SECT. 235.] OLFACTORY NERVE. 60J 



nerve possesses, in its tractus and bulbus, dark-bordered tubes and 

 nerve-cells, of which \vc have already spoken above (p. 236). On the 

 other hand, the nervi olfactorii, in man and in mammalia, nowhere 



Fig. '219. 



1 Ufaetory tabes of ttie ox; magnified 3.">f) times. 1. A thick crrey tube ; r<. envelope 

 of the same; b. effused contents with nuclei. 2. A fine dark-bordered tube, a. con- 

 tinuing from one of the foramina cribrosa into a pale nucleated fibre, b. 3. The empty 

 envelope of a grey tube, at one of its extremities appearing collapsed and like a 

 fibre. 



contain any white medullated fibres, even in the main brandies 

 which arise from the bulbs, but consist throughout of pale, slightly 

 granular, fiat tubes, 0"002" to 0003" in breadth, with elongated 

 nuclei ; these tubes are firmly connected and held together by 

 envelopes of connective tissue running between them, and these 

 are thicker and, consequently, whiter on the rami ad septum. The 

 fibres of the olfactory nerve are very similar to the embryonic 

 nervous elements, and a delicate, structureless envelope can be 

 easily seen on them in animals, distinct from the finely-granular 

 nucleated contents ; towards their terminations, they are continued 

 into finer and finer fibres, of cooi'" to 00005'" xn diameter, some 

 of which may even be found in the trunks of the nerve. As for 

 the origin of these nerve-fibres, nothing positive has yet been 

 ascertained, either in man or in the lower mammalia ; but from 

 the observations of Leydvj in the plagiostoma (Beitruqe, p. 34, 

 tab. i., fig. 6), and from other considerations, it is probable that 

 they arise from the cells of the olfactory bulb ; the details of their 

 union with these cells, however, still remains to be investigated. 

 The termination of the nerves is still more doubtful. This much 

 is easily seen, that the olfactory nerves, iu their course in the 

 mucous membrane of the olfactory region, divide frequently at 

 acute angles, become smaller and smaller as they descend, and 

 produce a plexus. In mammalian animals, also, they can be 

 traced over almost the whole of the proper olfactory region, but a 

 little in front of the border of it their plexus is always withdrawn 

 from view ; nor can any trace of terminal twigs be met with, so 



