MYODOME AND TRIGEMINO-FACIALIS CHAMBER 219 
and then practically disappears, but there is always a line con- 
necting the two ends of the parasphenoid, and hence also con- 
necting the ventral edges of the aortal groove. 
Approximately in the transverse plane of the foramen fa- 
ciale, a cartilaginous process projects mesially from the ventral 
end of each cartilaginous side wall of the myodome and meets 
its fellow of the opposite side in the median line, but it does not 
completely fuse with it, a slight line of separation always re- 
maining evident. These two processes thus together form a 
cartilaginous floor to the myodome, the parasphenoid lying 
against the ventral surface of this floor, but separated from it 
by the dense skeletogenous tissue above referred to. Imme- 
diately posterior to this point, the pharyngobranchial of the first 
branchial arch articulates with the dorsal portion of the side 
wall of the myodome, there lying between the vena jugularis 
and the external and internal carotid arteries; and immediately 
posterior to that, the pharyngobranchial of the second branch- 
ial arch articulates with the ventral surface of the parasphe- 
noid (fig. 7). The external and internal carotid arteries sepa- 
rate from each other slightly anterior to the latter point, both 
lying along the lateral surface of the lateral wall of the myo- 
dome. The external carotid runs forward and upward, ventral 
to the nervus facialis, and, joining the vena jugularis, traverses, 
with that vein, the short canal which represents the pars jugu- 
laris of the trigemino-facialis chamber. The internal carotid 
continues forward and downward along the side wall of the myo- 
dome until it reaches the hind edge of the ascending process 
of the parasphenoid, where it traverses a foramen which is, as 
in the adult, entirely enclosed in that bone. 
Beginning immediately posterior to this foramen (fig. 5) for 
the internal carotid artery and proceeding forward in the sec- 
tions, the cartilage forming the floor of the myodome, and also 
the ventral ends of its. side walls, gradually disappears and is 
replaced by the dense skeletogenous tissue already referred to 
several times, and in it a cavity appears, bounded on all 
sides by the tissue and lying between the parasphenoid and the 
myodomic cavity. The floor and side walls of this cavity 
