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.

3. _The Circulation_

Section 34. The next thing to consider is the distribution of the food material absorbed through the walls of the alimentary canal to the living and active parts of the body. This is one of the functions of the series of structures-- heart and blood-vessels, called the circulation, circulatory system, or vascular system. It is not the only function. The blood also carries the oxygen from the lungs to the various parts where work is done and kataboly occurs, and it carries away the katastases to the points where they are excreted-- the carbon dioxide and some water to the lungs, water and urea to the kidneys, sulphur compounds of some kind to the liver.

Section 35. The blood (Figure 4, Sheet 2) is not homogeneous; under the low power of the microscope it may be seen to consist of--

(1.) a clear fluid, the plasma, in which float--

(2.) a few transparent colourless bodies of indefinite and changing shape, and having a central brighter portion, the nucleus with a still brighter dot therein the nucleolus-- the white corpuscles (w.c.), and

(3.) flat round discs, without a nucleus, the red corpuscles (r.c.), greatly more numerous than the white.

Section 36. The chyle of the lacteals passes, as we have said, by the thoracic duct directly into the circulation. It enters the left vena cava superior (l.v.c.s.) near where this joins the jugular vein (ex.j.) (see Figure 1, Sheet 2, th.d.) and goes on at once with the rest of the blood to the heart. The small veins of the villi, however, which also help suck up the soluble nutritive material, are not directly continuous with the other body veins, the systemic veins; they belong to a special system, and, running together into larger and larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.) veins, which unite to form the portal vein (p.v.) which enters the liver (l.v.) and there breaks up again into smaller and smaller branches. The very finest ramifications of this spreading network are called the (liver) capillaries, and these again unite to form at last the hepatic vein (h.v.) which enters the vena cava inferior (v.c.i.), a median vessel, running directly to the heart. This capillary network in the liver is probably connected with changes requisite before the recently absorbed materials can enter the general blood current.

Section 37. The student has probably already heard the terms vein and artery employed. In the rabbit a vein is a vessel bringing blood towards the heart, while an artery is a vessel conducting it away. Veins are thin-walled, and therefore flabby, a conspicuous purple when full of blood, and when empty through bleeding and collapsed sometimes difficult to make out in dissection. They are formed by the union of lesser factors. The portal breaks up into lesser branches within the liver. Arteries have thick muscular and elastic walls, thick enough to prevent the blood showing through, and are therefore pale pink or white and keep their round shape.

Section 38. The heart of the rabbit is divided by partitions into four chambers: two upper thin-walled ones, the auricles (au.), and two lower ones, both, and especially the left, with very muscular walls, the ventricles (vn.). The right ventricle (r.vn.) and auricle (r.au.) communicate, and the left ventricle (l.vn.) and auricle (l.au.).