COMPARISON OF SENSE-ORGANS. 



155 



Lastly, the third tier of nervous elements is formed by the ganglion-cells of the retina 

 whose peripheral dendritic processes interlace with the central processes of the inner granules 

 and whose centrally directed processes are fibres of the nerve and have their central optic 

 terminations in the grey matter of the general nerve-centres. 



The comparison of the elements of the retina with those of the other sense organs is not 

 <;asy. If we compare the retina with the olfactory organ, we are at a loss to say whether we 

 are to place the rod- and cone-cells along with the olfactory cells, as in making this com- 

 parison is most frequently done, or whether we should not rather look upon the bipolar inner 

 granules as the homologues of those elements. In the latter case we should be led to 

 suppose that the nervous impulses originate in the peripheral process of the inner granules. 



Fig. 178. DIAGRAM OF THE ARRANGEMENT OF THE SENSORY NERVE-FIBRKS 



IN THE OLFACTORY ORGAN AND BULB. (After Gr. RetzillS.) 



being there set up by a stimulation received from the rod- 

 and cone-cells. And it is further open to us to suppose this 

 stimulation to be a mechanical one caused by the contraction of 

 the rod- and cone-elements under the influence of light. If on 

 the other hand we regard the rod- and cone-elements as representing 

 the olfactory cells, there appears to be no structure in the olfactory 

 apparatus homologous with the tier of inner granules of the retina. 

 A comparison of the retina with the remaining sense-organs is also 

 easier on the assumption that the inner granules represent the 

 actual sensory cells in which nervous impulses originate in response 

 to stimulation set up through the rod- and cone-elements. 



The connections of the olfactory cells (fig. 17H) more nearly 

 resemble the primitive arrangement of sensory structures which 

 occurs in Lumbricus, than is the case in any other of the sense 

 organs. For here, as in the epidermis of Lumbricus, the sensory 

 nerve-cells are at the free surface, lying between and supported by 

 columnar epithelium cells. And the sensory nerve-fibres are a 

 direct prolongation of the fixed ends of the olfactory cells, passing 

 to the nerve centre and there becoming interlaced with the 

 processes of the nerve-cells of the centre. The nature of the 



olfactory excitation, whether mechanical, chemical, or otherwise, is not known, but whatever 

 it be, we must assume that its result is to set up nervous impulses within the olfactory 

 cells, and that these impulses are then propagated along the fibres of the olfactory nerve to 

 the olfactory bulb : where, within the olfactory glomeruli, they are somehow transmitted to 

 the dendritic processes of the mitral cells, through the nerve-fibre processes of which they 

 are again passed on to other parts of the brain. 



It will therefore be seen that all the sense-organs have this in common, viz., a bipolar 

 sensory nerve-cell having (1) a peripheral process extending towards the surface and 

 penetrating between more or less modified epithelium-cells which cover that surface, and 

 (2) a central process which is in all cases recognizable as a nerve-fibre and the terminal 

 ramification of which interlaces with ramifications of nerve-cells within a nerve-centre. The 

 chief differences occur in the greater or less special modification of the epithelium-cells between 

 which the peripheral processes penetrate, such modification being very considerable in the 

 visual organ, less in the auditory and gustatory organs, very slight in the olfactory organ, - 

 and absent in the case of the general integument. 



