tions of expression and respiration. In consonance with this view he termed them respiratory nerves, and he gave to that part of the medulla oblongata from which they arise the name of respiratory tract.

FIRST PAIR. OLFACTORY.. The olfactory nerve arises by three roots; an inner or short root from the inner and posterior part of the anterior lobe close to the substantia perforata; a middle root from a papilla of gray matter (caruncula mamillaris), imbedded in the anterior lobe, its white fibres being derived from the corpus striatum; and an external or long root, which may be traced as a white streak along the fissure of Sylvius into the middle lobe, where it is continuous with the fibres of the anterior commissure and the exterior white substance of the thalamus opticus. The nervous cord formed by the union of the three roots is soft in texture, prismoid in shape, and imbedded in a sulcus between two convolutions on the under surface of each anterior lobe of the brain, lying between the pia mater and arachnoid. As it passes forwards it

OLFACTORY NERVE, WITH ITS DISTRIBUTION ON THE SEPTUM NASI. The nares have been divided by a longitudinal section made immediately to the left of the septum, the right naris being preserved entire. 1. Frontal sinus. 2. Nasal bone. 3. Crista galli process of ethmoid bone. 4. Sphenoidal sinus of left side. 5. Sella turcica. 6. Basilar process of sphenoid and occipital bone. 7. Posterior opening of the right naris. 8. Opening of the Eustachian tube in the upper part of the pharynx. 9. Soft palate divided through its middle. 10. Cut surface of the hard palate. a. Olfactory nerve. b. Its three roots of origin. c. Its bulb, from which the filaments are seen to proceed which spread out in the substance of the pituitary membrane. d. Nasal nerve, a branch of the ophthalmic, descending into the left naris from the anterior foramen of the cribriform plate, and dividing into its external and internal branch. e. Naso-palatine nerve, a branch from the spheno-palatine ganglion, distributing twigs to the mucous membrane of the septum nasi in its course to (f) the anterior palatine foramen. g. Branches of the naso-palatine nerve to the palate. h. Anterior and posterior palatine nerves. i. Septum nasi.

increases in breadth and swells at its extremity into an oblong mass of gray and white substance, the bulbus olfactorius, which rests on the cribriform lamella of the ethmoid bone. From the under surface of the bulbus olfactorius are given off the nerves which pass through the cribriform foramina, and supply the mucous membrane of the nares; they are arranged into three groups, an inner group, reddish in color and soft, which spread out upon the septum narium; an outer group, whiter and more firm, which descend through bony canals in the

outer wall of the nares, and are distributed on the superior and middle turbinated bones; and a middle group, which supply the mucous membrane of the roof of the nasal fossæ.

[FIG. 259.

SECOND PAIR. OPTIC.-The optic nerve, a nerve of large size, arises from the corpora geniculata on the posterior and inferior aspect of the thalamus opticus, from the thalamus itself, and from the nates. Proceeding from this origin it winds around the crus cerebri as a flattened band, under the name of tractus opticus, and joins with its fellow in front of the tuber cinereum to form the optic commissure (chiasma). The tractus opticus is united with the crus cerebri and tuber cinereum, and is covered in by the pia mater; the commissure is also connected with the tuber cinereum, from which it receives fibres, and the nerve beyond the commissure diverges from its fellow, becomes rounded in form, and is inclosed in a sheath derived from the arachnoid. In passing through the optic foramen the optic nerve receives a sheath from the dura mater, which splits at this point into two layers; one, which becomes the periosteum of the orbit; the other, the one in question, which forms a sheath for the nerve, and is lost in the sclerotic coat of the eyeball. After a short course within the orbit, the optic nerve pierces the sclerotic and choroid coats and expands into the nervous membrane of the eyeball, the retina. Near the globe, the nerve is pierced by a small artery, arteria centralis retinæ, which runs through the central axis of the nerve and reaches the internal surface of the retina, to which it distributes branches.

A VIEW OF THE SECOND PAIR OR OPTIC, and the origins of seven other pairs. 1, 1. Globe of the eye; the one on the left hand is perfect, but that on the right has the sclerotic and choroid removed to show the retina. 2. The chiasm of the optic nerves. 3. The corpora albicantia. 4. The infundibulum. 5. The pons Varolii. 6. The medulla oblongata. The figure is on the right corpus pyramidale. 7. Third pair, motores oculorum. 8. Fourth

pair, pathetici. 9. Fifth pair, trigemini.

10. Sixth pair, abducentes. 11. Seventh pair, auditory and facial. 12. Eighth

pair, pneumogastric, spinal accessory, and glosso-pharyngeal. 13. Ninth pair, hypoglossal.]

