RICHARDS. 
heart-shaped opening into the secondary tympanic cavity, 
which opening is seen as a black hole somewhat above and 
to the left of the central point of the picture. The large 
intra-petrous cell I am now describing does communicate 
therefore with the lower part of the tympanum through the 
medium of the secondary tympanic cavity, but it does not 
communicate with the chamber containing the ossicles ; vet 
it is probably the analogue of the mastoid antrum in man. 
a c 
b 
Fig. 2. 
The three cells whose anterior extremities are seen in the 
upper left hand corner of Fig. 2 belong to the system com- 
prised within the mastoid prominence of the exoccipital 
bone. The curved line of suture separating this bone 
from the squamosal (or it may be the tympanic at this pre- 
