Experiment 81. Object: To test the effect of pancreatic solution on starch.

Procedure: Repeat Experiments 76 to 78, using pancreatic juice instead of saliva.

Experiment 82. Object: To test the effect of pancreatic solution of protein.

Procedure: Repeat the Experiments with pepsin solution, using the pancreatic solution instead of the pepsin solution, and in Experiment 79 substitute sodium carbonate solution 2 per cent. for the 0.5% hydrochloric acid solution.

Experiment 83. Object: To test the action of pancreatic juice on fat.

Procedure: Into each of 2 test tubes put 10 c. c. of whole milk and enough litmus powder to color the milk a decided, but not dark, blue. To tube 2 add 2 c. c. of pancreatic juice. Put both tubes into a water bath and keep them at 40° C. After a short time, it will be noticed that the blue color of the milk in tube 2 changes to pink. Why does it do so?

CHAPTER XXIII

THE CHEMISTRY OF ABSORPTION AND METABOLISM

Changes that Occur in Food Substances during Absorption— Nature of Metabolism-Composition of the Blood-Changes that Occur in Food Substances during Metabolism-Some Causes and Results of Defective Metabolism-The Fuel Value of Foods-Food Requirements.

Absorption

The changes that occur in food substances during absorption. Of the substances formed during digestion from disaccharids, polysaccharids, proteins, and fats, only monosaccharids are usually found in the general circulation. Therefore, it is thought that during absorption and, probably, in the liver chemical changes the opposite of those occurring in digestion take place. These changes result in the transformation of the soap into fats and of peptones and amino acids into blood proteins-the serum albumin or globulin. The passage of these substances into the blood is thought to be partly due to osmotic pressure, but both their passage into the blood and their rebuilding are accomplished by the activity of cells in the mucous membrane of the intestine. Whether these cells form enzymes, or whether the enzymes with which the food became mixed in the process of

digestion by a reverse' action aid in this synthesis, are still debated questions.

Metabolism

Every living organism may be likened to a chemical laboratory, whether it be plant or animal, whether it be large and composed of millions of cells or so small that it cannot be seen without the aid of a microscope; for in the cells of all such organisms chemical reactions are constantly taking place whereby the tissues are built and repaired and, in the case of animals, heat and energy are generated to keep their body. machinery in motion. The classification of such processes in plants was mentioned in Chapter XVIII.; those occurring in animals are spoken of as metabolism. Some of the reactions occurring in metabolism can be easily performed in any chemical laboratory, but after a great many years of study, even the most skillful chemists and physiologists have but an indefinite knowledge of the nature of some of the processes.

Catabolism and anabolism.-The chemical reactions of metabolism, such as hydrolysis and oxidation, which result in the disintegration of complex substances into simpler compounds, are classed as catabolism and the changes are said to be catabolic (from the Gr. kata-down and metabole=change), and the synthetic changes in which simple substances are put together to form complex compounds are classed as anabolism and the changes are said to be anabolic (Gr. ana = up and metabole = change).

1 A common characteristic of enzymes is that an accumulation of the products of their action will check their activity and even cause them to reverse the nature of their action.

Composition of the blood.-Since the blood is the reservoir for the chemical supplies that the body uses for the numerous chemical reactions constituting metabolism, it may be well to recall its composition before going further with the study of the reactions.

The constituents of the blood are usually classified as follows:

Also, there are in the blood ferments, enzymes, internal secretions, antitoxins, and similar substances. The composition of the blood in the portal vein differs at times from that in the other vessels because

Substances, other than proteins, that may be extracted from dried residue of blood by water, alcohol, or ether.

it contains all the substances absorbed from the intestine and some of these the liver removes or changes before they enter the general circulation. For instance, glucose, over the amount necessary to provide the blood with a content of about 0.1 or 0.15 per cent., is removed by the liver cells and changed to glycogen, which is stored chiefly in the liver, but to some extent in the muscles. Also, putrefactive products, such as indol, that are absorbed from the intestine are changed to substances less likely to injure the system before they enter the general circulation.

I

Derivation of the blood constituents.-The constituents of the blood, other than the corpuscles, antitoxins, and similar substances, represent: (1) food matter absorbed from the intestines; (2) oxygen taken from the lungs; and (3) matter, useful and otherwise, that has been absorbed from the tissues and glands.

Fate of the food constituents of the blood.-There is a constant osmosis of substances from the blood, through the capillaries into the tissues, and this matter is either used by the cells for their nutrition or building or else it unites with oxygen and is broken down, just as fuel is when it combines with oxygen in a furnace. The result of this oxidation is the same as that which occurs in the furnace, viz., the production of heat. In the body, some of this heat is used for energy to keep the body machinery-heart, lungs, etc.-in action as well as to provide the body with power to perform its external work.

1 After the death of an animal the glycogen and glucose in the muscles and blood are soon oxidized.

2

This is contained principally in loose combination with the hemoglobin of the red blood-corpuscles.