SENSE ORGANS 



535 



periphery there are hardly any cones. Now the periphery is nearly 

 colour-blind, but more sensitive to weak light than is the fovea 

 (it is sometimes possible to see a faint star only by looking to one 

 side of it), so that it seems that while the rods are concerned 



^V- 



Fig. 418. — Examples of dififerent modes of ending of sensory nerve fibres 



of the frog. 



A, Cells from the olfactory epithelium. B, cells from the retina, much simplified. C, cells from one of the 

 patches of sensory epithelium in the labyrinth, with which the fibres of the auditory nerve are con- 

 nected. D, a portion of the epidermis, showing the ending of a nerve fibre. 



D is ordinary stratified epithelium. A, B, and C are true sensory epithelia — forms of colunmar epithelium 

 adapted to the purposes of special senses. In these latter there can be distinguished sense cells and 

 supporting cells. The sense cells bear processes of various kinds on the surface of the epithelium, and 

 at their other ends come into relation with nerve fibres. In A the sense cell is prolonged into a fibre 

 which runs in one of the olfactory nerves as a non-medullated nerve fibre (p. 512). In B also the sense 

 cells are prolonged into fibres though these are connected with the nerve by the intermediation of 

 other cells with whose processes their fibres interlock. In C, on the other hand, the sense cells are not 

 continued into fibres, but are embraced by branches of nerve fibres belonging to cells in the ganglion 

 of the auditory nerve. Thus they resemble D, where the nerve fibres have a similar relation to the 

 cells of the epithelium. In the lower animals, such as the earthworm, the sensory nerve endings io 

 the skin are usually of the type of A and B, rather than that of C and D. 



en.. Cone ; n.c, nerve ceUs ; n.f., nerve fibres ; rd., rod ; s.c, sense cells ; six. supporting cells. 



