COMPARISON OF SENSE-ORGANS. 



153 



is prolonged towards the central nervous system, where it terminates as in Lumbricus 

 (fig-. 175). Here, in Nereis, the body of the sense-cell, which, since it gives origin to a nerve- 

 fibre or fibres, may be termed the sensory nerve-cell, is beginning to move away from the 

 periphery. In Vertebrata, the only essential difference is that the same cell is further 

 removed from the periphery, and is nearer the central nervous system, viz., in the ganglion 

 of the posterior root. In all vertebrate embryos and in some Vertebrata throughout life, 

 the sensory nerve-cell is a spindle-shaped cell, with a peripheral process extending as the 

 sensory nerve-fibre towards the integument, and a central process, passing by the posterior 

 root into the grey matter of the nerve-centre. In most Vertebrata. the spindle-shaped 

 bipolar character of these sensory nerve-cells becomes lost, owing to the fact that as develop- 

 ment proceeds the two processes shift towards one another at their attachment to the cell 

 and ultimately come off from it by a common stem, but this is a mere secondary modification 

 and does not affect the validity of the comparison. In most other sense organs, the sensory 

 cells remain at or near the periphery, and the most important differences between them 

 consist in the relative extent of development of the centrally directed prolongation. In the 

 auditory organ (fig. 170), which is an involuted portion of the iiitegumental surface, the sense- 

 cells are represented by the hair-cells of the maculae and of the organ of Corti. Here a central 

 process is absent, and the connection with the central nervous system is effected by the 

 endings of the auditory nerve enveloping the cell-bodies by a close terminal ramification. 

 These hair-cells of the auditory organ are not in the same category of sensory cells with the 



Lumbricus 



Nereis 



Vertebrata 



Fig. 175. DIAGRAMS SHOWING THE RELATIVE POSITION OF THE SENSORY CELL IN LUMBRICUS, NEREIS, 

 AND VERTEBRATA. (After G. Retzius.) 



olfactory cells of the nasal membrane or the visual cells of the retina. For they are destitute 

 of a central or nerve-fibre process, and it may be doubted whether they transmute the auditory 

 vibrations into nervous impulses. On the other hand the manner in which the terminations 

 of the auditory nerve-fibres surround or abut against the enlarged deeper ends of these cells 

 suggests the possibility of a mechanical excitation of the nerve-terminations by a direct trans- 

 mission of the vibrations of the endolymph through the cells in question. If this view be 

 taken, viz.. that the hair-cells of the auditory organ are not true nerve-epithelium cells, but 

 only ordinary epithelium cells somewhat modified for the transmission of mechanical impulses, 

 then it will follow that the terminal sensory cells are to be looked for in the cells of the spiral 

 ganglion of the cochlea and in the cells in the vestibular branch of the auditory nerve. These 

 are bipolar cells having a peripheral process ending as we have seen in contact with the hair- 

 cells, and a central process (auditory nerve fibre) ramifying in the grey matter of the nerve- 

 centre. There will thus be a close analogy as regards nerve-ending between the auditory and 

 tactile organs. With regard to the gustatory organ our knowledge is still imperfect. It has 

 usually been supposed hitherto that the central processes of the gustatory cells are prolonged 

 into nerve-fibres, and if this were the case, these cells would come into the same category with 

 the olfactory cells of the nasal mucous membrane. But according to the most recent observa- 



VOL. III., PT. 3. M 



