It is not difficult to explain many of the apparent inconsistencies which attend the material theory. The varieties of the olfac tive power among human beings are quite sufficient to prove, that a substance which is strongly odorous to one individual may not produce any impression on the smell of an other, whose scent for other substances may nevertheless be very acute. And there is strong reason to believe that there is a great diversity in this respect•amongst dif ferent species of animals, some appearing en tirely insensible to odours which strongly affect others. That we do not appreciate an odour, therefore, is no proof of its non-exist ence ; and we have no right to say of any volatile or gaseous substances, that they are not odorous, but simply that they are not odorous to us. Again, the sense of smell, like the other senses, is rather relative than positive ; that is to say, it rather estimates a change in the condition of the surrounding medium, than its actual permanent state. This is fully proved by the fact that persons who habitually dwell amongst odours of any one kind, become, in time, entirely insensible to them, although their olfactive sense may re main of its full acuteness in regard to any different scent. This being the case, we at once perceive that water, oxygen, nitrogen, and carbonic acid could not, in accordance with the general laws of sensation, possess any odour to animals whose organs of smell are constantly imbued with them. We shall presently find that the moisture of the olfac tory membrane is a necessary condition of its functional power ; and thus neither fishes, which have their olfactory surface con stantly bathed in water, nor air-breathing ani mals, whose pituitary membrane is lubricated with it, could take cognisance of any odorous properties which it might really possess. In like manner, the nasal cavities of animals being continually filled with a mixture of oxygen, hydrogen, and carbonic acid, these gases cannot excite the olfactive sense; whilst on the other hand, we can easily imagine that if animals were adapted to breathe hydrogen or its strongly odorous compounds, they would be insensible to the latter, whilst they might distinguish oxygen, nitrogen, or carbo nic acid by their respective odours, just as readily as we distinguish phosphuretted, sul phuretted, or carburetted hydrogen.
Although it is through the atmosphere that odorous emanations are most readily conveyed, yet there can be no reasonable doubt that they may be transmitted through water also. Thus we find fishes provided with a complex organ of smell, which seems to be of considerable importance in directing them towards their prey, This may be infer red, not merely from the fact that the olfactive ganglia and nerves are of large size relatively to the rest of the encephalon, but also from the circumstance, well known to fishermen, that many fish are particularly attracted; by odorous bait. Some anglers are even in the habit of scenting their bait with essential oils, in order to render it more alluring.
The general structure of the organ of smell in man has already been described (NosE); but some particulars recently ascer tained by Messrs. Todd and Bowman re specting the minute anatomy of the pituitary membrane, and the structure and distribution of the olfactory nerve, are too important to be passed hy. That the true sense of smell is specially, if not exclusively, the endowment of the upper portion of the organ, has been inferred by anatomists, from the limited dis tribution of the olfactory nerve, and by phy siologists from the fact that odours are only perceived strongly when the odoriferous air is drawn into the upper part of the cavity. The lower part of the nasal cavity is pro perly to be regarded as the orifice of the respiratory passages : it is extremely sensitive to irritants, but it does not participate in the discrimination of odours properly so called ; and its mucous membrane is covered with a ciliated columnar epithelium. OD the other hand, the limits of the olfactive region " are distinctly marked by a more or less rich sienna-brown tint of the epithelium, and by a remarkable increase in the thickness of this structure compared with the ciliated region below ; so much so, that it forms an opaque soft pulp upon the surface of the membrane, very different from the delicate, very trans parent film of the sinuses and lower spongy bones. The epithelium, indeed, here quite alters its character, being no longer ciliated, but composed of an aggregation of superposed nucleated particles, of pretty uniform appear ance throughout ; except that, in many in stances, a layer of those lying deepest, or almost deepest, is of a darker colour than the rest, from the brown pigment contained in the cells. These epithelial particles, then, are not ciliated ; and they form a thick, soft, and pulpy stratum, resting on the basement membrane. The deepest layer often adheres after the others are washed away." The vessels of the olfactive membrane in the fcetus present a regular series of papillary loops ; but these cannot be seen in the adult. " The olfactory filaments form a considerable part of the entire thickness of the membrane, and differ widely from the ordinary cerebral nerves in structure. They contain no white
