Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

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. 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.

Section 24. After the food has undergone digestion in the stomach it passes into the duodenum, the U-shaped loop of intestine immediately succeeding the stomach. The duodenum is separated from the stomach by a ring-like muscular valve, the pylorus; this valve belongs to the class of muscles called sphincters, which, under ordinary circumstances, are closed, but which relax to open the circular central aperture. The valve at the anus, which retains the faeces, is another instance of a sphincter.

Section 25. The food at this stage is called chyme; it is an acid and soup-like fluid-- acid through the influence of the gastric juice. The temperature of the animal's body is sufficiently high to keep most of the fat in the food melted and floating in oily drops; much of the starch, has been changed to sugar, and the solid proteids to soluble peptones, but many fragments of material still float unchanged.

Section 26. It meets now with the bile, a greenish fluid secreted by that large and conspicuous gland the liver. The bile is not simply a digestive secretion, like the saliva or the gastric juice; it contains matters destined to mix in, and after a certain amount of change to be passed out of the body with, the faeces; among these substances, of which some portion is doubtless excretory, are compounds containing sulphur-- the bile salts. There is also a colouring matter, bili verdin, which may possibly also be excretory. If the student will compare Sections [10] and [11], he will notice that in those paragraphs no account is taken of the sulphur among the katastases, the account does not balance, and he will at once see that here probably is the missing item on the outgoing side. The bile, through the presence of these salts, is strongly alkaline, and so stops the action of the gastric juice, and prepares for that of the pancreas, which can act only in an alkaline medium. The fermentive action of the bile is trifling; it dissolves fats, to a certain extent, and is antiseptic, that is, it prevents putrefaction to which the chyme might be liable; it also seems to act as a natural purgative.

Section 27. The bile, as we shall see later, is by no means the sole product of the liver.

Section 28. The pancreatic juice, the secretion of the pancreas is remarkable as acting on all the food stuffs that have not already become soluble. It emulsifies fats, that is, it breaks, the drops up into extremely small globules, forming a milky fluid, and it furthermore has a fermentive action upon them; it splits them up into fatty acids, and the soluble body glycerine. The fatty acids combine with alkaline substances ([Section 26]) to form bodies which belong to the chemical group of Soaps, and which are soluble also. The pancreatic juice also attacks any proteids that have escaped the gastric juice, and converts them into peptones, and any residual starch into sugar. Hence by this stage, in the duodenum, all the food constituents noticed in [Section 17] are changed into soluble forms. There are probably, three distinct ferments in the pancreatic juice acting respectively on starch, fat, and proteid, but they have not been isolated, and the term pancreatin is sometimes used to suggest the three together.