632 MANUAL OF HISTOLOGY. 



The depth of this nervous coat, at the point of entrance of the optic 

 nerve, where it is greatest, is from O38 to 0'23 mm. Anteriorly it thins 

 off to about half of this, and at its foremost edge it is still 0'09 mm. in 

 thickness. Here it ends with an undulating border, the ora serrata. 

 External to the point of entrance of the optic nerve, and about 3 '4 mm. 

 from the centre of the latter, a yellow spot, the macula lutea, may be 

 observed, an oval patch about 3'4 mm. long and 1*13 mm. in breadth, 

 tinged with diffuse yellow pigment. In its centre it presents fh&fovea cen- 

 t ralis, an irregular depression corresponding to a great decrease in thick- 

 ness of the retina. The macula lutea is the point at which sight is 

 clearest. 



The retina (figs. 586 and 587) consists of the following layers, in suc- 

 cession from without inwards : (1) the layer of the rods and cones 

 (columnar layer, 1.1); (2) the membrana limitans externa (between 1 

 and 2) ; (3) external granular layer (2.2) ; (4) the wtergranular layer 

 (3.3) ; (5) the internal granular layer (4.4) ; (6) the so-called molecular 

 layer (5.5) ; (7) the ganglionic cell layer (6.6) ; (8) the expansion of the 

 optic nerve (7.7); (9) of the membrana limitans interna (10.10); we 

 add finally (10) of the pigmentary layer. 



We have recently been brought to consider this chaos of textural 

 elements as consisting of two essentially different constituents, although 

 it is still difficult to draw everywhere a sharp line between them. The 

 retina possesses in the first place (and in this it resembles the central nerv- 

 ous system) a connective-tissue framework. This springs up in the outer 

 part of the retina, soon attaining, at the inner side of the rod and cone 

 layer, considerable development as the membrana limitans externa. From 

 this it spreads itself throughout all the retinal strata lying internal to 

 this, and terminates by forming the membrana limitans interna. Between 

 these two membranes are stretched a number of vertical sustentacular 

 septa, the radial fibres, or fibres of Muller. The remainder of the retina 

 and in this is included a system of similarly radiating or oblique nervous 

 fibrillse -'may probably be reckoned' to the nervous tissue. 



REMARKS. Literature is very rich in new essays upon the structure of the retina. 

 The last are those of H. Muller and M. Schullze. 



315. 



We turn now, first of all, to the delicate sustentacular tissue of the 

 retina. 



For our acquaintance with this we are especially indebted to M. Schnltze^ 

 and any one who has thoroughly investigated the subject with an unbiassed 

 mind will be forced to concede the correctness of his views. 



The point from which this framework of the retina starts (fig. 588, 

 A) is a modified bounding layer, about O'OOll mm. in thickness, the 

 transparent membrana limitans interna (I). Anteriorly it is stronger than 

 posteriorly. Its internal surface is smooth, but not so the external, from 

 which a series of sustentacular fibres (e) take origin, which traverse 

 almost the whole thickness of the retina, perpendicularly to its internal 

 surface. These are known as the radiating fibres of Muller. Absent in 

 the macula lutea, they increase in quantity anteriorly. They spring from 

 the msmbrana limitans interna with a delicate triangular flat or conical 

 pedicle, or with several extremely fine fibrils, which unite at very acute 

 angles (e, below). In their course they give off numerous branches, by which 

 thev are connected one with another in a retiform manner. In addition 



