482 Vertebrata. 



The tympanic cavity which lies enclosed in the temporal bone, is often 

 of considerable size, so that the surrounding bony portions (especially the 

 tympanic bone) are swollen to form a vesicle (bulla) ; sometimes the tympanic cavity 

 is connected with air sinuses in the neighbouring bones (c/. Crocodilia and Aves). 

 The walls of the eustachian tubes ai-e usually partly ossified; they open separately 

 into the phai-ynx. In the Horse and Tapir, each Eustachian canal has a veiy large 

 thin- walled, saccular extension. 



The buccal cavity in young embryos, as in most Reptiles, 

 is undivided. Quite soon, however, a ledge develops laterally 

 and above, and unites with its fellow of the other side to form a 

 horizontal septum, the anterior end of which (covered, of course, 

 on both sides with mucous membrane) is the hard palate, whilst 

 the hinder portion remains soft, and forms the muscular soft 

 palate. The cavity above the hard palate unites with the nasal 

 cavities (the nasal septum growing down and becoming connected 

 with the hard palate) ; the cavity above the soft palate, which com- 

 municates freely at its front end with the nasal cavities, remains 

 single, and is termed the pharynx; it includes also the posterior region 

 of the primitive buccal cavity. The eustachian canals open above, the 

 trachea below into the pharynx (Pig. 396). The buccal cavity lies 

 below both hard and soft palates and encloses the teeth, tongue, etc. 



The teeth of Mammalia are chiefly remarkable in that their 

 number is small and fairly constant for a given species ; that their 

 form is usually relatively complex ; that they are implanted in sockets ; 

 and especially that the mode of replacement is peculiar, for 

 the teeth are not, as in other Vertebrata, replaced continuously 

 throughout life, but only two series, the milk and the permanent 

 dentitions, are present, following each other in regular sequence. 

 It may be noticed further that the teeth are used not only for the 

 prehension of food, but also very largely for its mastication. In 

 addition to the two ordinary components of teeth yet a third is 

 present, the cement, occurring chiefly at the root (see below). 

 It is simply a sheath of osseous matter deposited by the connective 

 tissue surrounding the tooth ; it lies external to the rest of the 

 tooth, and is formed last : it is not as hard as dentine. 



Two parts may be distinguished in a mammalian tooth, the crown 

 and the root. The root is the lower,* usually narrower part, and 

 is often split into several branches ; it is destitute of enamel, 

 but is covered with cem.ent. The crown is the upper enamelled 

 portion, which generally projects quite freely, and is usually clearly 

 demarcated from the root ; for instance, by a constriction. Cement 

 does not occur "in this region, excepting as an occasional layer, varying 

 in thickness, upon the surface of the enamel. The crown exhibits a 

 great diversity of form; it may be simply conical or chisel shaped. 



* The free end of the tooth is termed the upper, the opposite the lower, end ; 

 although this terminology is actually correct only for the teeth of the lower jaw. 



