104 SENSE ORGANS. 



from which spring the fibres of Miiller. These run outwards 

 (invisible beyond the third layer) to the outer limiting layer, and 

 form the sustentacular system of the organ. The nervous 

 elements are next in order from the first layer outwards (2) 

 fibrous layer of nerve fibres, (3) Ganglionic layer, nerve cells, 

 (4) inner molecular layer, (5) inner nuclear, (6) outer molecular, 

 (7) outer nuclear, (8) outer limiting, (9) rods and cones, (10) 

 pigmentary layer. Layers (5) and (6) are stained and so stand 

 out prominently. (//) Examine the different layers and 

 observe that the cones are much less numerous than the rods. 



Retina, Frog. Illuminated, (p. 8, s. 17, c. P., m. B.). (L) 

 Recognise the outer or pigmentary layer, also the two nuclear 

 layers (stained red), and the outer limiting layer, which sepa- 

 rates the external nuclear from the layer of rods and cones. 

 In the latter the rods are the prominent structures, particu- 

 larly on account of their large outer segments, which are 

 unstained, and between which prolongations from the pigment 

 cells pass nearly to the limiting layer. The external part of 

 the pigment cell contains less pigment granules, and the 

 nucleus is well seen. 



Retina, Frog. Killed after being kept 12 hours in dark- 

 ness, (p. same as before.) The distribution of the pigment 

 is the point of interest. Observe that it is retained in the 

 body of the cell itself, very little of it occurring between the 

 rods. It is now possible to find the cones ; these are small 

 spindle-shaped structures, which are placed at various distances 

 from the outer limiting layer, with which they remain con- 

 nected by a delicate process, and often project some way 

 towards the pigmentary layer. 



The Ear. Cochlea. Guinea pig. L.S. through the modiolus. 

 The bulla being exposed and opened, the cochlea is removed, 



