GENERAL CHARACTERISTICS 



ii 



Dental 



Formulae 



figured upper jaw of the dog the number falls short of this full complement, 

 owing to the circumstance that there are only two in place of three molars. 



As it would be exceedingly inconvenient 'always to have to de- 

 scribe the number of teeth in any given Mammal by writing them 

 down at length, a graphic formula has been invented by which the 

 number of teeth of each species can be shortly and clearly expressed. Thus, 

 taking only one side of each jaw, and indicating the incisors by the letter z', the 

 canines or tusks by c, the premolars by p, and the molars by m, and taking the num- 

 bers above the lines as representing the teeth of the upper, and those below the 

 same the teeth of the lower jaw, we may express the number and kinds of the 

 teeth of the dog by the 

 formula : zf, c\, p\, m\. 

 The total thus given is 

 2 1 , and double this num- 

 Taer will of course give 

 the entire number of 

 teeth on both sides of 

 the two jaws, which in 

 this case will be 42. 

 Structure of A few 



the Teeth words must 

 now be said regarding 

 the internal structure of 

 teeth, as without this it 



IS quite impossible tO TQ ghow distinction between incisors, tusks, and cheek-teeth. After Nehring. 



understand the modifica- 

 tions which they undergo in different groups of Mammals. Taking a simple more 

 or less conical tooth like the tusk of a lion or tiger, or any tooth of a sperm whale, 

 it may be observed that when such a tooth first appears above the gum it is 

 open at the base, where it forms a hollow cone. And in teeth like the tusks of the 

 elephant, which grow throughout the whole life of their owner, such a condition 

 remains permanent. Usually, however, a tooth ceases to grow after a certain 

 period, and the base of the root or roots then becomes completely closed, and 

 assumes a pointed shape. A tooth of this simple conical type is composed inter- 

 nally of a substance known as the ivory or dentine, coated externally with a thin 

 layer of a much harder nature and highly polished appearance, which is termed the 

 enamel. Moreover, outside the base of the crown there may be patches of a coarser 

 substance, called the cement. A model of such a tooth may be made by taking the 

 finger of a kid glove, filling it with beeswax, and putting some smears of sealing 

 wax at the base of the outer surface, when the beeswax will represent the ivory, 

 the kid the enamel, and the sealing wax the cement. If we then cut off the summit 

 of the finger we shall have a central disc of beeswax (ivory) surrounded by a circle 

 of kid (enamel), which will represent the condition of such a type of tooth when its 

 summit has been worn away by use against the opposing tooth of the opposite jaw. 

 If, however, before cutting off the end of our model, we indent the summit with 



