blindness of the 

 two right halves of the retinae, and, in accordance with the 

 laws of projection of retinal stimuli, in the two left halves of 

 the visual fields. There is, however, one very important excep- 

 tion. This is the fovea centralis, the area of most acute vision, 

 which is not affected by unilateral lesions of the occipital lobes. 



The paths over which visual impulses are carried to the brain 

 undergo more or less decussation in the chiasma. This crossing 

 is less complete in the mammalia than in the lower animals. 

 In fish, reptiles, and birds the crossing is said to be complete. 

 The fibers from the inner side of each retina cross, while those 

 from the outer side do not decussate but pass into the optic 

 tract of the same side. These fibers end mainly in the gray 

 matter of the external geniculate body, but some pass to the 

 pulvinar of the thalamus and some to the superior colliculus 

 of the corpora quadrigemina. These structures are known as 

 the primary optic centres. From these points the path is con- 

 tinued to the cortex through the occipitothalamic radiations 

 lying in the posterior limb of the internal capsule. There are 

 other fibers in connection with the optic nerves and tracts that 

 must be mentioned. The inferior or Gudden's commissure 

 consists of fibers passing from one optic tract to the other along 

 the posterior border of the chiasma, connecting the internal 

 geniculate and inferior colliculus of one side with those of the 



