222 SENSE OF SIGHT. 



modifying its own convexity, so as to adapt the eye to different dis- 

 tances. This was the opinion of Des Cartes ; and it has more recently 



Fig. 97. Fig. 98. Fig. 99. 



Lens, hardened in spirit and par- Front View of the Crys- Side View of the Adult 

 tially divided along the three interior talline Humour or Lens, in Lens, 



planes, as well as into lamellae. Mag- the Adult. 



nified 3* diameters. (After Arnold.) a/'lt^posSr get 



3, 3. Its circumference. 



been received, with modifications, by Dr. Young. 1 Its muscularity is, 

 however, by no means established, although its fibrous character is un- 

 questionable. 



The specific gravity of the human crystalline is said by Chenevix 2 to 

 be 1*0790. He considered it to be composed chiefly of albumen. Ac- 

 cording to an analysis, however, of Berzelius, 3 it would appear to con- 

 tain 35*9 parts in the hundred of a matter analogous to the colouring 

 matter of the blood. 



The vitreous humour, so called in consequence of its resemblance to 

 glass, occupies the whole of the cavity of the eye behind the crystal- 

 line. It is convex behind; concave before; and is invested by a deli- 

 cate, thin, transparent membrane, called tunica hyaloidea, which fur- 

 nishes prolongations internally, that divide it into cells. It is owing to 

 this arrangement of the membrane, and not to the density of the 

 humour, that it has the tenacity of the white of egg. Its density does 

 not differ materially from that of the aqueous humour ; their specific 

 gravities being stated at 1-0009, and 1-0003 respectively. The cells, 

 formed by the hyaloid membrane, are not all of the same shape and 

 size. They communicate freely with each other, and are well repre- 

 sented in Fig. 96. At the anterior part, where the hyaloid membrane 

 reaches the margin of the crystalline, it is separable into two laminae; 

 one of which is reflected over the anterior; the other over the posterior 

 surface of the lens. Between these laminae, and at their junction 

 round the crystalline, a canal exists, into which air may be introduced : 

 when it exhibits a plaited arrangement, and has been called bullular 

 canal of Petit; 4 ' and, by the French writers, canal godronne, or simply 

 canal of Petit. This canal is generally conceived to be devoid of 

 aperture; but Jacobson affirms, that it has, in its sides, a number of 



' Philos. Transact, for 1793, p. 169 ; and Med. Literature, p. 521, Lond., 1813. 

 2 Philos. Transact, for 1803, p. 195. 3 Medico-Chirurgical Transact, iii. 253. 



4 Mernoires de PAcademie des Sciences, Paris, 1723 and 1728; and Haller. Element. 

 Physiol., xvi. 2, 18. 



