CEREBRAL NERVES OF VERTEBRATA. 517 



branch. The ramus inaxillaris superior always runs on the floor of 

 the orbit, and gives off sensory branches to the maxillary region. Its 

 infraorbital branch is the largest, especially in the Mammalia. The 

 R. max. sup., with the ramus inaxillaris inferior, represents a ventral 

 trunk ; in the Selachii this is very clearly the nerve of the mandi- 

 bular arch, and appears as the largest portion of the trigeminus. It 

 is distributed to the muscles of the jaw, to the integument, and to a 

 large part of the mucous membrane of the mouth (ramus lingualis). 

 The intestinal trunk is represented by a palatine trunk of the second 

 branch, which, in Fishes, passes directly to the palate, but in the 

 higher Vertebrata only reaches the palate by means of its connection 

 with a sympathetic ganglion (sphenopalatine ganglion). 



The nerves of the optic muscles — the oculo-motorius and tro- 

 chlearis — in origin and distribution, belong to the trigeminus, and 

 seem to be parts which have been separated off from it. Although 

 the statements that the nerves for the optic muscles are given off from 

 the ophthalmic branch of the trigeminus in Lepidosteus and Lepi- 

 dosiren, and that in the Salamandrina the trochlear is replaced by a 

 branch of the same nerve, require confirmation, and whilst what really 

 has occurred in these cases is, perhaps, that the nerves of the optic 

 muscles have become united with the trigeminus, and not that they 

 are completely wanting, yet the supposition that processes of segre- 

 gation have been the cause of these nerves arising separately from 

 the myelencephalon in the other Vertebrata, cannot be rejected. 

 The fact that the ramus inferior of the trigeminus has no motor 

 elements, and that the muscles in its area are supplied by apparently 

 independent nerves, will always be of great importance. 



The second nerve of the trigeminal group is the facialis and 

 acusticus. The latter seems to be homologous with a purely sensory 

 dorsal branch of a spinal nerve ; its terminal area has been carried 

 below that plane in which we must suppose it to have primitively 

 ramified — that is, below the surface of the body ; and this change 

 has accompanied the removal of the vesicles of the labyrinth from 

 the integument, and their passage into the interior of the wall of the 

 skull (cf. infra, auditory organ). This points to a dorsal branch 

 having primitively passed upwards through the wall of the skull, and 

 agrees with the course of the dorsal branches of the other cephalic 

 nerves, and with the ophthalmic branch of the trigeminal. 



The facialis (Fig. 290, Fa) has the arrangement of a ramus 

 ventralis belonging to the hyoid arch. It supplies the integumen- 

 tary as well as the muscular portions of this segment, and is, there- 

 fore, primitively a mixed nerve. In the Teleostei it enters into con- 

 nection with the trigeminal; in many Sharks, also, it is fused 

 with it. So, also, in the anourous Amphibia it is united to the tri- 

 geminal. This union, however, is effected during their ontogenetic 

 development. In the Urodela, as in the higher Vertebrata, it is 

 always distinct, and in the Mammalia its sensory elements are 

 apparently absent. In these latter its area of distribution is very 

 great, owing to the increase in size of the facial musculature ; while 



