ORGAN OF VISION. 



219 



Frog, seen from 

 the side turned to- 

 wards the vitreous 

 humour. 



The four higher rows 



Fig. 93. 



Carpenter 1 thinks there can be little doubt that Fig. 92. 



they are identical with the globules of the retina of 

 Weber. The diameter of these globules in man, 

 according to Weber, is from the s^^th to s^Tjth 

 of an inch. 



About a sixth of an inch on the outside of the 

 optic nerve, and in the direction of the axis of the 

 eye, or of a line drawn perpendicularly through the Pa e 

 centre of the cornea, is a yellow spot, about a line 

 in extent, having a depression in its centre. This 

 spot and depression are the limbus luteus or ma- 

 cula lutea, and foramen centrale of Sommering. 2 

 The yellow spot does not exist in the foetus; 3 and 

 the folds, described by Sommering as surrounding 

 the yellow spot, would appear to be a post mortem appearance. In 

 the examination of two convicts, three hours after execution, the fora- 

 men was not seen satisfactorily. 4 



The retina receives many blood-vessels, 

 which proceed from the central artery of 

 the retina, or of Zinn. This vessel it 

 is important to observe enters the eye 

 through the centre of the optic nerve, the 

 porus options, and, before passing directly 

 through the vitreous humour, sends off late- 

 ral branches to the retina. 



2. Diaphanous parts of the Eye. The 

 parts that act as refracting bodies, are 

 either transparent membranes, or fluids con- 

 tained in capsules, which give them a fixed 

 shape. These parts are the cornea, aqueous 

 humour, crystalline, and vitreous humour. 



The cornea is the convex transparent 

 part of the eye, advancing in front of the 

 rest of the organ, as a watch-glass does 

 before the case ; and appearing like the 

 segment of a smaller sphere superadded 

 to a larger. It was, for a long time, con- 



sidered tO be a prolongation of the Scle- Plan of the Structures in the Fore Part 

 i ,-, L .,, ,, ,. . 



rotic ; but they are manifestly distinct 



of the Eye, seen in section. 



r , iii 1- Conjunctiva. 2. Sclerotica. 3. 



membranes, being separable by macera- cornea. 4. choroid. 5. 



tion. The posterior surface is concaye, ^fl/S^ 



and, between it and the iris, is the small %*$>y- f &$8??%g$: {?; 



Space OCCUpied by the aQUeOUS humOUr, Membrane of the aqueous humour (too 



11 j . 77 j? ,^ mi thick), a. Aqueous humour: anterior 



called anterior chamber of the eye. The chamber and fa) posterior chamber, b 



cornea is generally considered to be com- Cr y stalline lens - c - vitreous humour - 



1 Human Physiology, p. 262, Lond., 1842. 



2 Sommering, in Comment. Societ. Getting., torn. xiii. 1795-98; A. ab Ammon, de Genesi 

 et Usu Maculae Lutese, &c., Vinar., 1830. 



3 Rudolphi, Grundriss der Physiologic, B. ii. Abtheil, 1, s. 176, Berlin, 1823. 



< W. E. Homer, Special and General Anatomy, 5th edit., p. 426, Philad., 1839, and J. 

 Pancoast, in Wistar's Anatomy, 8th edit., Philad., 1842. 



