Olfactory organ

“ The olfactory organ in man appears to differ from that in other animals as his eyes differ from those of creatures in which they are hard; for man's eyes are furnished with a rampart, and a kind of sheath in lids, without the elevation and drawing asunder of which he cannot see, while hard eyes, having no such provision, are instantly sensible of whatever may be present in the diaphanous medium. In accordance with this, the olfactory organ is, in such creatures, like the eye, uncovered ”

“ Olfaction is, indeed, one of the primitive senses of animal life. And in man, as it happens, while vision has constructed for itself a highly complicated camera-like end-organ, and hearing has produced an apparatus even more elaborate, the olfactory organ, on the other hand, remains primitive, its essential structure having undergone no apparent evolutionary change from the simplest and earliest type. ”

“ At this stage, albeit early in our disquisition, we may appropriately turn to consider the curious fact that of all our senses that of smell is perhaps the most easily exhausted. The olfactory organ, under the continued stimulation of one particular odour, quite quickly becomes insensitive to it. ”

“ Odors, to be perceived, must be taken up by oxygen, and borne by it to the organ of smell. In a word, odor is the odoriferous particle itself, while light is not the light-giving body.
Does oxygen exert a chemical influence on those atoms of which it robs odorous substances? We do not know, neither do we know of what kind is the action which occurs on the contact of odor with the olfactory nerve, whether the phenomenon is a mere mechanical agitation, or whether some chemical decomposition takes place in the case. ”

“ After publishing in 1815 a preliminary work, he issued in 1821 his Osphrésiologie, ou Traité des odeurs, du sens et des organes de l'Olfaction, a complete monograph on the anatomy, physiology, psychology, and pathology of the olfactory organ and its functions, and a work that may still be consulted with profit, if indeed it can even yet be said to be at every point superseded. After Cloquet's time the study of the sense of smell seems to have fallen into some degree of discredit. For more than half a century no important progress was made in this field. ”

“ In Myxine the olfactory organ communicates with the mouth through the palate, but the meaning of this communication, which does not appear to be of the same nature as the communication between the olfactory pits and the mouth by the posterior nares in the higher types, is not known.
Fig. 308. Side view of the head of an embryo Chick of the third day as an opaque object. (Chromic acid preparation.)
C.H. cerebral hemispheres; F.B. vesicle of third ventricle; M.B. mid-brain; Cb. cerebellum; H.B. medulla oblongata; N. nasal pit; ot. ”

“ It is possible that the sexual associations of odors have been still further fortified by a tendency to correlation between a high development of the olfactory organ and a high development of the sexual apparatus. ”

“ The "mind" is not, so to speak, upon the other side of the epithelium. On the contrary, the inner ends of the olfactory cells are connected with nerve-fibers, and these nerve-fibers, passing into the cavity of the skull, at length end in a part of the brain, the olfactory sensorium. It is certain that the integrity of each, and the physical interconnection of all these three structures, the epithelium of the sensory organ, the nerve-fibers, and the sensorium, are essential conditions of ordinary sensation. That is to say, the air in the olfactory chambers may be charged with particles of musk ”

“ Perfumes or scents, however, exert not only an agreeable impression on the olfactory organ, but their effect extends to the entire nervous system, which they stimulate; when used in excess, they are apt to cause headache in sensitive persons; the laborers in the chemical factories where these substances are produced on a large scale, occasionally even suffer by reason of their stimulating action on the nerves. ”

“ The most accurate microscopical observations, however, have never been able to discover in such animals any distinct organ of Smell. They have a mouth and a stomach, but no nostrils. The organ of Taste, it is probable, has in them a sensibility of the same kind with that which the olfactory nerves have in more perfect animals. They may, as it were, taste at a distance, and be attracted to their food by an affection of the same organ by which they afterwards enjoy it; and Smell and Taste may in them be no otherwise distinguished than as weaker or stronger sensations derived from the same organ. ”

“ Not only is most of the air breathed in through the nose and warmed in its passage through, but the nose is the organ of smell and by means of the peculiar property of its nerves protects the lungs against deleterious gases and helps the taste discriminate. The olfactory or first cranial nerves, after emerging from the brain, lie on the under surface of the frontal lobe and rest on the ethmoid bone in what is known as the olfactory tract. ”

“ The sensation of smell, it should be borne in mind, is produced by a kind of odoriferous vapor, very fine and invisible, that flies off from nearly all bodies. The air which contains this vapor is drawn into the nose, and is in this way brought into contact with the very delicate nerves of smell that ramify the membrane which lines the air-passages of this organ. It is only when the exceedingly small particles of which the odor of various bodies is composed come in contact with the minute ramifications of the olfactory nerve that this sensation is produced. ”

