Section 20. In the month of the rabbit the food is acted upon by the teeth and saliva. The saliva contains ptyalin, a ferment converting starch into sugar, and it also serves to moisten the food as it is ground up by the cheek teeth. It does not act on fat to any appreciable extent. The teeth of the rabbit are shown in Figure XVIII., Sheet 4. The incisor teeth in front, two pairs above and one pair below (i.), are simply employed in grasping the food; the cheek teeth-- the premolars (pm.) and molars (m.) behind-- triturate the food by a complicated motion over each. Their crowns are flat for this purpose, with harder ridges running across them. Section 21. This grinding up of the food in the mouth invariably occurs in herbivorous animals, where there is a considerable amount of starch and comparatively little hydrocarbon in the food. By finely dividing the food, it ensures its intimate contact with the digestive ferment, ptyalin. In such meat-eaters as the cat and dog, where little starchy matter and much fat is taken, the saliva is, of course, of less importance, and this mastication does not occur. The cheek teeth of a dog ({Section 91}), and more so of a cat, are sharp, and used for gnawing off fragments of food, which are swallowed at once. Between the incisors and premolars of a dog come the characteristic biting teeth, or canines, absent in the rabbit. Section 22. The student will probably ask why the cheek teeth, which are all similar in appearance, are divided into premolars and molars. The rabbit has a set of milk molars-- a milk dentition-- which are followed by the permanent teeth, just as in man. Those cheek teeth of the second set, which have predecessors in the first series, are called premolars; the ones posterior to these are the molars. Section 23. After mastication, the food is worked by the tongue and cheeks into a saliva-soaked "bolus" and swallowed. The passage down the oesophagus is called deglutition. In the stomach it comes under the influence of the gastric juice, formed in little glandular pits in the stomach wall-- the gastric (Figure VIII. Sheet 3) and pyloric glands. This fluid is distinctly acid, its acidity being due to about one-tenth per cent {of a hundred} of hydrochloric acid, and it therefore stops any further action of the ptyalin, which can act only on neutral or slightly alkaline fluids. The gastric juice does not act on carbo-hydrates or hydrocarbons to any very noticeable degree. Its essential property is the conversion of proteids into peptones, and the ferment by which this is effected is called pepsin. Milk contains a peculiar soluble proteid, called casein, which is precipitated by a special ferment, the rennet-ferment, and the insoluble proteid, the curd, thus obtained is then acted on by the pepsin. In the manufacture of cheese, the rennetferment obtained, from the stomach of a calf is used to curdle the milk.