QUADRATE AND TYMPANIC RING 
In connexion with the 
ossicles it is very usual for 
mammals to possess a thin 
inflated bone, sometimes 
partly or entirely formed 
out of the tympanic bone, 
and known as the tympanic 
bulla. Whether this struc- 
ture is thin and inflated 
or thick and depressed in 
form it is characteristic of 
the mammals, and does not 
occur below them in the 
But it is not pre- 
sent in all mammals. It 
is absent, for example, in 
the Monotremes. When 
it is present it 1s some- 
times formed from other 
bones, as, for instance, from 
the alisphenoids. The tym- 
panic ring has been held to 
be the equivalent of the 
quadrate. It is more prob- 
ably the quadrato-jugal.' 
Ribs. — All mammals 
are furnished with ribs, of 
which the number of pairs 
differs considerably from 
group to group, or it may 
be even from species to 
species. The ribs are 
attached as a rule by two 
heads, of which one, the 
capitulum, arises as a rule 
between two centra of 
successive vertebrae. The 
other, the  tuberculum, 
series. 
springs from the transverse process. 
elaboration of the chain of auditory 
Mar 
apf 
pie Py . (0 Exe 
Dy NS 
Fig. 16.—Under surface of the cranium of a Dog. 
apf, Anterior palatine foramen ; as, pos- 
terior opening of alisphenoid canal; AS, 
1 
Xo 
alisphenoid ; BO, basioccipital ; BS, basi- 
sphenoid ; cf, condylar foramen ; eam, ex- 
ternal auditory meatus ; Hv.O, exoccipital ; 
fim, foramen lacerum medium ; jp, foramen 
lacerum posterius; /m, foramen magnum ; 
fo, foramen ovale; 7, foramen rotundum ; 
Fr, frontal; gf, glenoid fossa; gp, post- 
glenoid process; Mu, malar; Mx, maxilla ; 
oc, occipital condyle; op, optic foramen ; 
Per, mastoid portion of periotic ; pg7,, post- 
glenoid fossa; Pl, palatine ; PMa, pre- 
maxilla ; pp, paroccipital process ; pps, pos- 
terior palatine foramen; PS, presphenoid ; 
Pt, pterygoid ; sf, sphenoidal fissure or fora- 
men lacerum anterius ; sm, stylomastoid fora- 
men; SO, supraoccipital; Sy, zygomatic 
process of squamosal ; 7, tympanic bulla ; 
Vo, vomer. (From Flower’s Osteolog.) 
Only in the Monotremes 
1 Gegenbaur, Vergl. Anat. Wirbelth. Leipzig, 1898, p. 404. 
