The olfactory nerve-fibers are the central processes of the bipolar olfactory nerve cell-bodies situated in the olfactory region of the nasal mucous membrane. In man, the olfactory region comprises the epithelium upon the superior third of the nasal septum and that upon practically the whole of the superior nasal concha.
The area is relatively small as compared with that of other mammals and, as in other mammals, is characterized by an increased thickness of the epithelium and a yellowish brown colour in the fresh. The peripheral processes of the olfactory cell-bodies (the olfactory gangUon) are short and extend only to the surface of the olfactory epithehum. As the central processes pass upward from their cells of origin they form plexuses in the mucous membrane, and from the upper parts of these plexuses, immediately below the lamiiia cribrosa of the ethmoid, about twenty filaments issue on each side. These filaments comprise the olfactory nerve. They are non-medullated. They pass upward, through the foramina in the lamina cribrosa, into the anterior fossa of the cranium in two rows, and after piercing the dura mater, the arachnoid, and the pia mater, they enter the inferior surface of the olfactory bulb. They contribute to the superficial stratum of nerve-fibers on the inferior surface of the olfactory bulb and end in the glomeruli, which are formed by the terminal ramifications of the olfactory nerve-fibers intermingled with the similar ramifications of the main dendrites of the large mitral cells which lie in the deeper part of the grey substance of the olfactory bulb.
The olfactory nerve-fibers are grey fibers, since they do not possess medullary sheaths, and they are bound together into nerves by connective-tissue sheaths derived from the pia mater, from the subarachnoid tissue, and from the dura mater. Prolongations of the subarachnoid space pass outward along the nerves for a short distance.
The olfactory impulses are transmitted by way of the peripheral processes of the olfactory neurones through the cell-bodies and the olfactory nerve-fibers and through the glomeruli to the mitral cells. Thence they are carried by the central processes (axones) of the mitral cells, which pass backward along each olfactorj' tract and its three olfactory strise (see Rhinencephalon).
The terminal nerve (Nervus Terminalis)
In lower vertebrates and recently in those mammals whose sense of smell is relatively much more developed than in man, three nerves have been found concerned with the olfactory apparatus:
- The olfactory nerve proper whose fibers, as noted above, are the central processes of the nerve cell-bodies situated in the epithelium of the olfactory region of the nasal mucosa, and which terminate in the olfactory bulb;
- The vomero-nasal nerve, whose fibers are the central processes of nerve cell-bodies situated in the epithelium of the vomero-nasal (Jacobson's) organ and which pass caudalward in the submucosa and upward to join the filaments of the olfactory nerve proper and which, in the dog, cat, rabbit, rat, etc., terminate in the accessory olfactory bulb - a small protuberance possessed by these animals on the postero-median aspect of the olfactory bulb proper;
- The terminal nerve, a small plexiform nerve, which unlike the other two, is ganglionated.
In man, the vomero-nasal (Jacobson's) organ is rudimentary after birth and, therefore, the vomero-nasal nerve is not present, the only fibers for the vomero-nasal region being those of general sensibility from the trigeminus and sympathetic fibers common to the epithelium of the entire nasal fossa.
The terminal nerve has been recently described as present in the human foetus and it is mentioned here because of the expressed belief that it is present in the adult. From the observations recorded for human and rabbit foetuses and the adult dog and cat, the following description may be given: It is variably plexiform throughout its course. Its peripheral twigs are distributed to the mucosa of the nasal septum, some to the mucosa joining the olfactory region while other and larger twigs extend further forward and are distributed to mucosa of the vomero-nasal organ, accompanying and sharing in the distribution of the vomero-nasal nerve when this is present. Its central connections are in the form of two or three small roots which pass through the cribriform plate of the ethmoid bone in company with and mesial to the vomero-nasal nerve and then, still plexiform, extend caudalward over the infero-mesial aspect of the olfactory bulb and upon the olfactory peduncle or stalk (olfactory tract) beyond, a root often extending to near_the lamina terminalis and optic chiasma. The roots disappear in the mesial and infero-mesial aspect of the frontal portion of the brain at different localities caudal to the olfactory bulb and usually near the olfactory peduncle, but often one may disappear in the region corresponding to the anterior perforated substance of the adult human brain.
Numerous small groups of ganglion cells are found interposed along both the peripheral and intracranial course of the terminal nerve. A group, larger in size than the others and situated in the intracranial course of the nerve, is called the ganglion ierminale. The fibers of the nerve are non-medullated. Both the ganglion cells and the fibers of the nerve are described as having more the appearances characteristic of sympathetic neurones than of cranio-spinal. On the other hand, our conceptions of sympathetic neurones do not permit of their terminating within the central system except for the innervation of its bloodvessels. It may result that, instead of being an independent nerve as now claimed, the nervus terminalis is a part of the forward extension of the cephalic sympathetic, the larger ganglia and plexuses of which latter are well known, and that its neurones receive and convey impulses to the gland cells of the nasal mucosa and to the muscle of the blood-vessels of the mucosa and those supplying the infero-mesial part of the frontal end of the cerebrum.