ORGANS OF THE SENSES. 



and nerves disposed in two concentric rings, is attached 

 to the anterior part of the choroid around the ciliary liga- 

 ment, arid divides the cavity for the aqueous humour into 

 two unequal chambers which communicate by the pupil ; it 

 is variously coloured on its anterior surface, and supports 

 on its back-part the uvea which is covered with the black 

 pigment of the choroid. A little exterior to the entrance 

 of the optic nerve a small fold of the retina is observed, 

 and a yellow spot, with a minute round transparent portion 

 of the retina, the uses of which are unknown. The vitreous 

 humour (105.^.), of a gelatinous consistence and contained 

 within the concentric folds of the hyaloid membrane, occu- 

 pies about two-thirds of the axis of the eye, and its delicate 

 membrane embraces also the crystalline lens with its firm 

 capsule and forms around its edge the circular canal of 

 Petit. The crystalline lens, here comparatively small and 

 soft, has its posterior surface as usual more convex than 

 the anterior, and both are most convex at the earliest periods 

 of life. The aqueous humour, enveloped in a delicate capsule, 

 which forms the membrana pupillaris by extending over 

 the pupil in the embryo, occupies a large anterior and a 

 smaller posterior chamber, giving the necessary convexity 

 to the cornea and facilitating the free motions of the iris. 

 Thus we observe these complicated optical instruments, the 

 most universal and the noblest organs of sense, gradually 

 advancing to perfection from the monad to man, where all their 

 internal essential parts, and all their external accessary appa- 

 ratus are the most exquisitely finished and adjusted, and it 

 is chiefly through their means that he is enabled to provide 

 for his wants, to acquire the materials of thought, and to 

 enjoy the sublime spectacle of nature. 



THIRD SECTION. 



Organs of Hearing* 



The organs of hearing are next to those of vision in 

 their importance and in the universality of their occurrence 

 in the animal kingdom, where they are distinctly perceived 



T 2 



