The eye and its diseases / by D.B. St. John Roosa and Edward T. Ely.

  • Roosa, D. B. St. John (Daniel Bennett St. John), 1838-1908.
Date:
[between 1800 and 1899]
Catalogue details

Licence: Public Domain Mark

Credit: The eye and its diseases / by D.B. St. John Roosa and Edward T. Ely. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    when light enters the eye they do not perceive it at all. The point of entrance of the optic nerve itself, the optic papilla, forms a blind spot of corresponding size in our field of view. This is called the blind spot of Mariotte, after the man who first described it. Its existence can be proved by the following simple experi- ment : On a sheet of paper draw a small cross, and, three or four inches to the right of this, make a dot about half an inch in di- ameter. Hold this paper in front of the right eye, and, with the other eye covered, look stead- fastly at the cross. By moving the paper backward and forward a point will be found where the dot will disappear from view en- tirely. The reason of this is, that under the above conditions the image of the dot falls ex- actly upon the optic papilla; and, as it is not perceived, the experiment shows that the pa- pilla is really a blind spot. Be- cause the optic nerve-entrance is not at the centre of the eye- ball, the blind spot does not fall J[^%£^^^&1£j$ directly in OUr line Of Vision— optic nerve fibres ; No. 9, the layer of rods and that is, in the centre of our field of view. Hence it occasions no inconvenience, and centuries elapsed before it was even discovered. One portion of the retina is known as the layer of rods and cones, from the peculiar shape of the elements composing it. This layer is almost universally considered as the true percep- tive layer of the retina. This view is thought to agree best with anatomical facts and with the results of experiments. It is as- sumed that, in accurate vision, the image of the object is formed exactly in this layer of the retina (page 236). There is one point of the retina which is of great importance, and that is the macula lutea, or yelloiv spot. This is a region about 2^ to -fr inch in diameter, and situated about ^ to -fc inch to the outer side of the optic papilla. (See Fig. 45.) In its centre is a little