Carbohydrates. These reach the body, as already mentioned, by way of the blood, and in the form of monosaccharides or Simple sugars. It has been shown that if lactose be put into a lactase-free intestine, no absorption takes place, the lactose grad ually disappearing under bacterial action, whereas when the fer ment lactase is present, glucose and galactose, the products of its splitting, are absorbed as readily as cane-sugar and maltose. The body deals with its carbohydrate supply in the form of mono saccharides as if solutions of various sugars, mono- and disac charides be injected, it is found that the simple sugars are retained, whereas the double sugars are excreted in the urine. The only disaccharide which can be dealt with in the body is maltose, as there is a maltase present in the blood which splits it. Carbohy drates which are not absorbed from the intestine are disposed of by bacterial action.
Fats. Fats are absorbed from the intestine in the form of fatty acids and glycerol, i.e., in the form in which they exist after the action of the lipase. That a resynthesis takes place in the epi thelium is shown by the fact that as fat absorption goes on fat droplets are seen to grow in the protoplasm away from the free margin of the cells. As already mentioned, the fat is removed by the lacteals from the cells to the thoracic duct, and then to the general circulation. A small amount of the fat may pass into the body via the blood, but this is practically all retained by the liver. The amount of fat absorbed depends largely on the nature of the fat, especially with reference to its melting-point, fats of low melt ing-point being most readily taken up.
Protein. It is unquestionable in the light of modern work that the great bulk of the digested protein is absorbed from the intes tine via the portal blood stream in the form of amino acids. The work of Abel with his vivi-diffusion apparatus demonstrated very clearly that amino acids circulated in the blood and the work of Folin and van Slyke demonstrated that the amino acids were taken up by the various tissues.
In the large intestine, little absorption of nutrient matter takes place under normal conditions, mainly of course because most of the absorbable material is removed whilst the food is in the small intestine. The principal substance absorbed here is water; and thus the excreta become firm and formed.
Within the living protoplasmic mass of the tissue-cells activity is ceaseless. The whole series of changes has been given the name of metabolism and within this title are recognized the two phases of building up, anabolism, and of breaking down, catabolism.
There is a constant interplay between these two phases, sometimes rapid, sometimes slow, with a straining towards an equilibrium which is never reached whilst life goes on. When this metabolism is analysed further it is customary to divide it into two parts, the metabolism of energy and the metabolism of matter or mate rial. But it must be made clear that within the living cell metabo lism is one and indivisible. Further, although for descriptive pur poses it is necessary to discuss the metabolism of the different proximate principles, e.g., protein, carbohydrate, etc., individually the fact must be appreciated that the living protoplasmic mass which forms the content of the cell is not a mere conglomerate of protein, carbohydrate, fat, salts and water, where each constituent may be envisaged as a separate entity. Protoplasm, on the con trary, is a complex colloid—an intimate compound of its various constituents. No constituent can be metabolized without influ encing the metabolism of the others.