proteolysis in the living intestine,

109 Leucin and tyrosin products of the natural pancreatic digestion in the intestine, 112 The physiological significance of leucin and tyrosin, 113 Absorption of the main products of proteolysis, 116 Absorption of acid-albumin, alkali-albuminate, etc. 117 Absorption limited mainly to the intestine, very little absorption from the stomach, 119 The change which the primary products of proteolysis undergo in the process of absorption, 120 Peptones not present in the circulating blood, 121 The change which peptones and proteoses undergo by contact with the living mucous membrane of the small intestine, 122 Retrogression of peptones by contact with other living cells, etc., 125 Functional activity of leucocytes in absorption, 128 Digestive leucocytosis incited by nuclein, 131 Shore’s experiments on the ability of lymph-cells to assimilate either proteoses or peptones, 133 Lymph a true secretion from the blood-vessels, 134 Direct excitatory effect of peptones when present in the blood on the endothelial cells, 136 Selective activity of endothelial cells, 137

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DIGESTIVE PROTEOLYSIS

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LECTURE I.

THE GENERAL NATURE OF PROTEOLYTIC ENZYMES AND OF PROTEIDS.

INTRODUCTORY.

In digestive proteolysis we have a branch of physiological study which of late years has made much progress. Chemistry has come to the aid of physiology and by the combined efforts of the two our knowledge of the digestive processes of the alimentary tract has been gradually broadened and deepened. That which at one time appeared simple has become complex, but increasing knowledge has brought not only recognition of existing complexity, but has enabled us, in part at least, to unravel it.

By digestive proteolysis is to be understood the transformation of the proteid food-stuffs into more or less soluble and diffusible products through the agency of the digestive juices, or more especially through the activity of the so-called proteolytic ferments or enzymes contained therein; changes which plainly have for their object a readier and more complete utilization of the proteid foods by the system.

In selecting this topic as the subject for this series of Cartwright Lectures I have been influenced especially by the opinion that both for the physiologist and the physician there are few processes going on in the animal body of greater importance than those classed under the head of digestion. Further, few processes are less understood than those concerned in this broad question of digestive proteolysis, especially those which relate specifically to the digestion of the various classes of proteid food-stuffs, and to the absorption and utilization of the several products formed. Moreover, the subject has ever had for me a strong attraction as presenting a field of investigation where chemical work can advantageously aid in the advance of sound physiological knowledge; and certainly every line of advance in our understanding of the normal processes of the body paves the way for a better and clearer comprehension of the pathological or abnormal processes to which the human body is subject.

You will pardon me if I specially emphasize in this connection the fact that advance along the present lines was not rapid until physiologists began to appreciate the importance of investigating the chemico-physiological problems of digestion by accurate chemical methods. Something more than simple test-tube study, or even experimental work on animals, is required in dealing with the changes which complex proteids undergo in gastric and pancreatic digestion. The nature and chemical composition of the proteids undergoing digestion, as well as of the resultant products, are necessary preliminaries to any rightful interpretation of the changes accompanying digestive proteolysis; but physiology has been slow to appreciate the significance of this fact, and, until recently, has done very little to remedy the noticeable lack of accurate knowledge regarding the composition and nature of the proteid and albuminoid substances which play such an important part in the life-history of the human organism, either as food or as vital constituents of the physio­logically active and inactive tissues. This is to be greatly deprecated, since our understanding of the nature of proteolysis, of the mode of action of the enzymes or ferments involved, and of the relationships of the products formed, is dependent mainly