560 SIGHT. 



"The chorioid is internally coated by the retina* an ex- 

 pansion of the optic nerve after this has passed through the 

 sclerotic and chorioid f , of most beautiful textures, and per- 

 forated, in the imaginary axis of the eye, between the two prin- 

 cipal twigs of the central artery b , by the singular central foramen 

 of Sommerring 1 , which is surrounded by a yellow edge." k 



A delicate transparent membrane has been discovered by Dr. 

 Jacob of Dublin between the retina and chorioid, and adherent 

 to both. 1 



The anterior edge of the chorioid is terminated by a cellular 

 belt, called orbiculus ciliaris^ by which it adheres firmly to a cor- 

 responding groove in the sclerotic, and from which two other 

 membranes of a different kind, viz. the iris and ciliary processes, 

 are expanded in a circular form. 



e " B. S. Albinus, Anotat. Academ. 1. iii. p. 59. sq. 1. iv. p. 75. sq. L v. 

 p. 66. sq." 



f "Walter, De venis oculi, &c. Berol. 1778. 4to. tab. i. fig. 2. tab. ii. 

 fig. 2." 



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

 J. Mery to be visible in a living cat plunged under water, Mem. de PAcad. des 

 Sc. de Paris, avant 1699, t. x. p. 650. ; and 1704, p. 265. 



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

 by Zinn in an admirable engraving. Comm. Soc. Scient, Getting, t. iv. 1754, 

 tab, viii. fig. 3. 



By Montana, in the rabbit, Sur le venin de la vipere, voL ii. tab. v. fig. 12." 



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

 in the (Euvres de MARIOTTE, p. 527. fig. 1." 



1 " Sommerring, De Foramine centrati limbo luteo cincto retina humancc : in 

 the Comment. Soc. Reg. Scient. Getting, t. xiii. 



Ph. Michaelis, Journal der Erfindungen in der Natur-und Arzneywiss, 

 P. xv." 



k ~" As I have discovered this central aperture in the eye of no animal besides 

 man, except the guadrumana, the axes of whose eyes are, like the "human, parallel 

 to each other, I think its use connected with this parallel direction of the eyes, 

 and have endeavoured to explain the connection at large, in my Handbuch der 

 vergkichenden Anatomie, p. 4O2. sq. 2d edit. 



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

 both at the same time, and therefore more distinctly 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 dilates a little under this circumstance, and thus 

 removes the principal focus from the very sensible centre of the retina." 



1 Phil. Trans. 1819. Also, on various other points in the anatomy of the eye, 

 Med. Chirurg. Trans, vol. xii. P, 2. 



