VII 



EETINAL EXCITATION 



335 



tion of the three neurones takes place in the two molecular layers 

 by simple contact, or by anastomosis between the terminal 

 arborisations of the axons and the dendrites. In the outer mole- 

 cular layer the rods and cones come into connection with the 

 internal granules ; in the inner molecular 

 layer there is a series of synapses, by 

 which the elements of the internal granules 

 are brought into intimate connection with 

 the cells of the ganglion layer (Fig. 159). 



From the physiological standpoint it 

 is important to consider the structural 

 peculiarities of the retina at the fovea and 

 the macula lutea. We have already noted 

 the absence of rods in the fovea centralis 

 where only cones are present, and it must 

 be added that the cones are much longer 

 and narrower than elsewhere. All the 

 other layers of the retina are much 

 thinned, so that the cones are practically 

 in contact with the membrana limitans 

 interna. Towards the edge of the fovea 

 the layers increase rapidly in thickness, 

 and in the rest of the macula lutea they 

 are thicker than at any other point of 

 the retina. Within the fovea the cone- 

 fibres are disposed obliquely, and come 

 into relation with the elements of the 

 granules (Fig. 161). 



Investigations made by Salzer (1880), 

 in Briicke's laboratory, show that there is 

 not a corresponding fibre of the optic 

 nerve to each cone, the number of cones 

 being three times in excess of the number 

 of optic fibres. While the optic nerve 

 contains about a million fibres, the retina 

 possesses about three million cones, ex- 

 clusive of the rods which are 6-7 times 

 more plentiful. Salzer reckoned that there 

 are 13,200-13,800 to every sq. mm. of sur- 

 face in the fovea. 



The fact that not every cone has a 

 separate path to the cerebrum makes it difficult to account for the 

 clear perception of the visual images ; but it must not be for- 

 gotten that the diameter of the cones does not exceed 2'6 fj., and 

 that those of the fovea are the thinnest (2-2-5 /* according to M. 

 Schultze). 



It is also interesting to note that according to Ramon y Cajal 



inner g 



m.l.l. 



Fir.. 160. A fibre of Miiller from 

 tin 1 club's retina, shown hy 

 Gold's method. Highly mag- 

 nified. (R. y Cajal.) 1, nerve- 

 flbre layer; 2, ganglion -cell 

 layer ; 3, inner molecular layer; 

 4, inner nuclear layer ; 5, outer 

 molecular layer; ti,i>uternuele;n 

 layer; m.l.e., memlirana limitans 

 externa ; ni.l.i., membrana limi- 

 tans interna ; '>, nucleus of the 

 tibre ; a, process extending from 

 nucleated part into inner mole- 

 cular layer. 



