AUDITORY OSSICLES. DENTITION. 499 



Huxley considered the suprastap6dial( mammalian incus) to be homologous 

 with the hyomandibular or top of the hyoid arch of fishes (hence its con- 

 nection with the malleus which he regards as the proximal end of 

 the mandibular arch in mammals), the stapes or colvmiella affording 

 a secondary connection with the periotic capsule. By the up- 

 holders of the view that the malleus, incus, and stapes are all differentia- 

 tions of the columella auris of reptiles (top of the hyoid arch) the con- 

 nection of the processus gracilis of the malleus with Meckel's cartilage in 

 the mammalian embryo would be regarded as secondary, and as 

 another example of the arrangement in the Crocodilia in wliich 

 Meckel's cartilage is connected with the columella by a cartilagmous 

 band (p. 376). We are inclined to accept the last view so far as the 

 homologies of the ossicles are concerned, but we differ in thinking that 

 the quadrate has been absorbed into the squamosal in mammalia (pp. 399, 

 479) and has nothing to do with the tympanic, which is largely a 

 membrane bone. 



Dentition. Teeth are entirely absent in the adults of some 

 mammals, e.g. whalebone whales, the monotremes, many eden- 

 tates, but in many of these small calcified teeth, which do not cut 

 the gums but are absorbed early, are found in the foetus. Ecli idna 

 and some of the American ant-eaters appear to be as edentulous 

 as birds, no trace of teeth having been found even in the foetus. 

 When present they may be found on the premaxillae, maxillae 

 and mandibles, never on the palatal bones. They are imbedded 

 in sockets in the bone, (thecodont) the dental alveoli, which are 

 lined by a vascular membrane, the alveolar dental membrane. 

 They do not become ankylosed to the jaws. The part of the 

 tooth which projects above the gum is called the crown ; the 

 part below the gum and embedded in the socket is called the root 

 or fang. The neck of the tooth is where the root and crown join. 

 The back-teeth (grinders) commonly have more than one root 

 embedded in separate sockets, and this occasionally apphes to 

 some of the anterior teeth (some Insectivora, etc.). The tooth con- 

 tains a central pulp-cavity which has an opening at the apex of 

 the root, or, if there are more roots than one, at the apex of each 

 root (Fig. 264, ///, IV). This cavity contains a connective tissue 

 pulp with bloodvessels and nerves. In young growing teeth 

 and in the so-called rootless teeth which grow throughout life 

 (Fig. 264, /), the openings of the pulp cavity are wide, but tYiey 

 become narrow when growth has ceased and the pulp becomes 

 relatively less important ; pulp, however, usually persists 

 throughout life conferring sensibility on the dentine which is 

 traversed by the protoplasmic processes of the cells which line 



