CHEMICAL PHYSIOLOGY. 35 



presents the subject rather inadequately, but it will serve to show what 

 I mean. Further, the composition of the various juices is admirably 

 adjusted to the needs of the organism; when there is much proteid to 

 be digested, the proteolytic activity of the juices secreted is corre- 

 spondingly high, and the same is true for the other constituents of the 

 food. It is such general conclusions as these, the correlation of isolated 

 facts leading to the formulation of the law that the digestive process 

 is continuous in the sense I have indicated and adapted to the needs 

 of the work to be done, that constitute the great value of the work 

 from the Eussian laboratory. Work of this sort is sure to stimulate 

 others to fill in the gaps and complete the picture, and already has 

 borne fruit in this direction. It has, for instance, in Starling's hands 

 led to the discovery of a chemical stimulus to pancreatic secretion. 

 This is formed in the intestine as the result of the action of the gastric 

 acid, and taken by the blood-stream to the pancreas. Whether this 

 secretin as it is called may be one of a group of similar chemical stimuli 

 which operate in other parts of the body has still to be found out. 



The other series of researches to which I referred are those of 

 Ehrlich and his colleagues and followers on the subject of immunity. 

 This subject is one of such importance to every one of us that I am 

 inclined to place the discovery on a level with those great discoveries 

 of natural laws to which I alluded at the outset of this portion of my 

 address. I hesitate to do so yet because many of the details of the 

 theory still await verification. But up to the present all is working in 

 that direction, and Ehrlich 's ideas illustrate the value of bold theor- 

 izing in the hands of clear-sighted and far-seeing individuals. 



But when I say that the doctrine is bold, I do not mean to infer that 

 the experimental facts are scanty; they are just the reverse. But in 

 the same way that a chemist has never seen an atom, and yet he believes 

 atoms exist, so no one has yet ever seen a toxin or antitoxin in a state 

 of purity, and yet we know they exist, and this knowledge promises to 

 be of incalculable benefit to suffering humanity. 



It may not be uninteresting to state briefly, for the benefit of those 

 to whom the subject is new, the main facts and an outline of the theory 

 which is based upon them. 



We are all aware that one attack of many infective maladies protects 

 us against another attack of the same disease. The person is said to 

 be immune either partially or completely against that disease. Vac- 

 cination produces in a patient an attack of cowpox or vaccinia. This 

 disease is related to smallpox, and some still hold that it is smallpox 

 modified and rendered less malignant by passing through the body of 

 a calf. At any rate an attack of vaccinia renders a person immune to 

 smallpox, or variola, for a certain number of years. Vaccination is an 

 instance of what is called protective inoculation, which is now practised 



