THE RETINA. 17 



i rritation of this nerve produced contraction of the pupillary aperture, yet 

 in cats and rabbits such irritation, under similar circumstances, was suc- 

 ceeded by considerable enlargement of it. But before any weight can be 

 attached to this statement, which is so entirely opposed to the experience 

 of Valentin, Guarini, and others, it must be verified by the results of fur- 

 ther observations. 



Transparent Media of the Eye. Vitreous humour. It has been pointed 

 out by Pappenheim,* Ernst Briicke,t and more especially by Hannover, J 

 that the vitreous body of the eye of many animals, as the horse, sheep, 

 dog, and cat, is composed of concentric laminae of structureless membrane, 

 each of which forms a completely closed sac : the various sacs being 

 enclosed one within the other. But the vitreous body of the human eye 

 is shewn by Hannover to be composed of numerous sectors, the arcs of 

 which are directed to the surface, while the angles converge towards the 

 centre. Each sector consists of a fine textureless membrane derived from 

 the hyaloid membrane, and enclosing the fluid of the vitreous body. The 

 angles of the sectors do not quite reach the axis of the eye, but terminate 

 in a homogeneous structureless substance situated immediately around the 

 axis. 



Structure of the retina. Considerable addition has been made to our 

 knowledge of the minute structure of the retina since the publication of 

 the account given of it by Treviranus, and adopted by Professor Miiller. 

 It is essential to notice somewhat at length, the most recent information 

 on this subject, inasmuch as it shews to be erroneous the ingenious 

 explanation of the functions of the retina, founded on the supposed 

 termination of its nerve-fibres in distinct papillse. 



These so-called papillse, which are in reality the cylindrical or staff- 

 shaped bodies composing the membrana Jacobi, are quite distinct from 

 the nervous or sensitive portion of the retina; and, in the opinion of 

 Henle || and Briicke,^[ belong more to the choroid coat, or at least 

 to its pigmentary layer, than to the retina. A full description of 

 these cylindrical bodies, and of the singular changes which ensue in them 

 shortly after death, has been furnished especially by Hannover ** and 

 Henle.ff It will be sufficient here to state that these bodies are trans- 

 parent, highly refractive of light, and are arranged perpendicularly to the 

 surface of the retina, and that their outer extremities are imbedded, to a 

 greater or less depth, in the pigment of the choroid coat. The only 

 plausible suggestion which has been offered concerning the use of these 

 bodies, is one by Briicke,Jt who thinks it not unlikely that they may serve 



* Specielle Gewebelehre des Auges, 1842, p. 182. 

 t Mailer's Archiv. 1843, p. 345, and 1845, p. 130. 

 $ Miiller's Archiv. 1845, p. 471. Physiology, p. 1122. 



|| Allgemeine Anatomie, 1842, p. 662. 1 Miiller's Archiv. 1844, p. 444. 



** Muller's Archiv. 1840, p. 340. tt Op. cit. pp. 656 and 783. & Loc. cit. p. 444. 



C 