“ In Elasmobranchs they arise, not from the base, but from the lateral parts of the brain (fig. 263) , and become subsequently divided into a bulbous portion and a stalk. They vary considerably in their structure in the adult.
Fig. 263. Section through the brain and olfactory organ of an embryo of Scyllium. (Modified from figures by Marshall and myself.)
ch. cerebral hemispheres; ol.v. olfactory vesicle; olf. olfactory pit; Sch. Schneiderian folds; I. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain; pn. anterior prolongation of pineal gland. ”

“ This is the clearest instance we have of a neurone being actually excited under natural circumstances by an agent of the environment directly, not indirectly. The deep ends of these olfactory neurones having entered the central nervous organ come into contact with the dendrites of large neurones, called, from their shape, mitral. In the dog, an animal with high olfactory sense, the axone of each olfactory neurone is connected with five or six mitral cells. In man each olfactory neurone is connected with a single mitral cell only. ”

“ This is done that quantities of the other odorous substances large enough to allow for evaporation may be put into the solution without raising the intensity of the smell to the neighborhood of the terminal stimulus-intensity. [21] If, now, most smells are mixed, and if mixed smells alter in quality as the organ becomes fatigued, and if different olfactory qualities have not the same limina, then in quantitative work in smell, we are seeking to determine values which are continually changing according to laws which we do not know. ”

“ The organ of smell is not so easily demonstrated as that of hearing, though the evidence of their possessing the sense to an acute degree is readily attainable. A German naturalist inferred, from the fact of the nerve corresponding to the olfactory nerve in man being distributed to the antennæ, in insects, that the antennæ were the organs of smell in them. ”

“ An organ of smell is definitely situated behind or above the eyes. There two small projections, or, as frequently, two minute pits or depressions, occur. These pits are ciliated, and between the cilia "olfactory cells" are situated. These cells, in turn, represent the similar structures which occur in higher animals, and which, in man himself, form the characteristic terminations to his olfactory nerves. That the cuttle-fishes can literally scent their prey from afar off is an idea confirmed by the facts of their every-day life. ”

“ There is no doubt that the antennæ of these creatures are connected by nerve-stalks with the anterior part of their supra-œsophageal ganglia, i.e., the nervous centres corresponding to our brain.
But what kind and degree of power must such olfactory organs possess? The dog has, relatively to the rest of his brain, a much greater development of the olfactory nerves and ganglia than man has. His powers of smell are so much greater than ours that we find it difficult to conceive the possibility of what we actually see him do. ”

“ Olfactory development, high or low, is linked up with the natural habits of the different species. Thus, mammals which go about on all fours, whose visual outlook is restricted and whose muzzle is near the ground, are the most highly gifted ; those, again, like the seals, porpoises, whales, and walruses, which have reverted from a terrestrial to an aqueous environment, where smell is of less value to them, show poorly developed olfactory organs ”

“ Even in animals as high in the scale of life as fishes, there is a complete set of sense-organs—the muciparous canals, in the head and along the lateral line down the side, the function of which we can only guess. By some they are regarded as olfactory; by others, as fitted to respond to vibrations or shocks of greater wave-length than the auditory organ can appreciate ”

“ But these ocular ears or auditory eyes of the insect are not his only advantage over us. He has another sensory organ to which, with all our boasted intellect, we can claim nothing that is comparable, unless it be our olfactory nerve. ”

“ The different position of the olfactory tubercle has been already described as characteristic of some of the orders of fishes. In those fishes in which the tubercle is remote from the brain, the nerve which has entered the tubercle as a single stem leaves it split up into several or numerous branches, which are distributed in the nasal organ. In the other fishes it breaks up into branchlets spread into a fan-like expansion at the point, where it enters the nasal cavity. The nerve always passes out of the skull through the ethmoid. ”

“ The physical theory of odour, then, refers that quality to the vibration of the molecule. It suggests that the molecules of an odorous body passing in the gaseous or, in fishes, the liquid state into the olfactory region of the nose, are there received by the film of mucus in which the olfactory hairs lie, and stimulate these hairs by their molecular vibration. No chemical change is supposed to take place, only, as it were, a mechanical stimulation, comparable to the mechanical stimulation of the retina by the waves of light. ”

“ The olfactory system consists of the outer nose, which projects from the face, and the nasal cavities, contained in the skull, which support the olfactory mucous membrane for the perception of smell in their upper parts, and act as respiratory passages below. ”

“ Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on this earth, or could be reasonably imagined to have lived on it. ”