substance of Schwann, are not divisible into elementary fibrillm, are nucleated and finely granular in texture, and are invested with a sheath of homogenous membrane." These nerves thus rather correspond with the gela tinous fibres, than with the ordinary tubular fibres of the trunks and branches of true nerves ; and they are regarded by the authors as direct continuations of the vesicular matter of the olfactory bulb or ganglion. " Although these nucleated olfactory filarnents lie in great abundance under the mucous membrane of the olfactory region, we have been quite foiled in our attempts to trace their ultimate distribution in the membrane, and the diffi culty is attributable to their want of the characteristic white substance. Their elon gated nuclei render the larger branches un mistakeable ; but if these become resolved at last into fibrous elements, the nuclei cease to be distinct from those of the numerous nu cleated tissues which they traverse." " We are averse from speculating prematurely on the meaning of anatomical facts ; but as some hypothesis will intrude itself, we would ven ture to hint that the amalgamation of the elements of the peripheral part of the nervous apparatus in the larger branches, and probably in the most remote distribution, as well as the nucleated character indicative of an essential continuity of tissue viith the vesi cular matter of the lobe, are in accordance with the oneness of the sensation resulting from simultaneous impressions on different parts of this organ of sense, and seem to show that it would be most correct to speak of the first pair of nerves as a portion of the nervous centre put forward beyond the cra nium, in order that it may there receive, as at first hand, the impressions of which the mind is to become cognisant."* It has also been remarked by the same excel lent observers t, that on the septum narium and spongy bones bounding the direct pas sage from the nostrils to the throat, the lining membrane is rendered thick and spongy by the presence of ample and capacious sub mucous plexus of both arteries and veins, of vihich the latter are by far the larger and more tortuous. And they surmise, with much probability, that the chief use of these inay be to impart warmth to the air, before it enters the proper olfactive portion of the cavity ; as well as to afford a copious supply of moisture, which may be exhaled by the abundant glandulw seated in the membrane. " The remarkable complexity of the lower turbinated bones in animals with active scent, without any ascertained distribution of the olfactory nerves upon them, has given coun tenance to the supposition that the fifth nerve may possess some olfactory endowment, and seems not to have been explained by those who rejected that idea. If considered as accessory to the perfection of the sense in the way above alluded to, this striking arrangement will be found consistent with the view uhich thus limits the power of smell to the first pair of nerves."* The olfactive organ, in other air-breathing Vertebrata, corresponds with that of man in all the essential particulars of its structure ; being a cavity opening anteriorly upon the face by the external or anterior nares, and posteriorly into the upper part of the pharynx by the internal or posterior nares. It may thus be considered as the entrance of the respi ratory passages, which is dilated for the ex tension of the olfactive membrane ; or, per haps, it would be more correct to speak of it as a diverticulum from the commencement of the respiratory tube, since, as we have seen, the proper olfactive organ does not extend into that portion of the cavity which is placed in a direct line between the anterior and pos terior nares. The development of the olfac tive organ, as measured by the size of the olfactory ganglia and nerves, and by the ex tent of the surface over which these are dis tributed, varies greatly in different tribes ; and details must be sought on this subject under the respective names of the classes and orders of vertebrata. The chief departure from the ordinary type is observable in the case of the Cetacea, in which the nasal cavity is almost entirely devoted to the purposes of respiration, and to the ejection of the water taken in by the mouth with the food. To animals which seek their prey in the water, an organ of smell, adapted to take cognisance of odorous emanations contained in the inspired air, would obviously be entirely useless ; and it is probable that whatever olfactive power they possess is called into exercise by the passage of the water that is spouted through the nos tril. The ordinary statement that the Cetacea are entirely destitute of offactive ganglia and nerves, and that they must therefore be en tirely devoid of the sense of smell, is true only of the Delphznidca, or that division of the order which includes the dolphins and porpoises ; for the Bakenicke, or proper whales, do possess olfactive nerves, although these are comparatively of small size; and in the Manatido, or herbivorons whales, which properly belong rather to the Pachydermata than to the Cetacea, the olfactive apparatus is formed after the usual type.