The optic commissure rests on the processus olivaris of the sphenoid bone; is bounded by the lamina cinerea of the corpus callosum in front, by the substantia perforata at each side, and by the tuber cinereum behind. Within the commissure the innermost fibres of the optic nerves cross each other to pass to opposite eyes, the outer fibres continue their course uninterruptedly to the eye of the corresponding side; some fibres pass in an arched direction from one nerve to the other behind, and others taking a similar course in front connect the two retina. The neurilemma of the commissure, as well as that of the nerves, is formed by the pia mater.

THIRD PAIR. MOTORES OCULORUM.-The motor oculi, a nerve of moderate size, arises from the inner side of the crus cerebri, close to the pons Varolii, and passes forwards between the posterior cerebral and superior cerebellar artery. It pierces the dura mater immediately in front of the posterior clinoid process; descends obliquely in the external wall of the cavernous sinus; and divides into two branches which enter the orbit through the sphenoidal fissure

and between the two heads of the external rectus muscle. The superior branch ascends, and supplies the superior rectus and levator palpebræ. The inferior sends a branch beneath the optic nerve to the internal rectus, another to the inferior rectus, and a long branch to the inferior oblique muscle. From the latter a short thick branch is given to the ophthalmic ganglion, forming its inferior root. The branches of the third nerve enter the muscles on their ocular side.

The fibres of origin of this nerve may be traced into the gray substance of the crus cerebri,' into the longitudinal fibres of the pons Varolii or motor tract,' into the corpora quadrigemina and valve of Vieussens, and to a gray nucleus in

FIG. 260.

ISTHMUS ENCEPHALI, showing thalamus opticus, corpora quadrigemina, pons Varolii, and medulla oblongata, viewed from the side. 1. Thalamus opticus. 2. Posterior prominence of this body, tuberculum superius posterius or pulvinar. 3. Corpus geniculatum externum. 4. Corpus geniculatum internum. 5. Tractus opticus. 6. Pineal gland. 7. Nates. 8. Testis of one side. 9. Brachium anterius. a. Brachium posterius. b. Origin of fourth nerve, which may be seen descending over the crus cerebri. c. Processus e cerebello ad testem, or superior peduncle of the cerebellum. d. The band of fibres termed laqueus, the superior division of the fasciculus olivaris, crossing the superior peduncle of the cerebellum to enter the corpora quadrigemina. Through the small triangular space in front of this band, crossed by the fourth nerve, some of the fibres of the superior peduncle of the cerebellum may be seen. e. Superior portion of crus cerebri, termed tegmentum. f. Its inferior portion. g. Third nerve. h. Pons Varolii. i. Crus cerebelli, or middle peduncle of the cerebellum. k. Inferior peduncle derived from the corpus restiforme. The mass lying in the angular interval upon these is the superior peduncle. 1. Fifth nerve issuing from between the transverse fasciculi of the pons Varolii. m. Sixth nerve. n. Facial and auditory nerve. o. Corpus olivare crossed inferiorly by the superficial arciform fibres. p. Corpus pyramidale. q. Posterior pyramids of the medulla oblongata. r. Corpus restiforme. t. Fourth ventricle. the floor of the aqueduct of Sylvius. In the cavernous sinus it receives one or two filaments from the carotid plexus, and one from the ophthalmic nerve.

FOURTH PAIR PATHETICI (trochlearis). The fourth is the smallest cerebral nerve; it arises from the valve of Vieussens close to the testis, and winding around the crus cerebri to the extremity of the petrous portion of the temporal bone, pierces the dura mater below the third nerve, and passes forwards in the outer wall of the cavernous sinus to the sphenoidal fissure. In its course in the outer wall of the sinus it is situated at first below the motor oculi, but afterwards ascends and becomes the highest of the nerves entering the orbit by

1 Mayo.

2 Solly.

3 Stilling.

the sphenoidal fissure. On entering the orbit the nerve crosses the levator palpebræ muscle near its origin, and is distributed on the orbital surface of the superior oblique or trochlearis muscle; hence its synonym trochlearis.

The fibres of origin of the two nerves communicate with each other forming a kind of commissure, while those of the deep origin of the nerve may be traced in two fasciculi to a gray nucleus in the floor of the aqueduct of Sylvius in front; and to a similar nucleus in the floor of the fourth ventricle behind.

FIG. 261.

