154 



COMPARISON OF SENSE-ORGANS. 



tions the central process of the gustatory cell is not continuous with a fibre of the glosso- 

 pharyngeal nerve, and the connection of the gustatory cells with the fibres of that nerve is 



auditory 



gustatory 



tactile 



Fig. 176. DIAGRAM SHOWING THE MODE OP TERMINATION OF SENSORY NERVE FIBRES IN THE 



AUDITORY, GUSTATORY, AND TACTILE STRUCTURES OF YERTEBRATA. (After G. RetziuS.) 



indirect and not by continuity. If this is so. the arrangement in the gustatory organ would 

 appear to be somewhat similar to that which obtains in the general integument and in the 

 auditory organ (fig. 176). 



Fig. 177. DIAGRAM OF THE CONNEXIONS OF THE RETINAL 

 ELEMENTS. (After (r. Retzius.) 



gr.i., inner granules ; m.i., inner molecular layer ; </, ganglion 

 cell ; n, its nerve fibre process or neuron ramifying in the nerve 

 centre. 



The retina of the eye, as its development shows, is 

 rather to be regarded as an extension of the central nervous 

 system than a peripheral organ. It is composed of nerve- 

 elements (nerve -cells) which are arranged in three tiers 

 (fig. 177). Those which are placed most peripherally are 

 the visual cells or rod- and cone-cells, which resemble the 

 general type of sensory cells in consisting of a nucleated 

 enlargement or cell-body (outer granule) with a specially 

 modified peripheral process (rod or cone), and a centrally 

 directed ramified (nerve-fibre) process (rod- or cone-fibre). 

 In the case of the cone-elements the peripheral process has 

 been shown to be contractile, since it shortens under the 

 excitation of light (see p. 50). It is usually believed that in 

 these peripheral processes the nerve-impulses are produced 

 and that the impulses are thence conducted centrally by the 

 central or nerve-fibre process (rod- or cone-fibre), and by it 

 transmitted to the next tier of cells. The second tier of 

 nervous elements is afforded by the layer of inner granules. 

 Here there are bipolar nerve-cells, the peripheral processes 

 of which interlace with the central processes of the rod- 

 and cone-cells, but one peripheral fibre (fibre of Landolt) 

 extends in most vertebrates beyond the rest and comes to lie between the bodies of the 

 rod- and cone-cells. The central processes of the' bipolars interlace with the peripheral 

 processes of the cells of the next tier. 



