24 THE DOG'S SKULL. 
the dorsal border of the vomer, and 
forms part of the partition between 
the two nasal cavities ; in old skulls it 
is very largely ossified. 
vi. The vomer is a long narrow blade-like 
bone, posteriorly bounded by the 
anterior portion of the pre-sphenoid ; 
its ventral border lies upon the maxilla 
and palatine. It terminates anteriorly 
amltttle behind the "suture. of the 
premaxille. | 
b. The optic capsules. Those bones forming 
the boundaries of the orbit have already 
been described in their respective segments. 
c. The auditory capsules. In the embryo, as 
has been pointed out, the auditory organs are 
enclosed within a cartilaginous capsule which 
later becomes ossified, and is replaced by the 
otic bones. In the human skull ossification 
proceeds from three centres termed respec- 
tively pro-otic, epi-otic, and opisthotic.* 
i. The periotics are the series of fused 
otic bones, already referred to, situated 
between the occipital bones and the 
1 “Tn birds, the three periotic anchylose with one another, as well 
as with the adjacent supra-occipital and exoccipital, so completly, that 
even the Y-shaped suture becomes obliterated.” (Huxley). 
In the crocodile the epiotic is early united with the supra-occipital, 
and the opisthotic with the ex-occipital; the pro-otic alone remains 
» =. “incthe*turtle, the opisthotie- remains 
permanently distinct.” (Miall). 
separable throughout life. 
