GENERAL CHARACTERISTICS. 



1 1 



SKULL OF PIG. 



only one side of each jaw, and indicating the incisors by the letter i, the canines 

 or tusks by c, the premolars by />, 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 : i%, c [, j>|, -m f,. 

 The total thus given is 

 21, and double this num- 

 ber 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 Teetii. W ords must 



now be said regarding 



the internal structure of 



teeth, as without this it 



IS quite impossible to TO show 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 

 internally 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 bees-wax, and putting some smears 

 of sealing-wax at the base of the outer surface, when the bees- wax 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 bees-wax (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 several deep pits, and also mark the sides with one or more 

 grooves, and fill up such pits and grooves with sealing-wax, it is obvious that we 

 shall have a much more complex type of structure. This complex model will serve 

 to explain the type of tooth structure found in many of the Hoofed or Ungulate 

 Mammals : and it will be obvious that if we now cut off the summit of our model 

 we shall find a series of irregular discs of bees-wax (ivory), each surrounded by a 

 sinuous border of kid (enamel), in the folds of which will be masses of sealing-wax 