9

CEREBRAL NERVES WITHIN THE CRANIUM. 1. Olfactory nerve swelling at its anterior extremity into the bulbus olfactorius. 2, 2. Optic nerves, passing through the optic foramina. 3, 3. Motores oculorum, piercing the dura mater near the posterior clinoid processes. 4, 4. Pathetici; the nerve of the left side is seen piercing the dura mater; on the right side the dura mater is removed, and the nerve may be traced through the cavernous sinus. 5, 5. The trigeminus; on the left side the nerve is seen merely passing through the dura mater; on the right, the Gasserian ganglion and its three divisions are seen. 6. Abducentes. 7, 7. Facial and auditory nerve; on the right side the nerves are traced into the petrous portion of the temporal bone. 8, 8. Glosso-pharyngeal, pneumogastric, and spinal accessory nerve. 9, 9. Hypoglossal nerve, consisting of several fasciculi piercing the dura mater at the anterior condyloid foramen. 10. Dura mater attached to crista galli. 11. Cribriform plate of ethmoid bone. 12. Pituitary gland and infundibulum. 13, 13. Internal carotid arteries. 14, 14. Cavernous sinus. 15. Gasserian ganglion. 16. Ophthalmic nerve. 17. Superior maxillary nerve passing through the foramen rotundum. 18. Inferior maxillary nerve passing through the foramen ovale. 19. Intumescentia gangliformis of the facial nerve; the nerve joining it from the front is the nervus petrosus superficialis major. 20. Course of the facial nerve along the aquæductus Fallopii to the stylo-mastoid foramen. Branches. While in the cavernous sinus the fourth nerve gives off a recurrent branch, and sends a branch of communication to the ophthalmic nerve, the recurrent branch, consisting of sympathetic filaments derived from the carotid plexus, passes backwards between the layers of the tentorium, and divides into two or three filaments, which are distributed to the lining membrane of the lateral sinus. This recurrent nerve is sometimes a branch of the ophthalmic,

and occasionally proceeds directly from the carotid plexus. Sometimes the communication with the ophthalmic takes place in the orbit, in which case the lachrymal nerve has the appearance of arising by two roots.

FIFTH PAIR. TRIFACIAL (trigeminus.)-The fifth nerve, the great sensitive nerve of the head and face, and the largest cranial nerve, is analogous to the spinal nerves in its origin by two roots from the anterior and posterior columns of the spinal cord, and in the existence of a ganglion on the posterior root. It arises, or rather makes its appearance at the surface of the brain, on the anterior part of the lateral and constricted portion of the pons Varolii, and consists of a large and small fasciculus, separated by a narrow interspace, the larger fasciculus being the posterior or sensitive root; the smaller one, the anterior or motor root. The sensitive root is composed of from seventy to a hundred filaments, each inclosed in a neurilemma of pia mater, and the entire bundle is bound together into a single nerve and connected with the motor root by a sheath of arachnoid membrane. The nerve then passes through an oval opening in the border of the tentorium, near the extremity of the petrous bone, and spreads out into a large semilunar ganglion, the Gasserian. If the ganglion be turned over, it will be seen that the anterior root lies against its under surface without having any connexion with it, and may be followed onwards to the inferior maxillary nerve. The Gasserian ganglion divides into three branches, ophthalmic, superior maxillary, and inferior maxillary.

Following the nerve to its deep origin, the posterior root may be traced between the transverse layers of the pons Varolii, behind the corpus olivare, and between the corpus restiforme and fasciculus innominatus to the gray substance of the floor of the fourth ventricle; the anterior root being lost among the longitudinal fibres prolonged upwards from the corpus pyramidale, or, according to Stilling, also pursuing its course to the gray substance of the floor of the fourth ventricle. When the posterior root is torn from its attachment a cone of medullary substance is left behind, occasioned by the deeper embrace of the filaments by the pia mater at the periphery than in the centre of the fasciculus.

The OPHTHALMIC NERVE is the nerve of sensation of the eyeball, lachrymal gland, mucous membrane of the eye and nose, integument of the forehead and nose, and muscles of the eyebrow and forehead. It arises from the upper part of the Gasserian ganglion by a short trunk, about three-quarters of an inch in length; passes forwards in the outer wall of the cavernous sinus, lying externally to the other nerves, and divides into three branches. Previously to its division it receives several filaments from the carotid plexus, and gives off a small recurrent nerve, which passes backwards with the recurrent branch of the fourth nerve between the two layers of the tentorium to the lining membrane of the lateral sinus.

The Branches of the ophthalmic nerve are, the

The Frontal nerve enters the orbit immediately to the outer side of the fourth nerve, and passing forwards, for some distance, upon the levator palpebræ muscle, divides into a supraorbital and supra-trochlear branch.

The supraorbital branch, the proper continuation of the nerve, passes out of the orbit through the supraorbital notch, in company with the supraorbital artery, and after giving filaments to the upper eyelid, muscles of the forehead and pericranium, divides into two cutaneous branches, internal and external. The internal branch pierces the occipito-frontalis and is distributed to the integument as far as the summit of the head. The external branch of larger size communicates with the facial nerve, and piercing the occipito-frontalis supplies the integument as far back as the occiput.