. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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, w
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 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 pointed out the importance of the decussation of the commissural fibres. Dr. Alison arrives at the conclusions : first, that certainly in some, and probably in all animals, the structure of the optic nerve brings the impressions which form inverted images on the retina into the same order on the sensorium as those which might result from the touch of the same objects ; secondly, that in those animals which can direct both eyes to one point, the partial decussation of the optic nerves, generally if not universally present, enables the images produced by an object on the corresp
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