OP SIGHT. 169 



pigment, adhering, however, but loosely to its concave 

 surface in the form of mucus.* 



259. The chorioid contains the internal coat — the 

 retina,f — a medullary expansion of the optic nerve 

 which passes through the sclerotica and chorioid,^ of 

 most beautiful texture, § and perforated, in the imagi- 

 nary axis of the eye, between the two principal branches 

 of the central artery, || by the singular foramen of 

 Sommerring,** which is surrounded by a yellow 

 edge.f f (A.) 



* C. Mundini, in the Comm. Instil. Bononiens. T. vii. p. 29. H. F. Elsaesscr 

 (praes. G. C. Ch. Storr), De pigmento oculi Nigro. Tubing. 1800. 8vo. 



•f- B. S. Albinus, Annotat. Academ. L. iii. p. 59 sq. L. iv. p. 75 sq. L. v. 

 p. 66 sq. 



X Walter, De Venis Oculi, %c. Berol. 1778. 4to. tab. i. fig. 2. tab. ii. fig. 2. 



§ The extremely beautiful blood-vessels of the retina were first discovered 

 i>y T. Mery to be visible in a living cat plunged into water. Mim. de PAcad. 

 lea Sc. de Paris, avant 1699. T. x. p. 656 ; and 1704. p. 265. 



The most beautifully radiated surface of the retina in the hare is displayed 

 >y Zinn in an admirable plate. Comm. Soc. Scient. Golfing. T. iv. a. 1754. 

 ab. viii. fig. 3. 



By Fontana, in the rabbit. Sur le vinin de la viper e. vol. ii. tab. v. fig. 12. 



|| A plate accurately representing the course of these branches will be found 

 i l the CEuvres de Mariotte. p. 527. fig. 1. 



* * Sommerring, De Foramine centrali limbo luteo cincto retina humana : 

 i i the Comment. Soc. Reg. Scient. Gottingens. T. xiii. Ph. Michael Bosc, 

 . 'ournal der Erfindungen in der Natur-und Arzneywiss. p. xv. 



ft All have discovered this central aperture in the eye of no animal 

 i ;siu<ra man, except the quadrumana, the axes of whose eyes are, like the 

 Y iman, parallel to each other, I think its use connected with this parallel direc- 

 t in of the eyes, and have endeavoured to explain the connection at large, in 

 n y Handbuch der vergleichenden Anatomic p. 547 sq. 



As, on the one hand, this direction of the eyes renders one object visible to 

 b ith at the same time, and therefore more clearly visible ; so, on the other, 

 tl is foramen prevents the inconvenience of too intense a light, if there is a pro- 

 b bility that it expands and dilates a little and thus removes the principal focus 

 fi >m the verv sensible centre of the retina. 



