Olfactory nerve

“ The anterior surface of the olfactory lobe fills a large foramen in the anterior wall of the cranium and is pressed closely against the posterior surface of the nasal sac. Numerous small nerves, collectively forming the olfactory nerve, arise from the anterior face of the lobe, penetrate the membraneous wall of the olfactory organ, and are distributed to its highly folded surface. ”

“ When we electrify the eye, we call up in it an appearance of light; when we electrify the tongue, we produce in it a sensation of taste; when we electrify the inside of the ear, we provoke in it the effect of a sound. So, too, a similar excitement, electric or otherwise, of the olfactory nerves, creates in our mind the sensation of smell, even though no odorous molecule takes part in the phenomenon. Sensation, therefore, seems to depend chiefly on the nature of the sensitive nerve. The external world seems to contribute to it only by setting in motion the nerve-fibres. ”

“ 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 centers 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. ”

“ The peculiar development of the membrane and the complicated structure of the nasal cavities in the region occupied by the cells which are supposed to connect with the extreme divisions of the olfactory nerve are all that one would expect from the differences in endowment. ”

“ He included the olfactory nerves among the segmental nerves, and looked upon the olfactory pit, the orbito-nasal lacrymal duct, the mouth, and the spiracle as all modified gill-slits, so that he reckoned three pre-oral and oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and eight post-oral segments belonging respectively to the VIIth and VIth nerves, and to the IXth nerve, and six segments belonging to the Xth nerve. He pointed out that muscles supplied by the oculomotor nerve develop from the outer wall of the first head-cavity ”

“ Many persons will at first consider that we are asking too much, when we express a desire to have the same deference paid to the olfactory nerve, as to the other nerves that influence our physical pleasures and pains. ”

“ The anosmia of smokers cannot be wholly attributed to their catarrh, though a light, acute nicotine-poisoning does not seem to produce a loss of smell. Nervous anosmia may be congenital,—-i.e., due to imperfect development of the olfactory vesicle in the brain,—or may be senile,—due to degeneration of some of the nervous elements which condition the sense,—or may be due to exhaustion of the olfactory nerve, or to dryness of the epithelium. If we rule out exhaustion,we may say that respiratory anosmia is vastly more common than toxic or nervous. ”

“ When a bad smell causes a grimace, there is a reflex action through the same motor nerve, while the olfactory nerves constitute the afferent channels. In these cases, therefore, reflex action must be effected through the brain, all the nerves involved being cerebral. 'When the whole body starts at a loud noise, the afferent auditory nerve gives rise to an impulse which passes to the medulla oblongata, and thence affects the great majority of the motor nerves of the body. ”

“ Now, the chemical irritant, in the latter case ammonia gas, affects not the olfactory nerve, but certain naked nerve fibrils in the mucous membrane belonging to what is known as the fifth cranial nerve, a nerve of simple sensation. [2] And the simultaneous irritation of the eyelids, and in the case of the pollen and horse effluvia the bronchial tubes, shows that these 95resemble in their action the simple chemical irritants, and not the odours.
2. The difference between those two sensations becomes clearly evident when an anosmic person is peeling an onion. ”

“ Each nerve ends in a bulb-like termination called an olfactory bulb, which rests on the cribriform plate and sends little terminal fibers down through to be distributed to the nasal cavities, especially to the upper half of the septum of the nose, the roof of the nose, and the anterior and middle turbinated bones. For in the mucous membrane of the upper nasal cavity are specially modified epithelial cells called olfactory cells, which play an important part in the conduction of smell. Hence when one wishes to smell anything especially well he sniffs it up. ”

“ By neglecting to tutor the olfactory nerve, we are constantly led to breathe impure air, and thus poison the body by neglecting the warning given at the gate of the lungs. Persons who use perfumes are more sensitive to the presence of a vitiated atmosphere than those who consider the faculty of smelling as an almost useless gift. ”

“ The vena ophthalmica lies close to the point where the olfactory nerve passes out of the cranial cavity. The largest branch it receives is the recurrent ophthalmo-temporalis, which accompanies the lateral side of the optic nerve. It collects the blood from the base of the brain, from Harder’s gland, from the eye muscles, from the 237inner eye, the nose, the lacrimal gland, the skin of the frontal region, and the comb. ”

