234 
Having entered the acustico-trigemino-facialis recess, it in part im- 
mediately entered the externus muscle and in part issued with that 
muscle, from the recess, to innervate the remainder of the muscle, 
which portion of the muscle had its origin from the eye stalk. If the 
thin partition of cartilage that here separates the recess from the 
pituitary canal were to be resorbed, or broken down by the externus 
muscle, the recess would evidently become an upper lateral chamber 
of the pituitary canal similar to, but not the full homologue of, the 
upper lateral chamber of the myodome of Amia. 
The ramus ophthalmicus superficialis trigemini and the nervus 
profundus both issue from the acustico-trigemino-facialis recess with 
the truncus maxillo-mandibularis trigemini through the foramen 
trigeminum, this condition of the ramus ophthalmicus superficialis 
being considered by GEGENBAUR as a Secondary one (I. c. p. 68). 
According to that author, the ramus ophthalmicus trigemini, or first 
branch of the trigeminus, of his descriptions must primarily have 
run forward in a long canal in the cranial wall, not at any point enter- 
ing or being exposed in the orbit. This he considered established 
by the fact that the nerve has a separate and independent foramen 
of exit in many selachians; that in Hexanchus it first issues in the 
orbit with the truncus trigeminus and then passes under a bridge of 
cartilage that lies immediately anterior to the foramen trigeminum; 
that in certain selachians the nerve, in its course through the orbit, 
is enclosed in a firm connective tissue sheath; and that at the anterior 
end of the orbit it traverses a canal in the cartilage to reach the dorsal 
surface of the chondrocranium. The conditions found in the sup- 
posedly primitive Chlamydoselachus do not favor this conclusion. 
The acustico-trigemino-facialis recess of Chlamydoselachus is 
thus a more or less completely closed chamber in the lateral wall of 
the cranial cavity, and the fact that its membranous mesial wall is 
more or less complete in different specimens is not surprising if it 
be considered to be a structure in process of differentiation. In 
Lepidosteus the mesial membranous wall of the trigemino-facialis 
chamber, which would seem to correspond to the mesial wall of the 
recess in Chlamydoselachus, was not developed, according to VEIT 
(1911), in a 14 mm. larva of that fish, but was found completely 
developed in a 20 mm. one. The nervus acusticus, in Chlamydo- 
selachus, traverses the recess, but in certain other selachians a more 
or less complete separation of the acusticus from the trigemino- 
