CENTRAL ORIGIN AND RELATION OF THE CRANIAL NERVES.
The cranial nerves are twelve in number and are enumer ated from before backward, as : first, the olfactory, which con sists of the olfactory bulb, and nerve or commissure, which is lodged in the olfactory fissure upon the orbital surface of the frontal lobe of either hemisphere of the brain, parallel with the anterior median fissure of the cerebrum. The nerve divides behind into three roots, external, middle, and internal. The external root crosses the outer part of the anterior per forated space to the temporal lobe of the hemisphere. the lower portion of which is supposed to be the cerebral center of taste and smell. The middle root dips into the anterior perforated space, and apparently passes backward to the anterior commissure of the third ventricle. In the mole, an animal in which the sense of smell is highly developed, this root is large and appears to decussate with its fellow in the median line, and passes with the commissure to the tem poral lobe of the opposite hemisphere. The internal root pas ses inward behind the anterior extremity of the marginal con volution and is lost in the median fissure, probably forming, with the internal root of the opposite nerve, the commissure between the olfactory bulbs. The connection of the anterior commissure of the third ventricle with the opposite temporal lobes renders it probable that its function is a commissure be tween the cerebral centers of smell of the opposite hemi spheres. The central arrangement of the olfaetory nerves, if the above is correct, is similar to that of the optic nerves.
The second, or optic nerves of each side, unite to form the optic commissure, and again divide into the optic tracts. Each tract winds upon the external surface of the cerebral peduncle, near the posterior part of which it divides into two tracts which terminate: the inferior, in the internal geniculat,e body; aud the superior, a larger tract, in the external genicu late body and pulvinar of the thalamus. The divisions of the optic tract are connected with the corpora quadrigimina by the brachia, anterius and posterius. The posterior fibres of the optic commissure are commissural between the cerebral nerve centers, the middle fibres decussate, the anterior fibres are coramissural between the retinre, and the external fibres of each nerve are continued into the tract of the same side.
The third nerve, motor occuli, arises from the inner side of the crus cerebri near the bottom of the posterior perforated space. It pierces the internal margin of the crusta, and upon entering the tegmentum, its fibres spread into numerous strify as they pass through the anteiior extremity of the processus, to their destination in the grey matter beneath the iter.
The fourth nerve, pathetic, arises from the inner margin of the processus, behind the test,es, and is-connected with the grey matter surrounding the iter which lies immediately beneath its apparent origin.
The fifth nerve, the trigeminus, arises from the center of the side of the pons Varolii. It transfixes the middle pedun cle of the cerebellum as it passes obliquely backward, out ward, and downward to the inferior angle of the restiform trian gle, through which it reaches the grey matter beneath the floor of the fourth ventricle. It divides into several slips which are distributed to several grey nuclei as far down as the calamus scriptorius. Its motor root is small, arid at its origin is above and behind its sensory root.
The sixth nerve, the abducent, lies upon the pons, and, winding beneath the lower margin of this body, passes out ward above the olive to join the fillet at its outer margin, where it is in relation with the fifth, seventh, and anterior root of the eighth nerves. Above, it leaves the fillet to reach the grey matter in the floor of the iter.
The seventh nerve, facial, arises below the middle pedun cle of the cerebellum, internal to the flocculus and the audi tory nerve, and, passing upward behind the middle peduncle, between the restiform body and the outer margin of the fillet, it enters the fourth ventricle through the anterior angle of the restiform triangle. It crosses the floor of the ventricle obliquely downward and inward to the raphe, where it decus sates with the opposite nerve and joins the anterior pyramid of the opposite side beneath the lower margin of the pons. In the floor of the fourth ventricle it is connected with the nucleus of the seventh nerve. It forms the superior strife transverse of the floor of the fourth ventricle.