The third division of the fifth nerve, after its escape from the cranium, is situate in the superior, posterior, and internal part of the zygomatic fossa ; it is placed immediately be hind the external pterygoid muscle, before and somewhat internal to the styloid process of the sphenoid bone, internal to and on a line with the anterior margin of the temporo-maxillary articulation, and external to the Eustachian tube. So soon as the inferior maxillary nerve has entered the fossa, it gives off, immediately beneath the superior wall of that fossa, a set of branches remarkable for their source and destination ; they proceed from the front of the nerve; their regular number is five, but they present variety in this respect, being in some instances not so many at their origin, in others amounting to six ; they vary also in the mode in which they arise ; for the most part they are given off separately and branch off, as rays, from the nerve, but at times the nerve divides into two branches, a smaller anterior one, and a larger posterior ; in such case the anterior divides immediately into the branches, which otherwise arise from the nerve itself. These branches are the masseteric, the deep temporals, the buccal, and the pterygoid nerves, and they are ranged in succession from behind forward, and from without inward ; the first is external and posterior ; to it succeed the tempo mls, then the buccal, and lastly the pterygoid.
1. The massetcric branch proceeds from the anterior and outer part of the nerve; it passes out ward, nearly transversely, beneath the superior wall of the temporal fossa, and in front of the articular surface of the temporal bone; it crosses obliquely over the external pterygoid muscle, at its outer extremity, between the muscle and the wall of the fossa, and then inclines down ward through the sigmoid notch of the lower jaw, in front of its neck, and of the insertion of the external pterygoid muscle; and posterior to the coronoid process and the tendon of the temporal muscle. Having traversed the notch it descends forward, external to the ramus of the jaw, and passing between the two portions of the massetcr, divides into numerous ramifica tions, which are distributed altogether to that muscle : while between the portions of the masseter, it inclines from its posterior toward its anterior margin, and its terminating filament can be traced to the latter at the inferior part of the muscle. This branch gives off, during its course, some minor branches ; while in front of the articulation of the jaw it gives one or more filaments to the articulation ; in the next place it gives a small branch to the posterior part of the temporal muscle, and lastly it fre quently gives off the external or posterior deep temporal nerve.
2. The deep temporal branches are two; they are distinguished into posterior and anterior or external and internal. The anterior is the larger. They present varieties in their num ber and mode of origin ; at times there is but nne, at others there are three ; in sonic instances they arise by a common origin ; in others, and for the most part, separately, and in others again the posterior or lesser branch is given off either by the masseteric or the buccal nerve. They both pass outward, in front of the tem poro-maxillary articulation, between the exter nal pterygoid muscle and the superior wall of the zygomatic fossa; they then change their direction and ascend in the temporal fossa, be tween the muscle and the surface of the fossa, and divide into branches, which attach them selves to the temporal muscle, on its deep sur face, and are distributed, those of the posterior to its posterior, and those of the anterior to its middle and anterior parts. The two branches frequently anastomose with each other as they leave the zygoniatic fossa. The anterior also frequently communicates with or receives a branch from the buccal nerve, and by one of its anterior filaments it anastomoses with the nerve resulting from the junction of the tempo ral branches of the lachrymal nerve and the temporo-malar branch of the second division of the fifth. This communication between the
three divisions of the fifth is however, accord ing to the elder 1\ leckel, subject to variety; lie states that he has seen the communicating branch of the anterior deep temporal at times enter the orbit either through the malar bone, or through the spheno-maxillary fissure, and there unite with the conjoined branch of the other two.
3. The buccal nerve is the largest and the principal of these branches ; it arises from the front of the inferior maxillary nerve, next in or der after the anterior deep temporal, for the most part a distinct and single branch ; but it is not unusual to find the buccal nerve give off one or both of the deep tempomls, or in rare cases all the three former branches: in some instances also it arises double, the two filaments, of which it is then composed, being separated by a portion of the external pterygoid muscle. It runs downward and forward, passing at first either and for the most part through the exter nal or between the two pterygoid muscles, be neath the external and external to the internal ; having traversed the pterygoid it descends in front of its inferior part, internal to the coronoid process of the lower jaw, and the inferior part of the temporal muscle, next between the ten don of the temporal and the buccinator, then between the anterior margin of the masseter and the latter muscle, and finally emerging from between them it inclines toward the angle of the mouth, superficial to the buccinator and beneath the dense expansion by which that muscle is coveted. During its descent it is immersed in the fat which occupies the lower part of the zygomatic fossa. The ramifications which it gives off are numerous; first while traversing and immediately after escaping from the pterygoid it gives branches to the muscle ; at the same time it gives off a fasciculus of branches which pass outward, in front of the external pterygoid to the internal surface of the temporal muscle, at its inferior part ; some of these descend with the muscle toward its insertion, and are distributed to it at that point, others ascend in the temporal fossa, between the muscle and the bone, penetrate the muscle, and are distributed, along with the branches of the anterior deep temporal, with which they anastomose freely, to the muscle at its inferior anterior part. In the next place, while between the masseter and the buecinator, the nerve gives off backward several branches, three or four, which are distributed to the bue einator at its origin, to the buccal glands, and to the membrane of the mouth ; as it lies upon the last-named muscle, between the rannus of the jaw and the angle of the mouth, it gives filaments to it at its middle and anterior part, which, like the former, both supply the muscle, pass through its fibres, and are distributed also to the buccal glands and membrane. Finally, as the nerve approaches the angle of the mouth, it divides into two, occasionally three, branches; these two branches pursue the direction of the nerve toward the angle, passing beneath the facial vein and inclining, one upward, the other downward; after a short course they are united both to branches of the portio dura, the inferior to a branch of the inferior or cervico-facial divi sion, the superior to one of the superior or tern poro-faeial division of that nerve. By their union they form conjoined branches or loops, from each of which are given off several fila ments to the muscles of the mouth at their in sertion into the angle; from the superior, more particularly, to the buecinator, the zygomatic, and levator anguli; and from the inferior to the buecinator and depressor anguli oris.