METHODS OF INVESTIGATION 31 



In regard to the chemical nature of the depressor substance 

 in tissue extracts, nothing positive has yet been discovered. 

 It has been suggested that it may be histamine (ft Iminazol- 

 ylethylamine) but more recent investigations have rendered 

 this hypothesis untenable. 



It is doubtful whether the presence of these depressor sub- 

 stances in tissue extracts has any physiological significance. 

 Recent literature supplies many contributions from authors 

 who become impressed by the fact that some organ (such as a 

 lymphatic gland) yields a depressor substance to extracts, and 

 therefrom rashly assume that it is the function or one of the 

 functions of the organ in question to pour out this depressor 

 substance as. an internal secretion to minister to the needs of 

 the economy. 



Abelous and Bardier find in normal urine a substance which 

 raises the blood-pressure. This substance they call " uro- 

 hypertensin," but it is satisfactory that so far there has been 

 no suggestion that this is any kind of * ' hormone ' ' or evidence 

 of an "internal secretion." 



The physiological activity of the amines which are formed 

 when carbon dioxide is split off from amino-acids, as, for 

 example, by putrefaction, ought perhaps to be referred to 

 here. ABelous first noted the presence of pressor principles in 

 putrid meat. These have since been identified by Barger and 

 Walpole as isoamylamine, phenyl-ethylamine, and p. hydroxy- 

 phenylethylamine . 



But, of course, the effects upon the blood-pressure are not 

 the only actions of tissue extracts which have been studied. 

 The influences which such extracts exert upon the heart are 

 studied by recording directly the heart movements either in 

 the body or isolated in the lower vertebrata, and in mammals, 

 and various forms of tonometers and plethysmographs have 

 been employed for registering changes in the volume of the 

 heart. The muscle-nerve preparation may be used to demon- 

 strate the action of the extracts upon muscle and upon nerve. 

 The volume of organs, the rate of flow of secretions, and the 

 movements of muscular tissues, are also observed. In fact, all 

 the modern methods of observing and recording changes in 

 physiological conditions are constantly employed for the study 

 of the action of tissue extracts. 



An interesting method of studying the action of animal 



