. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1118 VISION. being seen nearly in a straight line drawn per- pendicularly to the retina from that point of it where its image falls and this law of visible direction may be regarded as exercising an important influence on the production of single vision. In reference to this phenomenon, it is ne- cessary to advert to the remarkable structure of the commissure of the optic nerve. The chiasma results from the junction of the optic tracts, in front of and inferior to the tuber cinereum. The fibres which constitute the inner
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1118 VISION. being seen nearly in a straight line drawn per- pendicularly to the retina from that point of it where its image falls and this law of visible direction may be regarded as exercising an important influence on the production of single vision. In reference to this phenomenon, it is ne- cessary to advert to the remarkable structure of the commissure of the optic nerve. The chiasma results from the junction of the optic tracts, in front of and inferior to the tuber cinereum. The fibres which constitute the inner margin of each tract, B, are continued Fig. across from one side of the brain to the other, forming no connection with the optic nerves, and existing where those nerves do not exist, as in the mole. These fibres may be con- sidered as commissural between the thalami of opposite sides. The remaining fibres of the tracts go to form the optic nerves, the cen- tral passing across to the nerve of the oppo- site side, and the outermost, c c, passing to the optic nerve of the same side. Besides these, the two retinas are brought into direct connection by the fibres, A, which form the anterior border of the chiasma. From this arrangement it appears that corresponding parts of the two retinas are brought into relation one to the other, in the same manner as corresponding parts of the cerebral con- volutions are linked together by the various commissures: the right side of each retina appears also to be continuous with the right optic tract, and the left side of each with the left ;thus each side of the central apparatus is brought into communication with its1 own side of both retinal images, which may be supposed to favour their conception as one. The learned Arabian Alhazen supposed that when corresponding points of the two retinas are affected, the mind perceives one image ; this opinion has found favour ; but Dr. Todd and Mr. Bowman, and also Dr. Alison have especially pointe
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