428 NERVOUS SYSTEM. 



lens, is the canal of Petit, and is that part of the tunica hya- 

 loidea which is impressed by the ciliary striae and ridges of the 

 choroides. At intervals, passing in a radiated manner from 

 the exterior to the interior circumference of the canal, there is 

 a sort of shortening or constriction of it, producing partial 

 septa in its cavity; so that when the canal is inflated, it seems 

 to consist of a series of small cells, arranged circularly. The 

 cells of the colon will give some idea of this arrangement, though 

 they are produced in an entirely different manner. 



The fluid part of the vitreous humour, by analysis, gives out 

 98.40 water, .16 albumen, and the remainder is saline. In con- 

 sequence of the very small quantity of albumen in it, neither 

 acids nor heat coagulate it to a striking degree. 



The vitreous humour is supplied with a branch from the cen- 

 tral artery of the retina. This branch does not convey red 

 blood, but only serum, except in the fcetal eye. It may be in- 

 jected, at almost any age, with size, coloured with vermilion; 

 but is then, of course, put very much on the stretch. It has 

 been well described by Zinn. It penetrates the vitreous hu- 

 mour near the optic nerve, and is disseminated by very fine 

 branches on the periphery, and on the internal cellular struc- 

 ture of the tunica hyaloidea. M. J. Cloquet has described par- 

 ticularly one branch, which, running through the centre of the 

 vitreous humour, in an appropriate canal, is spent by small ra- 

 mifications upon the posterior part of the capsule of the lens. 



Some anatomists* speak of a fluid between the tunica hya- 

 loidea and the retina: when it does exist, it in all probability is 

 the fluid of the vitreous humour, which has strained through 

 the tunica hyaloidea after death. 



The Lens (Lens Crystattina) or the Crystalline Humour, as 

 it is very generally called, is placed immediately behind the 

 pupil, in a depression on the front of the vitreous humour. Its 

 shape is that of a doubly convex lens, of which the posterior 

 convexity is greatest, being the section of a sphere whose dia- 

 meter is from four to five lines, white the anterior convexity 

 is in the proportion of a sphere of from six to nine lines. The 

 usual breadth of the lens is about three and a half lines. It, 



* Bichat, Anat. Descrip. 