“ As we proceed downward, the hornlike projection of the olfactory nerves beyond the central hemispheres goes on extending more and more, and the relative magnitude of the terminal ganglia or olfactory lobes increases in similar order. ”

“ For instance, the introduction of too concentrated a saline solution into the nasal cavity will set up rhinitis and destroy the terminations of the olfactory nerves. Pure water, on the other hand, is itself a caustic. ”

“ Olfactory apparatus similar to that of fishes, viz. a small aperture near the extremity of the snout leads into a cavity or cul de sac, lined by a delicate membrane, plentifully supplied by the fibrillæ of two slender olfactory nerves, which go off from the anterior end of each lobe of the cerebrum. The brain is of an oblong figure, the cerebrum is formed of two lobes, the cerebellum of one lobe situate directly posterior, not much thicker than the medulla oblongata. ”

“ The long-continued action of such impurities on the olfactory nerves may ultimately induce, through the central nervous system, alterations in the respiration, circulation, and nutrition. When moderately vitiated air is breathed more or less continuously, the individual becomes pale and loses her appetite; after a time there is a decline in the muscular strength and animal spirits. The aëration and nutrition of the blood is interfered with, and the general tone of the system falls below par. ”

“ In some quadrupeds with great power of smell, the olfactory nerves extend so far forward as to protrude beyond the front of the hemispheres of the brain, with bulbous terminations relatively very much larger than those of man. ”

“ The vertebrate olfactory apparatus commences as a single median tube which terminates dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and terminates blindly at the infundibulum in Ammocœtes. The dorsal position of the nasal opening is not the original one, but is brought about by the growth of the upper lip. The nasal tube originally opened ventrally, and was at that period of development known as the tube of the hypophysis. ”

“ That they must be connected with the respiratory apparatus would seem, judging by analogy, to be indisputable, for, so far as we know, an odor cannot be appreciated unless the air containing it be allowed to pass more or less rapidly over the olfactory nerves. ”

“ The waders, which have the largest olfactory nerves, are also those birds that display the greatest keenness of scent. Reptiles have very large olfactory lobes, leading us to believe that they discern odors readily, but at present we know little of the impressions they are sensitive to in this respect. Fish also have an olfactory membrane. Fishermen have always remarked that they may be attracted or driven off by throwing certain odorous substances into the water. ”

“ The olfactory nerves are very large, and distributed over a great extent of surface. Professor Owen concludes, from certain anatomical peculiarities, that some Fishes, as the Rays, which have the olfactory nerves greatly developed, "scent as well as smell ”

“ A recent development of the theory which we owe to Heyninx, a Belgian scientist, brings the process very closely into harmony with what occurs in the eye. According to this authority, olfaction is in reality a perception of ethereal undulations of the same character as the undulations of light, these undulations being provoked by the intra-molecular vibrations of the odorous vapour in the nasal mucus and transmitted to the olfactory hairs not by immediate contact, but through the medium of the ether. ”

“ 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. ”

“ Gleg has estimated that the reaction time for auditory sensation is from 0’12 to 0’15 of a second, whereas the reaction time for smell is as much as 0’5 of a second, only one sensory stimulus being slower, that of pain, namely, which occupies 0’9 of a second.
Odours are conveyed to the olfactory end-organ in the air we breathe. Before they can rise into the air from the odorivector (the odorous body) and be transported they must, it is clear, pass into the vaporous or gaseous state. (In the case of fish, of course, the odour must undergo solution, that is pass into the liquid state.) ”

“ When one odour destroys the other, the obliteration must take place in the cerebral centre. Certain odours are antagonistic, such as musk and oil of bitter almonds, volatile oils and iodoform, ammonia and acetic acid. It is not unlikely that when one odour predominates among many, this may be due not to any chemical action of one substance over another, but that the missing sensations may be accounted for by their failure to excite the olfactory region of the cerebrum in the presence of a stronger stimulus. ”

“ We are now able to follow the undulations we term light, not only into the eye, but into the brain itself, locating their central station in the occipital lobe, whence their effects radiate all over the organism. And in the case of olfaction Pawlow has taught us that its chief vegetative function, the result of radiations from the olfactory central station in the brain, is the arousing of the digestive glands to activity. ”