INTRODUCTORY 9 



after death are due to their activity. It has been suggested 

 that life is nothing more than the sum-total of the activities of 

 the enzymes contained within the living matter. A theory 

 related to this has been suggested in a tentative form by Bayliss 

 and Starling. In a paper on the mechanism of pancreatic 

 secretion (a subject which will be referred to again later on, 

 vide infra, p. 59), these authors tested the hypothesis that the 

 products of metabolism of certain tissues would be found to act 

 as vasodilators only for certain tissues in functional relation to 

 those in which they arise, or, at all events, would act to a 

 greater degree on these tissues than on the rest of the body in 

 general. Results were obtained which tended to confirm their 

 view. Vincent and Sheen, however, obtained different results, 

 and suggested that the subject may be complicated by the 

 existence not only of specific vasodilators, but also of specific 

 vasoconstrictor substances, whose effects might be looked for 

 on those occasions when the injection of a tissue extract pro- 

 duces a rise of blood-pressure. This line of work has, however, 

 not led to any definite conclusions. 



In some modern textbooks the conception of internal secre- 

 tion is extended beyond the limits which appear reasonable. 

 Thus, it is stated that the lymphatic glands "form an internal 

 secretion which consists of lymph cells, and these furnish the 

 blood with a supply of certain kinds of colourless corpuscles." 

 It has already been mentioned (supra, p. 3) that definite 

 morphological elements should be excluded from the category 

 of the secretions. Thus, the ova and spermatozoa are not 

 included among the external secretions, and the cells manu- 

 factured by the spleen, lymph glands, and bone-marrow must 

 be excluded from the group of internal secretions. 



The conception of internal secretion has had far-reaching 

 effects in the realms of both physiology and pathology. Patho- 

 logists are now able to recognize the existence of new forms of 

 stimuli which influence growth and metabolism, and this either 

 in a positive or in a negative direction that is to say, either in 

 the direction of augmentation or inhibition. Thus, to mention 

 one example, dwarfism and gigantism may be explained by 

 reference to certain internal secretions, or, at any rate, a plaus- 

 ible hypothesis may be furnished by such reference. It is 

 further to be noted that the function of each particular internally 

 secreting gland may be conceived as varying, or capable of 



