MYODOME AND TRIGEMINO-FACIALIS CHAMBER 221 
compartment of the myodome that is occupied by the inter- 
nal carotids (fig. 3). In its passage along the mesial surface of 
the lateral wall of the myodome it lies between the wall and a 
delicate layer of connective tissue which everywhere lines the 
myodomie cavity, thus apparently not definitely entering the 
central cavity of the myodome. 
Slightly anterior to the point where the prootic bridge is per- 
forated by the nervi abducens and palatinus facialis, and slightly 
anterior also to the transverse plane of the internal carotid 
foramina, the median cartilaginous portion of the prootic bridge 
ceases, and the roof of the myodome is then perforated by what 
is, in the prepared cranium of the adult fish, the pituitary open- 
ing of the brain case (fig. 1 to 3). This opening is closed, in 
fresh specimens by a portion of the dura mater that projects 
ventrally into the myodome and so forms a pit-like depression 
in the floor of the cavum cerebrale cranii, in which the pituitary 
body lies; thus forming the actual pituitary fossa. It will, how- 
ever, be best to call it the pituitary sac, for the term pituitary 
fossa, and its equivalent sella turcica, has been given to the de- 
pression that, in the floor of the cartilaginous or osseous cranial 
cavity, lodges this pituitary sac, and the two are not always 
coincident. The sac forms the roof of this part of the myodome 
of Hyodgn, and a median vertical membrane descends from its 
ventral and anteroventral surfaces. Anteriorly this membrane 
is directly continuous with the membranous interorbital septum; 
posteroventrally it is continuous with the anterior edge of the 
median portion of the horizontal myodomic membrane, the 
lateral portions of the latter membrane here being so broken up 
and interrupted by the muscles and vessels entering or leav- 
ing the myodome that they cannot be followed in the sections. 
The vertical membrane does not at this point extend ventrally 
to the floor of the myodome, but in the transverse plane of the 
hind edge of the basisphenoid (fig. 1) it becomes the interorbital 
septum, and there its flaring ventral edges are each attached to 
a ridge on the related lateral edge of the dorsal surface of the 
parasphenoid. In the triangular space enclosed between the 
latter bone and the V-shaped ventral end of the septum lies, 
JOURNAL OF MORPHOLOGY, VOL. 32, No. 2 
