LESSON 94.] ORGANS OF VISION IN THE HIGHER ANIMALS. 303 



lar, triangular folds, formed (apparently) by the plaiting of tlie in- 

 ternal layer of the choroid. They are about sixty in number (in 

 man), and may be divided into large and small, the latter being 

 situated in the spaces between the former. These processes are 

 covered by a thick layer of pigmentum nigrum. 



1374. The retina is the expanded portion of the optic nerve, and 

 is'the medium, equivalent to the ground glass of the Camera obscura, 

 upon which all images seen by the eye are painted. Notwithstand- 

 ing its extreme delicacy, it consists of three layers ; these are, the 

 external, or Jacob's membrane ; middle, or nervous membrane ; in- 

 ternal, or vascular membrane. 



1375. The zonula ciliaris is a thin vascular layer, which con- 

 nects the anterior margin of the retina with the circumference of the 

 lens. It presents upon its surface a number of small folds corres- 

 ponding with the ciliary processes, between which they are received. 



1376. The HUMORS of the eye are also three ; these are, the 

 aqueous humor, situated in the anterior and posterior chambers of 

 the eye. 



1377. The anterior chamber is the space between the cornea in 

 front, and the iris and pupil behind, 



1378. The posterior chamber is the narrow space bounded by 

 the posterior surface of the iris and pupil in front, and by the ciliary 

 processes and lens behind. 



1379. The crystalline humor, or lens, is situated behind the 

 pupil, and is ' surrounded by the ciliary processes, which slightly 

 overlap its margin. It is more convex on the posterior than on the 

 anterior surface, and is embedded in the anterior part of the vitreous 

 humor, from which it is separated by the y IG . 429. 



hyaloid membrane. It is invested by a 



proper capsule, which contains a small FIG. 430. 



quantity of fluid, and is retained in its 



place by the attachment of the zonula 



ciliaris. In its ultimate structure the 



crystalline lens consists of a multitude of 



fibres, the edges of which are wavy, or 



undulate (Fig. 429). In other animals 



they are serrated, and by this means lock 



J ' J Fibres of the crys- 



into each other, and form a tissue. This taiiine lens, human. 



can be seen, in the higher animals, in the fibres of the Ox (Fig. 430). 



1380. The vitreous humor forms the principal bulk of the globe 

 of the eye. It is enclosed in a delicate membrane, the hyaloid, 



