HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 87 
anterior part of the head; (b) the communicating branch run- 
ning direct to the ciliary ganglion; (c) the mandibular branch, 
which runs down between the second and third head cavities, 
and then along the mandibular arch. By stage L two or more 
slender non-ganglionic roots appear in front of the original gan- 
glionic root, and the maxillary nerve is given off as a branch 
from the mandibular. 
The Development of the Seventh (Facial) Nerve——The 
facial and auditory nerves at stage K arise by a single root 
from the hind brain, a short distance behind the fifth nerve. 
This root has the same ganglionic character as the original root 
of the fifth, and has a similar bulging outwards of the hind 
brain opposite its point of origin. The nerve divides almost 
immediately into an anterior part—the facial, and a posterior— 
the auditory nerve. The latter runs almost directly backwards, 
and becomes applied to the anterior wall of the auditory vesicle 
(figs. 7, 8, and 9 vitt awd.). The facial nerve gives off three 
main branches: (a), from its upper and anterior part a very 
large stout nerve arises, which runs forward along the dorsal 
surface of the head to its anterior end, lying immediately above 
the ophthalmic branch of the fifth, and immediately beneath the 
superficial epiblast; this I shall speak of as the ophthalmic 
branch of the seventh; (b) the second branch arises from the 
anterior part of the facial nerve just below the root of the oph- 
thalmic branch; this, which is also a stout nerve, runs down- 
wards and forwards, lying parallel to and immediately superficial 
to the maxillary branch of the fifth; it may be spoken of as the 
palatine nerve; (c) the main stem of the seventh is continued 
downwards as a stout nerve which runs along the posterior or 
hyoidean border of the spiracular or hyomandibular cleft. 
From this hyoidean branch an anterior or mandibular branch is 
given off at a slightly later stage, which runs over the top of the 
spiracular cleft, and then down in its anterior or mandibular 
wall. 
These branches of the seventh nerve are well shown at stage 
M in figs. 15 to 18. The large ophthalmic branch (vita.) is 
seen at its origin in fig. 17, while figs. 16, 18, 19, show other 
portions of its course ; its relation to the ophthalmic branch of 
the fifth (v a.) is seen in fig. 19. The root of this ophthalmic 
branch of the seventh is also well shown at a slightly earlier 
stage (between L and m) in fig. 25 (vita.), which shows how 
very closely it lies to the surface. It is seen in transverse sec- 
tion in figs. 26, 27, and 28 (vi1a.), and finally, is shown along 
the whole of its course at stage o in figs. 32 to 35, these figures 
