1Verves of the nose.—The olfactory nerve, or, as it may be more properly called, the olfactory lobe of the brain, arises from the posterior, inner, and inferior part of the an terior lobe of the cerebrum. It lies in the groove between the two most internal of the convolutions of this part of the brain, and may be divided into three parts;— the posterior, or pyramid, the anterior, or bulb, and the middle, or proper trunk of the nerve or lobe. At its origin it may be traced backwards into three roots. Of these, the °O'er or long root appears first in the fissura Sylvii at the junction of the anterior and middle lobes of the cerebrum, just above the trunk of the middle cerebral artery. llence, its chief portion proceeds in wards, forwards, and a little downwards on the under surface of the anterior lobe and in front of the substantia peiforata antica ; and on coming near the other roots it turns more directly forwards and unites with them at the beginning of the groove between the two convo lutions. In this course it receives on its outer border one or more separate fasciculi, which come from the deeper part of the lobe and are sometimes completely concealed by grey mat ter covering them.
The inner or short root is first visible at the inner and posterior part of the anterior lobe of the cerebrum, in front of the beginning of the fissura Sylvii, and just outside the great me dian division of the cerebrum. It consists of one or more fasciculi, and passes outwards and forwards to the commencement of the groove, where, curving round like the preceding, but in the opposite direction, it joins the other roots to form part of the pyramid.
The middle root' arises between the two preceding roots, and first appears in two or three bands of fibres just in front of the sub stantia perforata entice, whence they proceed forwards to join the other roots.
Each of these roots is connected with grey matter prolonged from the surface of the ante rior lobe and the fissura Sylvii, and especially from two slight elevations, one of which lies in the concavity of the internal, the other in that of the external, root, as they severally turn for wards where they join the middle root. The grey matter connected with the external root covers part of its origin, is continued along both its sides, and conceals more or less the filaments which join its outer border. Then connecting itself with the grey matter around the fibres of the middle root, they pass forwards together,and are lost near the apex of the pyra mid ; part of them entering its substance, but a greater portion forming a thin layer ( propago cinerea externa) which dovers its surfaces, especially the upper one. The grey matter connected with the inner root conceals many of the fasciculi of its origin, and two streaks pro ceed from it, of which one passes into the inte rior of the pyramid, separating the internal and middle roots, and the other, of much larger size, is continued over its surface. This latter
is the middle or grey root of Cloquet, &c. (the propago einerea interim.) It has a somewhat pyramidal form, and covers a part of both sur faces of the pyramid, but chiefly the upper surface. Its deepest edge penetrates to the middle root, and its outer edge sometimes joins the superficial layer covering the other roots. As it proceeds forwards it becomes more and more slender ; and it is prolonged further on the upper than on the lower surface of the pyramid, near the apex of which it ceases.
Thus, the pyramid of the olfactory nerve is fumed by three fasciculi of white filaments, separated by streaks of grey matter, by which also it is covered on both its borders and on a great part of its upper surface. It is between two and a half and three lines long ; its base lies in the angle where the two internal convolutions of the anterior_lobe dii erge; and at its anterior extre mity, becoming gradually smaller and flatter, it is continued into that which may be called the trunk of the nerve, the tractus ollaciorius. This is nearly flat : it is grooved aloni, the middle of its under surface, which rests on trie upper part of the body of the sphenoid bone, and has a ridge, or is altogether convex upon its upper surface, which lies in the groove between the convolu tions. It is striated in its whole length, and nearly white; though some grey matter is col lected within the meshes of the plexuses, which its fibres form as they proceed forwards and a little inwards towards the bulb in which they expand. After long immersion in spirit, the groove on the under surface of the trunk, which, in the recent state, varies much in depth in different persons, always becomes deeper and more distinct. Valentin' suggests that it indicates the course of the canal which in the human embryo passed from the lateral ventricle to the end of the olfactory bulb. The analogue of this canal is persistent in the olfactory nerves of lower AIammalia; but there is no sufficient evidence of its having been ever seen in the human adult; at /east in this part of the nerved The bulb of the olfactory nerve is a nearly elliptical flat body, about half an inch long, slightly furrowed above, convex on its lower surface, and evenly rounded in front. It rests upon the dire mater covering the cribriform plate; its inner margin is in contact with that covering the crista galli, and with the anterior part of the falx cerebri ; by which alone it is separated from the bulb of the opposite side. It is of a greyish-red colour from the quantity of grey nervous matter which is placed upon its surface and among the plexuses formed by the nervous filaments within it. In its interior a small cavity or ventricle may be generally detected by a vertical antero-posterior section : it is the remains of the embryonic conditio just alluded to.