THE CRANIAL NERVES.

IN the previous chapter many points pertaining to the superficial and deep attachments of the cranial nerves have been incidentally mentioned whenever different regions of the brain from which they arise have been considered. The nuclei of origin of some of the nerves have been discussed in detail. It seems to me advisable, however, to call attention again to some scattered hints, which have been dropped respecting these nerves, before they are individually considered from a physiological and clinical standpoint.

The nerves which arise from the brain are arranged as twelve pairs (according to Soemmering), which, from before backward, are called the olfactory, optic, motor-oculi, trochlearis, trigeminus, abducens, facial, auditory, glosso-pharyngeal, pneumogastric, spinal accessory, and hypoglossal. All of these, excepting the ninth, tenth, and eleventh pairs, are confined in their distribution to the head; while the other three have a distribution to the structures of the neck and trunk.

Willis has divided the cranial nerves into nine pairs, grouping the seventh and eighth nerves as one pair, and the ninth, tenth, and eleventh as one pair.

The OLFACTORY TRACT AND BULB must be regarded in the light of a constituent part of the brain rather than as a true nerve. This is demonstrated by its method of development, as well as by certain peculiarities in its structure. During fœtal life the olfactory lobe or tract consists of a diverticu

lum from the hollow globe of the cerebral hemisphere, and its cavity then communicates with the lateral ventricle. Its cortical layer is continuous superiorly with that of the rest of the brain.

The olfactory bulb forms a cap which embraces the prolongation of the brain substance. In its interior, rounded masses-the so-called "glomeruli" of the stratum glomerulosum-are found. These are peculiar to this region.

FIG. 68.-The base of the skull and the cranial nerves. The nerves are indicated by Roman numerals: 1, attachment of the tentorium; 2, sella turcica; 3, carotid artery.

Each olfactory nerve fiber appears to be wound into a knot, as it were, by the aid of nerve cells that are inserted in its course. The cortical substance of the olfactory tract or lobe presents, moreover, peculiarities in respect to the cells which compose it. These are in marked contrast to those which compose the cerebral cortex.

DEEP ORIGIN OF OLFACTORY FIBERS.

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The olfactory nerve has three peduncles or roots by means of which its fibers find their deep origins within the brain substance. The external or long root passes along the front border of the anterior perforated space, and enters the temporo-sphenoidal lobe. The middle or gray root arises from the cortical gray matter covering the anterior perforated space. The internal root passes into the substance of the frontal lobe.

The view is held, from some results obtained by Gudden's method, that the olfactory sense is presided over by a center or nucleus which is situated in the substance of the temporo-sphenoidal lobe, and that decussating fibers can be proven to pass between the olfactory tracts and also between the centers of smell of each hemisphere. The diagram introduced (Fig. 69) will help to illustrate in a rough way the conclusions advanced by late observers.

When the optic thalamus was under consideration, the opinion of Luys, that a center of smell could also be demonstrated within the substance of that ganglion, was commented upon. Some pathological facts, as well as clinical experimentation (to which that author refers), appear to lend credence to the view that the optic thalamus is associated, to a greater or less extent, with the special senses of smell, sight, and hearing. Its anatomical connections with this special sense are, however, a subject of pure conjecture as yet. Gudden's method of research does not sustain the opinion of Luys.

The late investigations of Flechsig have led him to the conclusion that the olfactory tracts can be traced backward as three bundles. One of these passes to the base of the frontal lobe, and probably terminates in the gyrus fornicatus; one goes to the cortex of the gyrus uncinatus; and one can be traced to the internal capsule of the cerebrum, by means of the anterior perforated lamina. Ganser and Gudden (by means of the latter's method of observing the atrophic changes that follow the extirpation of certain parts in the newly-born rabbit) seem to have arrived at the conclusion that the "olfac

tory portion" of the anterior commissure of the brain connects the two bulbs of the olfactory nerves, and the so-called "temporal portion" of the anterior commissure connects the temporal lobes of the two hemispheres.

FIG. 69.-A diagram designed by the author to illustrate the probable decussation of the fibers of the olfactory tracts.

O. B., olfactory bulbs; O. T., olfactory tracts; 1, 2, 3, internal, middle, and external roots of same; C. S., corpus striatum of each hemisphere of the cerebrum; a, fibers connecting the olfactory tracts; b, fibers connecting the centers of smell; a-b, anterior commissure, with its two sets of fibers.

The fact that the so-called anterior commissure of the brain consists of two strands (the temporal and olfactory fasciculi of Ganser) is not new. It is only the verification of the view that the olfactory apparatus can be proven to have an anatomical relationship with the anterior commissure that deserves special notice. In man and the monkey tribe, the temporal fasciculus is much larger than the olfactory. In the lower mammals (particularly in the rabbit, hedgehog, and mole), the olfactory strand seems to be developed in excess of the temporal.

The fibers of the OPTIC NERVE have been already considered at some length when the corpora quadrigemina were