ORAL EQUIPMENTS. 51 



thoroughly vegetable eaters are classed in accordance with the 

 peculiar structure and size of the nose or proboscis, and called 

 " Proboscidia," or the hoofed animals called Ungugulata, and 

 so on with the Sirenia, Hyracoidea, &c. 



In this class the skull and jaws are ponderous in propor- 

 tion to the size of the animal, and of great antero-posterior 

 length ; the glenoid cavity and condyle of the lower jaw are 

 broad, round and flat, so as to admit of the grinding action 

 necessary for reducing vegetable fibre to a pulp; and the 

 molar teeth are broad, and very rough on the grinding surface 

 caused by the uneven wearing of tissues of a different degree 

 of hardness ; the hardest substance, the enamel, standing out 

 from above its investing tissue. 



If you will oblige me by looking at the diagram of a 

 horse's teeth, you will see typically displayed what I am 

 endeavouring to describe. 



Let us take for the sake of the better comprehension of 

 the complex pattern of the molar teeth, the incisor or front 

 tooth represented in longitudinal section. Try to imagine 

 what would be seen if we took the finger of a kid glove, filled 

 it with plaster of Paris, and before it set hard pushed in the 

 tip. We should thus have the kid which had been outside 

 before the process, inside after it, and cut down longitudinally 

 it would look very like our section of a tooth. We should 

 see, passing from without inwards, first kid, next a thick body 

 of plaster of Paris, and next to this, kid, and then the space 

 or tube, made by pushing the finger tip in ; and so it is with 

 the tooth before us, first enamel, next a thick body of dentine 

 or ivory, and next enamel, bounded on the inside by a cavity. 

 Look at the same tooth in transverse section and we see a ring 

 of enamel, then a broad ring of dentine; and then another 

 ring of enamel enclosing an empty space. When the tooth 

 wears down, the " mark " is eradicated, and we see the condi- 

 tions as represented by the other figure on the diagram. The 

 tube in the tooth of a young horse gets filled with debris of 

 food, and indicates according to its depth the age of the 

 creature. This is what is known to horse-dealers as "the 

 mark," and dishonest dealers often drill a hole into the teeth 

 of an old horse and fill it with dirt ; but this fraud could only 

 deceive a novice in the trade, as there would be no ring of 

 enamel intervening between the tube and the dentine. 



Multiply these conditions with the molar tooth, and fill 

 up the interspaces between the pillars or cusps and ridges 

 with cementum, and you can easily perceive how the complex 

 pattern of the surface is produced. The teeth are constantly 



