54 



THE EYE. 



The yellow tint of the macula is absent at the centre of the f ovea : it is said to be due to 

 a diffuse colouring- matter which is seated in the interstices between the elements of the four 

 or five inner layers. The yellow colour of this part of the retina is peculiar to Primates : in 

 man it develops after birth. A corresponding area is found however in all mammals, 

 characterized by a lack of dark pigment in the pigmentary layer, while in some mammals 

 (Chievitz) as well as in birds, reptiles and amphibia (H. Mliller, Hulke, W. Krause), a 

 Jovea has been described within such a central area. In some birds two foveas are present, 

 one being near the ora serrata : in some cases several fovese are found (Chievitz). The central 

 area is always characterized by containing relatively smaller visual cells. The occurrence and 

 relative development of the central area and f ovea in vertebrata has been specially investigated 

 by Chievitz (see Bibliography). 



Structure of the ora serrata and pars ciliaris retinae. At the line of the 

 ora serrata the numerous complex layers of the retina disappear, and in front of it, 

 the retina is represented merely by a single stratum of elongated columnar cells with 

 the pigmentary layer external to it (pars ciliaris retina}. The transition is, in man, 

 somewhat abrupt, all the changes being met within a zone of about 0*1 mm. in 

 breadth. The layer of rods and cones (fig. 61, #) first disappears as a complete 

 layer, the cones continuing rather further than the rods, but being imperfectly 

 formed and lacking the outer segments ; the nerve- and ganglionic layers, which 

 were already very thin and incomplete, cease altogether at the ora, the inner 



Fig. 61. VERTICAL SECTION THROUGH THE CHOROID AND RETINA NEAR THE ORA SERRATA (Kolliker). CO 



DIAMETERS. 



a, hyaloid membrane ; b, limiting membrane and nervous layer of the retina ; c, ganglionic and 

 inner molecular layers with closely set Miillerian fibres ; d, inner nuclear ; e, outer molecular ; /, outer 

 nuclear layer ; g, columnar layer ; h, pigment ; i. Jc, choroid ; I, part of one of the ciliary processes ; 

 TJi, pars ciliaris of the retina. (The recess shown at a' is not constant. ) 



Fig. 62. A SMALL PORTION OF THE CILIARY PART OP THE RETINA (Kolliker). 350 DIAMETERS. 



A, human ; B, from the ox ; 1, pigment-cells ; 2, columnar cells. 



molecular layer (c), which is now largely occupied by Miillerian fibres, retains its 

 thickness up to a certain point, and then abruptly terminates (a 1 ), as do also the 

 nuclear layers, outer and inner (f, d,). The columnar cells of the pars ciliaris, 

 which appear directly to continue these layers of the retina, are at first of con- 

 siderable length, but become gradually shorter anteriorly ; they are finely striated 

 (fig. G2, 2), and each cell has a clear oval nucleus at the outer part of the cell, near 

 the pigmentary layer. The inner end may be rounded, pointed, square, or even 

 branched ; the sides of the cells, too, are sometimes uneven. 



This double layer of cells is continued as before said over and between the ciliary 

 processes to join the uveal layer upon the posterior surface of the iris (pars iridica 

 retince). On the ciliary processes, and especially their anterior aspect, glandular 

 depressions of the epithelium occur which may be solid or may be provided with a 



