QUADRATK AND TYMPANIC RING 



29 



In fonnexion with the elahorafcion of the chain of auditory 

 ossiole.s 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 

 series. But it is not pre- 

 sent in all mammals. It 

 is absent, ftir 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 

 cpiadrate. It is more prob- 

 ably the (juadrato-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, 

 springs from the transverse process. Only in the Monotremes 



' Gegenbaur, Vr.njl. Anat. Wirlellh. Leipzig, 1898, \>. 404. 



Fig. 16. — Under surface of the crauiuiii o\ a Dog. 

 X ^. ct^, Anterior palatine for.amen ; ffs, pos- 

 terior opening of alisphenoid canal ; ^l-S', 

 alisphenoid ; BO, basioccipital ; 7->'/\ basi- 

 sphenoid ; cf, condylar foramen ; eam^ e.\- 

 ternal auditory meatus ; E.r.O, exoccipital ; 

 flm, foramen laceriini medium ; /fjj, foramen 

 lacenim posterius ; fin, foramen magnnni ; 

 'b, foramen ovale ; //•, foramen rotunduni ; 

 Fr, frontal ; gf, glenoid fossa ; yp, post- 

 glenoid process ; M'l, nialar ; Mt, maxilla ; 

 uc, occipital condyle ; op^ optic foramen ; 

 Per, mastoid portion of periotic ; -pr/f, post- 

 glenoid fossa ; PJ, palatine ; PMx, pre- 

 maxilla ; pp, paroccipital process ; 2^P.f\ pos- 

 terior palatine foramen ; P*S', presphcnoid ; 

 Pt, pterygoid ; .«/', sphenoidal fissure or fora- 

 men lacerum anterins ; sin, stylomastoid fora- 

 men ; SO, supraoccipital ; Sq, zygomatic 

 process of squamosal ; Ti/, tympanic IniUa ; 

 Vo, vomer. {From Flower's Ostcolo!/'/.) 



