ourse, chemically
equivalent but soluble and diffusible the peptones; and fats and oils
undergo a more complicated, but finally similar change.

Section 19. We shall discuss the structure and action of -a gland-
[glands] a little more fully in a subsequent chapter. Here we will
simply say that they are organs forming each its characteristic fluid
or secretion, and sending it by a conduit, the duct, to the point
where its presence is required. The saliva in our mouths, tears, and
perspiration, are examples of the secretions of glands.

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. T