Practical anatomy: a manual of dissections / by Christopher Heath.

  • Christopher Heath
Date:
1870
Catalogue details

Licence: Public Domain Mark

Credit: Practical anatomy: a manual of dissections / by Christopher Heath. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

    553/600 (page 543)
    is the yellow spot of Soemmering, the most sensitive point, which consists of a small elevation, in the centre of which is a minnte depression, the fovea centralis. The retina consists of three layers: 1. The internal or fbrous lixyev [generally called nervous'] is continuous with the optic nerve (the fibrillffi of which here lose their white substance of Schwann [or myeline]), and has also nume- rous ganglionic cells and nuclei developed in it. 2. The middle or granular layer consists of oval bodies collected into two sets between which is granular material. 3. The outer la3'er \_Bacillar layer] or Jacobus menihrane consists of a peculiar arrangement of particles to which the names rods and bulbs [or cones] have been given. These rods are continuous with some fibres which pierce the whole depth of the retina, and are called the fibres of Miiller. [The retina, if the eye be fresh, is by no means such a delicate membrane as is often represented. It can be handled, brushed, etc., with considerable rudeness. By shaking the eye in the water, the outer layer of rods and cones (Jacob's membrane) can sometimes be detached.] [To uncover the vitreous humor the retina may be washed away by a camel's hair brush, or the eye may be seized by the thumb and fingers, and by gentle pressure the vitreous humor may be squeezed out of the retina.] The Vitreous Humor (Fig. 243, 13) is the transparent body filling all the posterior part of the ej'eball. It is a gelatinous substance contained in a transparent membrane called the hyaloid membrane, and is traversed by nume- rous delicate and perfectly transparent septa. The exist- ence of these may be demonstrated by crushing the vitre- ous humor with the fingers, when the fluid portion will drain away. On the eye from which the vitreous humor has been removed the retina will fall together, leaving the inner surface of the choroid exposed, and in the bullock's eye [sheep's, calves', etc.] the tapetum lucidum will be seen. This is the colored appearance which is peculiar to the lower animals, and is due to the presence of a thick laj^er of wavy fibrous tissue outside the choroidal epithelium. The object of this is to reflect the rays through the retina a second time, and thus to enable the animal to see with a very small amount of light.