Volume 2

The physiological anatomy and physiology of man / by Robert Bentley Todd and William Bowman.

Date:
1845-1856
Catalogue details

Licence: Public Domain Mark

Credit: The physiological anatomy and physiology of man / by Robert Bentley Todd and William Bowman. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    double sensation is excited. Thus, in squinting, two impressions are excited, unless, by long habit, one eye ceases to be adjusted and employed, and gradually loses its excitability: but when two similar objects are presented to the eyes of a squinting person, one carefully in the axis of each, their images coincide and they are seen as one. The double vision of drunkenness, and of certain cerebral affections, is explicable partly on the same ground, but in such cases con- \ siderable allowance must be made for the disordered state of the ^ sensorium. Again, if corresponding points of the two retinae are pressed by the finger, a single luminosity is perceived; but a double f- one, if the points touched are non-symmetrical. Something similar to this blending of two impressions in one sensation exists in the sense of hearing, and, perhaps, also in taste and smell. The corresponding points of the two retinae are such as would be i in contact, if the two retinae were adapted to one another: the i upper and lower parts correspond with the upper and lower, and the inner side of one with the outer side of the other. As we are entirely ignorant of the mode in which the mind , takes cognizance of a single impression on an organ of sense, we ■ cannot hope to understand how a single sensation can result from a double impression. But it is most interesting to remark a - structural peculiarity in the course of the optic nerves, which , : certainly allies itself with this wonderful part of their function. Their partial decussation in the chiasma, or commissure, con- nects each retina with both optic tracts, and with the cor- responding portions of the cerebrum ; and it is not improba- ble, as Dr. Wollaston conceived, and Mr. Mayo has described, that the right side of both retinaj is continuous with the right optic tract, and the left side of both with the left. This would place each side of the central apparatus in connexion with its own side of both the symmetrical images, and might be sujiposed to | favour their conception as one. Dr. Wollaston relates, that on different occasions he lost the power of seeing one half of an object to which he directed both eyes; and others have experienced similar temporary attacks. Thus, Abernethy would humorously affirm that he could sometimes see only his ne and thy, having lost the other members of his name. Such phenomena are readily explained by supposing the anatomical arrangement of the sides of the retinae, with regard to the optic tracts, to be such as above described, since any derangement of one optic tract would then affect the same part of both optic images. Indeed, in Dr. Wollas- ton’s own case, a tumor was found involving one of the optic