QUADRATE AND TYMPANIC RING 



In connexion with the elaboration of the chain of auditory 

 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 Ma 

 the mammals, arid does not 

 occur below them in the 

 series. But it is not pre- 

 sent in all mammals. It 

 is absent, for example, in 

 the Monotremes. When 

 it is present it is 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- T 



FIG. 16. Under surface ot the cranium ot a Dog. 

 ably the quadrato-jugal. 1 x ^. apf, Anterior palatine foramen ; as, pos- 



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, 

 springs from the transverse process. Only in the Monotremes 



1 Gegenbaur, Vergl. Anat. Wirbelth. Leipzig, 1898, p. 404. 



terior opening of alisphenoid canal ; AS, 

 alisphenoid ; 0, basioccipital ; 8, basi- 

 sphenoid ; cf, coudylar foramen ; earn, ex- 

 ternal auditory meatus ; Ex.0, exoccipital ; 

 fim, foramen lacerum medium ; flp, foramen 

 lacerum posterius ; fm, foramen magnum ; 

 f o, foramen ovale ; fr, foramen rotund ura ; 

 Fr, frontal ; gf, ^rlenoid fossa ; yp, post- 

 gleuoid process ; Ma, malar ; MX, maxilla ; 

 oc, occipital condyle ; op, optic foramen ; 

 Per, mastoid portion of periotic ; pgf, post- 

 glenoid fossa ; PI, palatine ; PMx, pre- 

 maxilla ; pp, paroccipital process ; ppf, pos- 

 terior palatine foramen ; PS, presphenoid ; 

 Pt, pterygoid ; sf, sphenoidal fissure or fora- 

 men lacerum anterius ; sm, stylomastoid fora- 

 men ; SO, supraoceipital ; Sq, zygomatic 

 process of squamosal ; Ty, tympanic bulla; 

 Vo, vomer. (From Flower's Osteology.) 



