Henry Gray (18251861). Anatomy of the Human Body. 1918.
They form a plexiform net-work in the mucous membrane, and are then collected into about twenty branches, which pierce the cribriform plate of the ethmoid bone in two groups, a lateral and a medial group, and end in the glomeruli of the olfactory bulb (Fig. 772). Each branch receives tubular sheaths from the dura mater and pia mater, the former being lost in the periosteum of the nose, the latter in the neurolemma of the nerve.
The olfactory nerves are non-medullated, and consist of axis-cylinders surrounded by nucleated sheaths, in which, however, there are fewer nuclei than are found in the sheaths of ordinary non-medullated nerve fibers.
The olfactory center in the cortex is generally associated with the rhinencephalon (page 826).
The olfactary nerves are developed from the cells of the ectoderm which lines the olfactory pits; these cells undergo proliferation and give rise to what are termed the olfactory cells of the nose. The axons of the olfactory cells grow into the overlying olfactory bulb and form the olfactory nerves.
5b. The Optic Nerve
(N. Opticus; Second Nerve)
The optic nerve(Fig. 773), or nerve of sight, consists mainly of fibers derived from the ganglionic cells of the retina. These axons terminate in arborizations around the cells in the lateral geniculate body, pulvinar, and superior colliculus which constitute the lower or primary visual centers. From the cells of the lateral geniculate body and the pulvinar fibers pass to the cortical visual center, situated in the cuneus and in the neighborhood of the calcarine fissure. A few fibers of the optic nerve, of small caliber, pass from the primary centers to the retina and are supposed to govern chemical changes in the retina and also the movements of some of its elements (pigment cells and cones). There are also a few fine fibers, afferent fibers, extending from the retina to the brain, that are supposed to be concerned in pupillary reflexes.
The optic nerve is peculiar in that its fibers and ganglion cells are probably third in the series of neurons from the receptors to the brain. Consequently the optic nerve corresponds rather to a tract of fibers within the brain than to the other cranial nerves. Its fibers pass backward and medialward through the orbit and optic foramen to the optic commissure where they partially decussate. The mixed fibers from the two nerves are continued in the optic tracts, the primary visual centers of the brain.
The orbital portion of the optic nerve is from 20 mm. to 30 mm. in length and has a slightly sinuous course to allow for movements of the eyeball. It is invested by an outer sheath of dura mater and an inner sheath from the arachnoid which are attached to the sclera around the area where the nerve fibers pierce the choroid and sclera of the bulb. A little behind the bulb of the eye the central artery of the retina with its accompanying vein perforates the optic nerve, and runs within it to the retina. As the nerve enters the optic foramen its dural sheath becomes continuous with that lining the orbit and the optic foramen. In the optic foramen the ophthalmic artery lies below and to its outer side. The intercranial portion of the optic nerve is about 10 mm. in length.