616 



MANUAL OF HISTOLOGY. 



In order the better to understand the nature of these remarkable olfac- 

 tory cells, we must now make ourselves acquainted with the arrangement 

 of the branches of the first cerebral nerve. 



In one of the foregoing sections ( 299) we have already described the 

 olfactory lobes, and seen that the nerves of scent 

 take their origin from peculiar lumpy masses in the 

 lower surface of the latter, in the form of bundles of 

 pale fibres. A few dark-edged fibres, which have 

 been met with by Remak and Schultze in the olfac- 

 tory nerves, are probably derived from anastomosis 

 with the trigeininus. 



The true olfactory fibres are pale elements, O0045- 

 0'0074 mm. in thickness, enclosed within a nucleated 

 sheath. The contents of the latter are not constituted 

 by a single axis cylinder, however, but, as was found 

 by Schultze, by a bundle of extremely delicate vari- 

 cose primitive fibrillae, '002 3-0 -0002 mm. in dia- 

 meter, presenting a secondary nucleus formation 

 (comp. 175). Similar fibres are to be found 

 in the grey matter of the bulbus olfactorius ( Walter, 

 Schultze). 



In the mucous membrane of the regio olfactoria 

 several other bundles may be recognised (fig. 572, 

 E, f). These spring at acute angles from branches 

 of the olfactory twigs, and give origin in their further 

 course to the true (complex) nerve tubes. The latter 

 are for a short distance enclosed within their nu-. 

 cleated sheaths, until eventually the delicate varicose 

 fibrillas of the interior stream out free into the tissues 

 around (Schultze). 



The mode in which these filaments ultimately ter- 

 minate is not yet fully ascertained. It seems, how- 

 ever, extremely probable that the varicose fibrillce 

 are continuous with the descending fibres of the olfac- 

 tory cells, so that these bodies, with their narrow rods, 

 may be looked upon as the terminal structures of the 

 nerves of smell. 



In fig. 574 we have given a diagram of the pro- 

 bable arrangement of parts here, which appears very 

 nearly allied to the mode of final distribution of the 

 gustatory nerves in the tongue of the frog ( 305). 

 The fact, however, cannot be concealed, thr.t the most recent observations 

 by Exner on the structure of these parts have pointed to other conclusions. 

 According to him there is no such very sharp line of distinction 

 between the two species of elements of the regio olfactoria, the olfactory 

 and columnar epithelial cell; they are, he says, connected by inter- 

 .mediate forms. 



Further, underneath these cells there exists a ("subepithelial") band- 

 work of protoplasm, whose interstices are filled with nuclei. Into this 

 (in man) thin network the ramifications of both kinds of cells sink from 

 above, and become fused. From below, also, the olfactory fibres ascend 

 into it. Thus we have in it an intermediate nervous plate. 



The course of development of the olfactory organs in the embryo, 



Fig. 574 Probable mode 

 of termination of the 

 olfactory nerves in the 

 pike (after Schultze). a, 

 olfactory cells ; 6, rods ; 

 c, deeper varicose 

 threads; e, axis fibrillae 

 in the sheath/; d, dis- 

 tribution of the latter. 

 At missing con- 

 nection with the corre- 

 sponding fibres, c. 



