OF SIGHT. 1.65 > 



pigment, adhering, however, but loosely to its concave 

 surface in the form of mucus.* 



259. The chorioid contains the internal coat the 

 retina,^- 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. Instit. Bononiens. T. vii. p. 29. H. F. Elsaesser 

 (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. 



J Walter, De Venis Oculi, Sfc. Berol. 1778. 4to. tab. i. fig. 2. tab. ii. fig. 2. 



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

 by T. Mery to be visible in a living cat plunged into water. Mem. de I'Acad. 

 des 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 

 by Zinn in an admirable plate. Comm. Soc. Scient. Gotting. T. iv. a. 1754. 

 tab. viii. fig. 3. 



By Fontana, in the rabbit. Sitr le ve'nin de la viper e. vol. ii. tab. v. fig. 12. 



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

 in the CEuvres de Mariotte. p. 527. fig. 1. 



** Sommerring, De Foramine centrali limbo litteo cincto retina; humane? : 

 in the Comment. Soc. Reg. Scient. Gottingens. T. xiii. Ph. Michael Bose, 

 Journal der Erfindungen in der Natur-und Arzneywiss. p. xv. 



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

 besides man, except the quadrumana, the axes of whose eyes are, like the 

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

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

 my Handbuch der verglcichenden Anatomic, p. 547 sq. 



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

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

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

 bability that it expands and dilated a little and thus removes the principal focus 

 from the vcrv sensible centre of the retina. 



