238 PLYMOUTH INSTITUTION. 



layer becomes thick, spongy, opaque, and of a reddish colour, in 

 order to supply this third substance. Thus, all the different 

 substances of the tooth are the products of one and the same 

 membrane ; for even the pulp itself is no more than a secretion, 

 so to call it, of the vessels at the lower part of the capsule ; and 

 that it is not impossible for two or more substances, differing in 

 their structure, to be deposited by the same membrane is demon- 

 strated by an instance on record of the head of a human thigh 

 bone having been found, with a portion of its surface, an inch and 

 a half in length and an inch in breadth, covered with highly 

 polished enamel, somewhat resembling that of the teeth, More- 

 over, tumours have been found in the frontal sinuses of the human 

 head, having a perfect resemblance to ivory: and two instances 

 of this kind have been met with, and recorded by Sir Everard 

 Home. 



The hare and the rat are animals belonging to the same order, 

 Rodentio. The under incisors of the former have a straight 

 edge; those of the latter a curved and rather more pointed one. 

 Those of the hare are adapted for cutting the tender blades of 

 wheat and other vegetables, while the rat's are better suited to 

 separate into fine rtolecules the hard substances which it is gene- 

 rally destined to feed upon. The common bat and the mole are 

 both insectivorous; and have canines which are irregularly 

 conical and very large. The angular surfaces of those of the bat 

 render it easy for them to penetrate the hard wing covers of the 

 coleopterous insects : but the mole has need of a different form 

 of teeth to cut or tear softer and more flesh like substances. Its 

 under jaw is consequently provided with two canines which 

 have posteriorly a sharp edge, and are very much flattened later- 

 ally ; so that a transverse section of one of those teeth would 

 resemble in form a similar section of a razor. By means of these 

 canines the mole skins the common earth worm in an exceedingly 

 curious manner, by first slitting the skin from end to end, and 

 then squeezing out the contents of it. The horse and the ox are 

 frequently seen grazing together in the same pasture. But the 

 former animal can also with his flat gTinders tritui-ate hard com, 

 as wheat, barley and oats ; and the latter is almost indispensable 

 to him, in a domesticated state, at least. On the other hand, the 

 irregular surface observable in the molars of the ox renders it 

 difficult for him to feed at all on grain. 



All the substances which, in any degree, supply the place of 

 teeth are nearly allied to horn, rn their structure. They appear 

 in common with that substance, to resemble a mass of agglutinated 

 hairs; although in some instances their texture is rather lamin- 

 ated than fibrous. Such are the beaks ofthe whole class of birds, 

 and of the chelonian division of the class of reptiles, or the 

 tortoises. 



There is another substance of a similar structure, but perfectly 

 anomalous in form, which is the only otmj to be noticed in the ^ 



