The Alimentary Canal of the Rabbit

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. The Alimentary Canal of the Rabbit

The Alimentary Canal of the Rabbit

Section 16. Figure 1 represents the general anatomy of the rabbit, but is especially intended to show the alimentary (= food) canal, shortened to a certain extent, and with the proportions altered, in order to avoid any confusing complications. It is evidently simply a coiled tube-- coiled for the sake of packing-- with occasional dilatations, and with one side-shunt, the caecum (cae.), into which the food enters, and is returned to the main line, after probably absorbent action, imperfectly understood at present. A spiral fold in this cul-de-sac {bottom-of-sack}, which is marked externally by constrictions, has a directive influence on the circulation of its contents. The student should sketch Figure 1 once or twice, and make himself familiar with the order and names of the parts before proceeding. We have, in succession, the mouth (M.), separated from the nasal passage (Na.) above the palate; the pharynx (ph.), where the right and left nasal passages open by the posterior nares into the mouth; the oesophagus (oes.); the bag-like stomach, its left (Section 6) end being called the cardiac (cd.st.), and its right the pyloric end (py.); the U-shaped duodenum (ddnm.) and the very long and greatly coiled ileum (il.). The duodenum and ileum together form the small intestine; and the ileum is dilated at its distal end into a thick-walled sacculus rotundus (s.r.), beyond which point comes the large intestine. The colon (co.) and rectum (r.) continue the main line of the alimentary canal; but, at the beginning of the large intestine, there is also inserted a great side-shunt, the caecum (cae.), ending blindly in a fleshy vermiform appendix (v.ap.). The figure will indicate how the parts are related better than any verbal description can. Between the coiling alimentary tube and the body walls is a space, into which the student cuts when he begins dissecting; this is the peritoneal cavity (pt.). A thin, transparent membrane, the mesentery, holds the intestines in place, and binds them to the dorsal wall of this peritoneal space.

Section 17. The food stuffs of an animal, the unstable compounds destined ultimately to be worked into its life, and to leave it again in the form of katastases (Section 13), fall into two main divisions. The first of these includes the non-nitrogenous food stuffs, containing either carbon together with hydrogen and oxygen in the proportion of H2O (the carbo-hydrates), or carbon and hydrogen without oxygen (the hydrocarbons). The second division consists of the nitrogenous materials, containing also carbon, hydrogen, a certain amount of oxygen, sulphur, and possibly other elements. Among the carbohydrates, the commonest are starch and cellulose, which are insoluble bodies, and sugar, which is soluble. The hydrocarbons, fats, oils, and so on, form a comparatively small proportion of the rabbit's diet; the proverb of "oil and water" will remind the student that these are insoluble. The nitrogenous bodies have their type in the albumen of an egg; and muscle substance and the less modified living "protoplasm" of plants, a considerable proportion of the substance of seeds, bulbs, and so on, are albuminous bodies, or proteids. These also are insoluble bodies, or when soluble, will not diffuse easily through animal membranes.

Section 18. Now the essential problem which the digestive canal of the rabbit solves is to get these insoluble, or quasi-insoluble, bodies into its blood and system. They have to pass somehow into the circulation through the walls of the alimentary canal. In order that a compound should diffuse through a membrane, it must be both soluble and diffusible, and therefore an essential preliminary to the absorption of nutritive matter is its conversion into a diffusible soluble form. This is effected by certain fluids, formed either by the walls of the alimentary canal or by certain organs called glands, which open by ducts into it; all these fluids contain small quantities of organic compounds of the class called ferments, and these are the active agents in the change. The soluble form of the carbohydrates is sugar; proteids can be changed into the, of course, 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. 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.

Section 29. A succus entericus, a saliva-like fluid secreted by numerous small glands in the intestine wall (Brunner's glands, Lieberkuhnian follicles), probably aids, to an unknown but comparatively small extent, in the digestive processes.

Section 30. The walls of the whole of the small intestine are engaged in the absorption of the soluble results of digestion. In the duodenum, especially, small processes, the villi project into the cavity, and being, like the small hairs of velvet pile, and as thickly set, give its inner coat a velvety appearance. In a villus we find (Figure IX., Sheet 3) a series of small blood-vessels and with it another vessel called a lacteal. The lacteals run together into larger and larger branches until they form a main trunk, the thoracic duct, which opens into the blood circulation at a point near the heart; but of this we shall speak further later. They contain, after a meal, a fluid called chyle.

Section 31. Emulsified fats pass into the chyle. Water and diffusible salts certainly pass into the vein. The course taken by the peptones is uncertain, but Professor Foster favours the chyle in the case of the rabbit-- the student should read his Text-book of Physiology, Part 2, Chapter 1, Section 11, if interested in the further discussion of this question.

Section 32. The processes that occur in the remaining portions of the alimentary canal are imperfectly understood. The caecum is so large in the rabbit that it must almost certainly be of considerable importance. In carnivorous animals it may be so much reduced as to be practically absent. An important factor in the diet of the herbivorous animals, and one absent from the food of the carnivora, is that carbohydrate, the building material of all green-meat- [food], cellulose, and there is some ground for thinking that the caecum is probably a region of special fermentive action upon it. The pancreatic juice, it may be noted, exercises a slight digestive activity upon this substance.

Section 33. Water is most largely absorbed in the large intestine, and in it the rejected (mainly insoluble) portion of the food gradually acquires its dark colour and other faecal characteristics.

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