18 STRUCTURE OF THE RETINA. 



to conduct back to the sensitive portion of the retina, those rays of light 

 which, having traversed that membrane, are not entirely absorbed by the 

 black pigment of the choroid ; and he supposes that the individual rays 

 returning through these highly refracting bodies may be directed to the 

 same portion of the retina through which they had previously passed, 

 and that their dispersion by mere reflection, which would tend to interfere 

 with the distinctness of vision, is thus prevented. This view is rejected 

 by Volkmann * as improbable. 



Within the above described layer of staff-shaped bodies, is placed the 

 expansion of the optic nerve in the form of a fine fibrous membrane, the 

 individual fibres of which radiate from the point of entrance of the optic 

 nerve, and pursue a tolerably straight course towards the- anterior margin 

 of the retina. At first the fibres run in distinct bundles, but these, by 

 subdivision and a plexiform interchange of the individual filaments, speedily 

 disappear, and for the remainder of their course the fibres are disposed in 

 the form of a fine fibrous membrane, in which it is difficult to distinguish 

 the several filaments. According to Dr. Todd and Mr. Bowman, | this 

 expansion of the optic nerve appears to be composed of the gray or central 

 portion alone of the individual fibres : the external white substance ceasing 

 at the point where the optic nerve perforates the sclerotica. In the 

 rabbit, indeed, the white substance is continued for a short distance 

 within the globe, but even here the fibres speedily lose their white lustre, 

 and assume for the remainder of their course the grey appearance ob- 

 served in the fibrous layer of the retina elsewhere. 



In defiance of the numerous attempts to determine the mode of ter- 

 mination of the nerve-fibres in the retina, the subject still remains in 

 obscurity. It was supposed that Treviranus had discovered the true ar- 

 rangement and ultimate disposal of the fibres, each of which was considered 

 to terminate in a rod-shaped papilla on the internal surface of the retina : 

 but it appears to have been clearly proved of late that this view is 

 quite erroneous. The nature of the papillae, as already observed, is 

 quite different from that described by Treviranus, who, as well as others 

 by whom his view was at once adopted, appears to have overlooked the 

 peculiar structure of the fibrous expansion of the optic nerve. Several 

 later observers, including Valentin, Bidder, and Pappenheim,J are of 

 opinion that the fibres terminate in loops at the anterior margin of the 

 retina : Krause says he has observed these loops at every part of the 

 retina, both in front and behind : whilst Hannover || states that they end in 

 free extremities, but never in loops. Nothing, therefore, can at present 

 be stated positively on this point, except that at the posterior part of the 



* Wagner's Handworterbuch, Art. Sehen, p. 272. 

 t Physiological Anatomy of Man. Part iii. p. 28. 



I Canstatt's Jahresbericht, 1843, p. 108. Canstatt's Jahresbericht, 1842, p. 169. 



II M'tiller's Archiv. 1840. 



