SENSE OF SMELL. 



711 



FIG. 257. 



inodorous. Nearly all the vegetable odors belong to one or other of three 

 categories namely, hydrocarbons, aldehydes, or ethers. Many of them are 

 capable of being artificially formed, and they present good examples of 

 Isomerism. Thus the essences of turpentine, citron, bergamot, neroli, juni- 

 per, savin, lavender, cubebs, peppermint, and cloves, all have the same com- 

 position, C 10 H 16 . Amongst the odoriferous essences that are ranged by 

 chemists in the class of aldehydes are those of mint, rue, bitter almonds, 

 cummin, anise, fennel, canella, and meadow sweet. The ethers are very 

 varied and complex in constitution. Liunteus arranged odors into seven 

 classes: Aromatic (laurel leaves); fragrant (jasmine); ambrosial (musk); 

 alliaceous (garlic); fetid (stinking goose-foot); repulsive (solauacese); and 

 nauseous odors. Such an arrangement is very imperfect, not including, for 

 example, such well-marked odors as those of tar gas. 1 



606. The tractus olfactorius is exclusively composed of medullated nerve- 

 fibres which have a sheath of Schwann. It terminates on each side in the 

 Olfactory ganglion, from which the Olfactory nerves pass down in the form 

 of very numerous minute threads, which form a plexus upon the surface of 

 the Schneideriau or pituitary membrane (Fig. 257). 

 The filaments composing this plexus are described 

 by Messrs. Todd and Bowman 2 as differing widely 

 in structure from those of the ordinary cephalic 

 nerves ; they contain no white substance of Schwann, 

 are nucleated and finely-granular in texture, and 

 altogether bear a close resemblance to the gelatinous 

 form of nerve-fibres (Fig. 257). The mode in which 

 these nerves terminate has recently been the subject 

 of close investigation by Hoyer, 3 Schultze/ Lock- 

 hart Clarke, 5 Babuchiu, 6 Paschutiu, 7 Cisoff, 8 and 

 Martin. 9 Their distribution appears to be limited 

 to the membrane covering the upper third of the 

 septum of the nose, the superior turbiuated bone, 

 and perhaps the upper part of the middle turbinated 

 bone; together with the upper wall of the nasal Fibresofultimatera ; n ' ificationg 

 cavities beneath the cribriform plate of the ethmoid O f Olfactory Nerve of Dog. 

 bone ; all which surface is covered (as Messrs. 



Todd and Bowman have pointed out) with an epithelium of a rich sepia- 

 brown hue, constituting the regio olfactoria of Babuchin. According to 

 Schultze these epithelial cells are divisible into two sets : one of these 

 (a, Fig. 258) may be described as terminating externally by truncated flat 

 surfaces which cannot be observed to be covered by any membrane separate 

 from the contents of the cell. The contents themselves appear to consist of 

 protoplasm, presenting a yellowish granular appearance in the outer part, 

 whilst at the lower part an oval nucleus lying in clear protoplasm can be 

 readily distinguished. Towards their attached extremity these cells become 



1 See an interesting paper on Odors, by F. Papillon, in Pharmaceut. Journ., Nov. 

 16th, 1872, p. 383; also Lancet, 1870, p. 847. 



2 Physiological Anatomy, vol. ii, p. 9. 



3 Henle and Meissner's Bericht, 1857, p. 27. 



4 Untersuchungen uber den Bau der Nasenschleimhatit, Halle, 4to. 



5 Zeits. f. "VVissens. Zool., Bd. xi. For an abstract of this paper, see Med.-Chir. 

 Review for 1862, vol. i, p. 621. See also Exner, Sitz. d. k. Akad. zu Wien, 1870 

 and 1872. Exner denies the distinctness of the two forms of cells described below. 



6 Strieker's Human and Comp. Histology, Syd. Soc. Trans., 1873, p. 201, vol. iii. 



7 Leipziger Physiol. Arbeiten, 1873, p. 41. 



8 Centralblatt fiir die Med. Wissenschaften, 1874, p. 689. 



9 Journ. of Anat. and Physiol., vol. viii, 1874, p. 39. 



