The Cerebral Nerves

The greatest number of pairs of nerves ever given off from the vertebrate brain is twelve, including the so-called olfactory nerves, and the optic nerves, which, as has been seen, are more properly diverticula of the brain, than nerves in the proper sense of the word.

The olfactory "nerves" (olfactorii) constitute the first pair of cerebral nerves. They always retain their primary connection with the cerebral hemispheres, and frequently contain, throughout life, a cavity, the olfactory vertricle, which communicates with the lateral ventricle.

The optic "nerves" (optici) are the second pair of cerebral nerves. In the Lampreys and Hags (Marsipobranchii) these nerves retain their embryonic origin from the thulamencephalon, and each goes to the eye of its own side. In other Vertebrate, the nerves cross one another at the base of the brain (Teleostei), or are fused together into a chiasma (Ganoidei, Elasmobranchii, and all the highet Vertebrata). In the higher Vertebrata, again, the fibres of the optic nerves become connected chiefly with the mesencephalon.

All the other cerebral nerves differ from these in arising, not as diverticula of any of the cerebral vesicles, but by histological differentiation of the primitive brain - case, or lamince dorsales of the skull.

The third (motores oculorurn) and fourth. (pathetici) pairs of nerves are distributed to the muscles of the eye; the third to the majority of these muscles, the fourth to the superior oblique muscles. The third pair of nerves issues from the crura cerebri, or inferior division of the metencephalon, upon the base of the brain; the fourth pair, from the fore-part of the upper division of the metencephalon, immediately behind the optic lobes, upon the superior surface of the brain. This region is known as the Valve of Vieussens in the Mammalia.

All the other cerebral nerves originate in the posterior division of the hind-brain - the myelencephalon. The great fifth pair (trigemini) passes out from the sides of the metencephalon, and supplies sensory nerves to the integument of the head, and motor nerves to most of the muscles of the jaws, by its three divisions - the ophthalmic, the superior maxillary, and the inferior maxillary, nerves.

Of these divisions the two latter are, very generally, closely connected together, while the ophthalmic division remains distinct. The ophthalmic division passes to the cleft between the trabecula and the maxillary process (which nearly corresponds with the orbit, and might be termed the orbito-nasal cleft), and is distributed to the inner and the outer side of that cleft. Hence its main branches are nasal and lachrymal. The two maxillary nerves, on the other hand, are distributed to the inner and outer sides, or anterior and posterior boundaries, of the buccal cleft. Hence the superior maxillary belongs to the posterior, or outer, side of the maxillary process, while the inferior maxillary appertains to the anterior region of the first visceral arch. The superior maxillary commonly unites with the outer, or lachrymal, division of the ophthalmic ; the inferior maxillary with the anterior division of the facial.

In the higher Vertebrata, the trigeminal nerve usually has two very distinct roots, a dorsal sensory, provided with a gan glion (the Casserian ganglion), and a ventral motor, non-ganglionated. The fibres of the latter pass almost exclusively into the inferior maxillary division. In addition, the ophthalmic division may have a ganglion (ciliary); the superior maxillary another (sphenopalatine or Meckelian), and the inferior maxillary a third (otic).

The sixth pair (abducentes) issues from the inferior surface of the brain, at the junction of the myelencephalon with the metencephalon. It supplies the external straight muscles of the eye; with the muscles of the nictitating membrane, and the retractor bulbi, or musculus choanoides, when such muscles exist.

The seventh pair (faciales) supplies the superficial facial muscles, and ultimately divides into two branches, one of which is in relation with the mandibular, and the other with the hyoidean arch.

The five nerves which have just been mentioned are often intimately connected together. Thus, in the Lepidosiren, the three motor nerves of the eyeball are completely fused with the ophthalmic division of the fifth. (I am greatly disposed to think that the motor nerves of the eye more nearly retain their primary relations in Lepidosiren than in any other vertebrated animal; and that they are really the motor portions of the nerves of the orbito-nasal cleft, the third and fourth appertaining to the inner division of the ophthalmic, the sixtth to its outer division.) In the Myxinoid fishes there are no motor nerves of the eyeball; but, in the Lamprey, the rectus externus and inferior, and the obliquus inferior, are supplied by the ophthalmic, while the oculomotor and the pathetic unite into a common trunk, which gives branches to the rectus superior and internus, and obliquus superior. The oculomotor, the pathetic, and the abducens, are more or less confounded with the ophthalmic in the Amphibia; but in Teleostei, Ganoidei, Elasmobranchii, and in all the higher Vertebrata, the nerves of the muscles of the eye are distinct from the fifth pair, except where the oculomotor unites with the ophthalmic into the ciliary ganglion.

The facial and the trigeminal nerves have common roots in fishes. In Amphibia, though the roots are distinct, the facial may be completely united with the ganglion of the trigeminal, as in the Frog. In all abranchiate Vertebrata the two nerves are quite distinct.

Whether the nerves are distinct or not, a palatine, or vidian, nerve (which, in the higher Vertebrata, is especially connected with the facial), runs through, or beneath, the base of the skull, parallel with its long axis; and, after uniting with the superior maxillary, and usually contributing to form the sphenopalatine, or Mechkelian, ganglion, is distributed to the mucous membrane of the roof of the mouth; and the mandibular division of the seventh, or chorda tympani, unites with the inferior maxillary division of the fifth nerve.

The eighth, pair (auditorii) is formed by the nerves of the organ of hearing.

The ninth pair (glossopharyugei) is especially distributed to the pharyngeal and lingual regions of the alimentary canal, and, primarily, supplies the boundaries of the second visceral cleft.

