894 



THE CENTRAL NERVOUS SYSTEM 



pulvinar (or posterior portion) of the optic thalamus, and the anterior 

 corpus quadrigeminum. In young animals all these structures undergo 

 atrophy after extirpation of the eyeball. The visual path is continued 

 from the pulvinar and the external corpus geniculatum by the axons 

 of these nerve-cells, which proceed in the optic radiation (p. 855) to the 

 occipital cortex. The fibres which pass from the retina to the anterior 

 corpus quadrigeminum are distinguished by- their small size, and 

 probably constitute the path of the impulses which cause contraction 

 of the pupil when light falls on the retina. The reflex arc is schematic- 

 ally shown in Fig. 360, where optic nerve-fibres are represented as 

 forming synapses with cells in the anterior corpus quadrigeminum 

 whose axons pass to the nucleus of the third nerve and arborize around 



some of its cells (Figs. 344, 346, 

 and 360). At the chiasma the 

 fibres of the optic nerve de- 

 cussate, partially in man and 

 some mammals, as the rabbit, 

 dog, cat, and monkey, com- 

 pletely in animals whose visual 

 field is entirely independent for 

 the two eyes, as in fishes and 

 birds. In man the fibres for 

 the nasal halves of both retinae 

 cross the middle line at the 

 chiasma, those for the temporal 

 halves do not. This does not 

 mean, however, that exactly 

 half of the optic nerve-fibres 

 decussate. The number of un- 

 crossed fibres is smaller than 

 il Nerve that of crossed. The chiasma 

 also contains fibres in its pos- 

 terior portion, which extend 

 from one optic tract to the 

 other, but are not connected 

 with the retinae or the optic 

 nerves. They are commissural 

 fibres which connect the two 

 mesial geniculate bodies across 

 the middle line, and are called 

 Gudden's commissure. A suffi- 

 ciently extensive lesion involv- 

 ing the occipital cortex on one 

 side, or the posterior portion 

 of the optic thalamus, or the 

 optic tract, causes hemianopia* or defect of the visual field on the 

 side opposite to the lesion, with blindness of the corresponding halves 

 of the two retinae. Thus, a lesion equivalent to complete section of the 

 right optic tract would cause blindness of the nasal half of the left, and 

 of the temporal half of the right eye, and the left half of the field of 

 vision would be blotted out the patient would be unable, with his eyes 

 directed forwards, to see an object at his left. Such a complete 

 hemianopia is much rarer in disease of the cortex than in disease of the 

 * The terms ' hemiopia/ ' hemianopia,' ' hemianopsia/ are used with refer- 

 ence sometimes to the blind side of the retinae, but ordinarily to the half of the 

 visual field which is deficient We shall always use the word ' hemianopia ' 

 in the latter sense. 



Fig. 360. Scheme of the Visual Path (after 

 Schafer). 



