26 THE DOG'S SKULL. 
aqueductus Fallopii; through it the 
facial nerve passes. 
The mastoid portion forms the posterior 
and external portion of the periotic. 
Its most posterior portion is directed 
downwards, and forms the mastoid 
process; between the mastoid and 
the auditory bulla is the stylomastoid 
foramen, transmitting the main trunk 
of the facial nerve. 
ii. The ¢ywpanzcs lie on the outer side of 
the periotics, they are somewhat flask- 
shaped. The portion, which forms in 
nearly all the Carnivora such a con- 
spicuous prominence on the ventral 
surface of the skull, is the bulla:? 
laterally it is slightly produced ; its 
edges being uneven, and supporting 
the cartilage of the external ear. The 
lateral” opening; of the bulla,- the 
meatus auditorius externus, has at 
its base an incomplete bony ring, the 
tympanic ring, which rises up as a 
part of the septum ; in life it is covered 
by the tympanic membrane. The 
septum lies behind the tympanic ring, 
aud divides the bulla into an inner 
1 In Zhylicinus (a Marsupial) whose skull resembles in very many 
particulars that of the Dog, the tympanics are rudimentary and do not 
anchylose with the cranial elements, and there is no bulla. 
