The course of the second division of the fifth within the cranium is short ; it is directed for ward, slightly outward and downward, toward the superior maxillary or the foramen rotund um of the sphenoid bone ; having reached that foramen it enters the canal, of which it is the aperture, and escapes through it from the cranium. While within the latter the nerve is contained in a sheath of dura mater, and rests in a shallow channel on the body of the sphenoid bone, at its junction with the great ala. From the cranium it enters the spheno-maxillary fossa, and crosses that fossa at its superior extremity, from behind forward, inclining still downward and outward, though but slightly ; its course across the fossa is also very short, extended between the root of the pterygoid process behind and the highest part of the posterior wall of the maxillary antrum before; having traversed the superior part of the fossa it enters the infra-orbital canal, through which it is transmitted, in company with the in fra-orbital artery, to the face. In the canal it is situate in the floor of the orbit or the roof of the antrum, separated from each cavity, more or less perfectly, by a thin lamina of bone ; its course within the canal is by much its longest stage; as the nerve approaches the anterior extremity of the canal, it inclines inward, and thus its course is rendered a curve, convex outward. In this respect, however, it pre sents varieties, dependant upon the transverse dimensions of the face, which being great, the course of the nerve is more curved and vice versa, it being sometimes nearly straight. From the time that the nerve enters the canal, it has been called infra-orbital; but, inasmuch as that part of it is manifestly but the con tinuation of the trunk, and names are already rather too numerous than otherwise, it would be better if that one were discarded. From the infra-orbital canal the nerve escapes through its anterior aperture into the face ; that aperture corresponds, for the most part, to the point of junction of the two external with the internal third of the inferior margin of the orbit, and is from a quarter to half an inch below it ; its situation, however, is not uniform; in some skeletons it will be found to correspond nearly to the middle of the margin, and this circum stance is worthy of attention, in consequence of its relation to the operation for the division of the nerve.
At its escape from the canal the nerve is concealed by the lower margin of the orbicu laris palpebrarum and by the levator labii supe rioris muscle, beneath which it is placed, and it is above the upper extremity of the origin of the levator anguli oris : immediately after its escape it separates into a number of branches, which go off in different directions to their several destinations, but principally downward.
The branches which the second division gives off are the temporo-malor, the .spheno palatine, the posterior superior dental, the an terior superior dentol, and the facial branches. While within the cranium the nerve gives off no branch.
1. The first branch given off by the second division, the temporo-malar, has been called cutaneous molar by the elder Meckel ; it has been also called orbitar, but without good reason ; the name temporo-malur fully expresses its distribution. This branch is given off by
the nerve, either while yet within the canal, through which it escapes from the cranium, or after it has entered the spheno-maxillary fossa ; it is one of its smallest branches ; it passes for ward through the fossa, toward the spheno maxillary cleft, enters the orbit through the cleft, and then pursues its course forward and outward, along the floor of that region, beneath the inferior rectus muscle, and about the mid dle of it divides into two branches ; an exter nal, the temporal, and an anterior, the malar.
Before entering the orbit it sometimes gives off a small branch, which enters that cavity through the periosteum of the posterior part of the orbitar process of the sphenoid bone, and joins the lachrymal branch of the first division, presenting one of the instances of a second root to that branch, as described by the elder Meckel.
The external temporal branch passes toward the outer wall of the orbit, ascends between it and the external rectus muscle ; then becomes attached to the wall, and continues its course either through the periosteum, or in a groove, or at times through a canal in the orbitar pro cess of the malar, or occasionally of the sphe noid bone; here it is joined by the posterior temporal branch of the lachrymal nerve, the third branch of the first division : the conjoined branch is then transmitted into the temporal fossa, through an aperture on the temporal sur face of the orbitar process of the malar bone ; there it is joined by a small branch of the an terior deep temporal branch of the inferior maxillary or third division of the fifth, and plunging among the fibres of the temporal muscle, it is distributed to them in common with the filaments of the deep temporal; a filament or filaments of it gain the superficial surface of the muscle, perforate its aponeurosis, become subcutaneous, and are distributed su perficially upon the temple, communicating with filaments of the portio dura, and of the superficial temporal branch of the third divi sion. The temporal branch of the temporo malar is sometimes double, or divides into two, one communicating with the branch of the lachrymal, the other transmitted to the temple.
The molar branch pursues the course of the original nerve, until it has reached nearly to the anterior margin of the orbit, at its inferior external angle ; then it enters, either single or divided into two, the corresponding canal or canals, by which the malar bone is perforated, and through them is transmitted outward and forward to the malar region of the face. Its ramifications are distributed to the inferior ex ternal part of the orbicularis palpebrarum, and to the integuments of the malar region ; they communicate with those of the portio dura, of the superficial temporal and lachrymal nerves, and of the palpebral branches of the second division. Before reaching the malar canals, the malar branch frequently gives off one or more filaments, which ascend to the lachrymal gland, unite with those of the lachrymal nerve, and follow a similar distribution.