140 PHYSIOLOGY. 



draw towards a point at the base, but always keep open or conical, yet 

 do not always continue to grow, as the tusks. 



Some teeth are wholly composed of bony substance, which is a mixture 

 of two different substances, viz. a mixture of animal substance and 

 calcareous earth : such are those of the ray-kind, alligator ; as also 

 some peculiar teeth of some animals whose teeth in general are not so 

 simple, such as the elephant's tusk, boar's tusk, &c. The teeth of many 

 animals are composed of tbe two above-mentioned substances, but in one 

 degree in a different manner, viz. one part being composed of bony 

 substance, the other of calcareous earth alone [called ' enamel ']. The 

 teeth composed of bone and enamel belong to man, the cat, the hare, 

 the horse, ruminating animals, &c. 



The bony part of both genera are formed upon and by a pulpy sub- 

 stance ; therefore the whole of the first genus [with teeth wholly 

 composed of bony substance] is formed by this pulp, but only the bony 

 part of the second ; the enamel is formed by an opposite pulp, which 

 makes it complicated. How far homy substance may be so shaped as 

 to deserve the name of teeth, I do not yet know 1 . 



Teeth continually growing I have divided into two species : first, the 

 ' dentes scalprarii ; ' and second, the ' tusks.' The first belongs to the 

 hares, &c, the second to the boar-tribe ; as also to the narwhal, and 

 probably many more. 



Reasons for a vacant space between the Cutters and Grinders. 



Most animals have avast length of mouth from the symphysis of the chin 

 to the posterior grinder or the root of the coronoid process. The incisors 

 or cutters must be placed at the fore part, where the opening into the 

 mouth is, because there the food must enter and be divided from what 

 is to remain out of the mouth. The mouth being properly filled with 

 food, it is then thrown back, or up, to near the centre of motion of the 

 jaw where the grinders are placed. Now, in all long-jawed animals 

 there is a space in which teeth of any kind of shape can be of no use ; 

 they could not, from the position, separate the internal food from the 

 external, and the space is too far forward for grinders to be of any use. 

 This vacant place is shorter in proportion as the jaws are shorter, [and 

 more so] in some animals than in others ; and in some there is very 



1 [The substance resembling bone in the proportions of earth and gelatine, is 

 divided into ' dentine ' and ' cement,' which latter Hunter had recognized as ' a 

 second kind of bone.' There is a minute proportion of gelatine in enamel, and this 

 substance covers the crown of the teeth of the alligator. Since Hunter wrote the 

 above, the Ornitlwrhynchus has afforded the reply to his remark on horny teeth.] 



