From the lower surface of the bulb procee, the numerons branches of the olfactory nerve They vary much in number and size both i different persons, and on the two sides of th same individual ; a want of symmetry which may often be seen in the perforations of th:. cribriform plate. The ordinary number o branches is from fifteen to twenty on each side. Each of them, invested by a very delicate neu rilema, passes through an aperture in the cribri form plate, through which also a tubular pi., longation of the dura meter passes and becomes continuous with the periosteum of the nasal fossm. The nerves, which become rather firmer when they have passed through the cribnform plate, ramify between the periosteum and the mucous membrane, and are divisible into tw chief sets, some being placed upon the septum, others upon the outer wall of the nose.
The internal or septual branches are abou twelve in number. After passing through th• cribriform plate, they diverge a little as they descend; the anterior going somewhat forwards, the posterior backwards. The trunks soon, and often while within the foramina of the cribri form plate, break up into tufts of filaments, which unite into plexuses with long and narrow quadrilateral meshes; and frotn these, smaller branches proceed which aaain form finer plex uses. They may be traced nearly to the lower fourth of the septum.
The external or labyrinthic branches are rather more numerous and smaller. They diverge and ramify like the preceding, lying in the channels and grooves upon the upper two turbinated bones. They have been traced to the lower border of the middle turbinated bone, but not to its outer concave surface, nor to any part below it; yet the similarity of the structure and arrangement of the inferior turhinated bone and the mucous membrane over it makes it very probable that they ramify on it also. The middle branches are the longest; the posterior ones form curves directed backwards towards the sphenoidal sinuses, but not entering thetn. At the posterior angle of the middle turbinated bone some of them are described by Mr. Swan* and by Soemmering as anastomosing with a branch from the spheno-palatine ganglion ; but Valentin / could not find any such communica tion. A few branches in addition to these are said to be distributed in the membrane covering the cribriform plate iiself (Cloquet).
How the primitive filaments of the olfactory nerve terminate has not yet been ascertained ; their softness and the density of the tissue in which they lie have hitherto prevented an accu rate observation of thern in this part of their course.
Compared with the other nerves the olfactory present many peculiarities of structure and arrangement, especially in the part which is within the skull. I. They are the softest of the nerves within the skull, possessing only the most delicate neurilema; and a rather less de gree of this softness is characteristic of their branches, so that their dissection is more diffi cult than that of any others of equal size. 2. They have grey nervous matter both upon and between their fasciculi, and their bulbs are not like the ganglions of other nerves, but like portions of the. brain. 3. They are not, as other nerves, cylindrical, but triangular in one and tlat in another part of their course. 4. Their trunks converge, while those of all otheis diverge from their origins. 5. They lie in deep furrows on the surface of the brain, and they leave the skull by several distinct orifices. In many of these characters.they are rnore like portions of brain than nerve and, as Valentin observes, there is no other`nerve in the adult human body which shows its origin as an immediate prolongation of the central nervous mass so plainly as these do. It was on account of these characters that the olfactoty nerves were regarded by the an cient anatomists as processes of the brain (mamillary or papillary processes), through the central canals in which they supposed that the pituitary humour was carried from the lateral ventricles to the nose, and air wits drawn into them by the nostrils. And al though this notion was derived from dissect ing the nerves of animals in which the trunks remain hollow, yet their true nature was doubted on the same gr, ounds by many, even after Willis had demonstrated their structure in man.* In iccordance with its numerous offices, the nose receives, in addition to these,—the nerves of its peculiar sense,—others for common sen sation, for the movements of its muscles, and for the government, in some degree at least, of the organic processes which are carried on in it. Its sensitive and organic nerves are de rived from the internal nasal or ethinoidal branch of the first or ophthalmic division of the fifth, from the naso-palatine and numerous other branches, from the sphenolialatine ganglion, the nasal branches of the Vidian, palatine, anterior dental, and infra-orbital .nerves; all of which are described under the title FTFTII PAIR OF NERVES. IIS motor nerves are supplied by the facial or seventh pair [SEVENTII PAIR OP NERVES].