The tenth pair (pneumogastrici or vagi) consists of very remarkable nerves, which pass to the gullet and stomach, the respiratory and vocal organs, to some parts of the integument of the body, and to the heart. In the Ichthyopsida they give off, in addition, long lateral nerves to the integuments of the sides of the body. In the higher Vertebrata, these lateral nerves are represented only by small branches distributed chiefly to the occipital region. The ninth and tenth pairs are both motor and sensory in function, and are often so intimately connected as to form almost one nerve.

The eleventh pair (accessorii) are cerebral only by courtesy, as these nerves take their origin from the spinal cord, by roots which issue between the proper anterior and posterior roots of the spinal nerves, and, joining together, form, on each side, a nerve which passes out with the pneumogastric, partly joining it, and partly going to muscles which arise from the head and anterior vertebrae, and are inserted into the pectoral arch.

The spinal accessory exists in no Ichthyopsid vertebrate, but is found in all Sauropsida, with the exception of the Ophidia, and in the Mammalia.

The twelfth and last pair (hypoglossi) are the motor nerves of the tongue, and of some retractor muscles of the hyoidean apparatus.

In the Ichthyopsida the first cervical nerve supplies the distributional area of the hypoglossal; but in all the abranchiate Vertebrata there is a hypoglossal, which traverses a foramen in the ex-occipital, though it oftens remains closely connected with the first cervical, and may rather be regarded as a subdivision of that nerve, than as a proper cerebral nerve.

Thus the nerves arising from the hind-brain, in all the higher Vertebrata, fall into three groups: 1st, a sensori-motor, pre-auditory, set (3d, 4th, 5th, 6th, 7th); 2d, the purely sensory auditory nerve (8th); 3d, the sensori-motor, post-auditory, set (9th, 10th, 13th).

The apertures by which several of these nerves leave the skull, retain a very constant relation to certain elements of the cranium on each side. Thus:

1. The filaments of the olfactory nerve always leave the cranium between the lamina perpendicularis, or body of the ethmoid, and its lateral or prefrontal portion.
   
   
2. The optic nerve constantly passes out behind the centre of the orbitosphenoid and in front of that of the alisphenoid.
   
   
3. The third division of the trigeminal, or fifth nerve, always leaves the skull behind the centre of the alisphenoid and in front of the prootic.
   
   
4. The glossopharyngeal and pneumogastric always make their exit behind the centre of the opisthotic, and in front of the centre of the ex-occipital.

The apertures for the exit of the cranial nerves denoted in the paragraphs a, b, c, d, when surrounded by bone, and well defined, are called respectively: a, the olfactory foramen; b, the optic foramen; c, the foramen ovale; d, the foramen lacerum posterius. The adjacent bones may take equal shares in bounding these foramina, or the foramina may be altogether in one bone; but their positions, as here defined, never change.

Another point to be especially considered respecting the general disposition of the cranial nerves, is the relation which some of them bear to the visceral arches and clefts, and which has already been incidentally mentioned. Thus, the seventh nerve is distributed to the posterior part of the first visceral arch, and to the anterior part of the second visceral arch, its two branches enclosing the first visceral cleft. In like manner, the ninth (glossopharyngeal) nerve is distributed to the hinder part of the second arch and to the front part of the third, its branches enclosing the second visceral cleft. The first branch of the pneumogastric has similar relations to the third and fourth arches and to the third cleft; and, in branchiate Vertebrata, the other anterior branches of the pneumogastric are similarly distributed to the successive branchial arches, the two divisions of each branch enclosing a branchial cleft.

The second and the third divisions of the trigeminal are distributed, in an analogous manner, to the anterior region of the first visceral arch, and to the posterior or outer region of the maxillo - palatine process - the gape of the mouth representing a visceral cleft between the two. The inner and outer portions of the first division of the trigeminal are similarly related to the inner, or anterior, region of the maxillo - palatine process, and the outer side of the trabecula cranii - the orbito nasal fissure representing the cleft between the two.

Considerations of this kind suggest that the trabeculae and the maxillo - palatine processes may represent pre - oral visceral arches, which are bent forward; and, in the case of the trabeculce, coalesce with one another. Such an hypothesis would enable us to understand the signification of the naso - palatine canal of the Myxinoid fishes, which would be smiply the interspace, or passage, between the trabeculae (which must have originally existed it ever they were distinct visceral arches) not yet filled up; and the anomalous process of the roof of the oral cavity, which extends toward the pituitary body in the embryos of the Vertebrata in general, might be regarded as the remains of this passage.

On this hypothesis, six pair of inferior arches belong to the skull - namely, the trabecular and maxillo - palatine, in front of the mouth; the mandibular, the hyoidean, and two others (first and second branchial), behind it. For, as there are three cranial nerves embracing the first three visceral clefts which lie behind the mouth, there must be four post - oral, cranial, visceral arches.

Supposing that the occipital segment in the brain - case answers to the hindermost, or second branchial, cranial, visceral arch, the invariable attachment of the proximal ends of the mandibular and hyoidean arches to the auditory capsule leads me to assign the parietal and the frontal segments to the maxillo - palatine and trabecular visceral arches. And thus the ossifications of the auditory capsule, alone, are left as possible representatives of the neural arches of the three anterior postoral visceral arches.

But these speculations upon the primitive composition of the skull, however interesting, must not, as yet, be placed upon the same footing as the doctrine of its segmentation, which is simply a generalization of anatomical facts.