METHODS OF INVESTIGATION 33 



the intestine may be suspended from the short arm of a writing 

 lever in a cylinder containing Locke's fluid at the proper 

 temperature. Then the fluid may be replaced by any other 

 (such as an organ extract) whose action it is desired to record. 

 Various organs yield extracts which will produce distinct 

 effects upon such muscle preparations. Many of these have 

 been shown not to be specific. The more important of them 

 will be referred to again under adrenal bodies, pituitary, etc. 

 This method is one of extreme delicacy, and great care should 

 be taken that proper control experiments are carried out. The 

 preparations are very susceptible to slight changes of tem- 

 perature, slight accidental mechanical stimuli, and in all 

 probability the rate of change of dosage of the active substance 

 makes a great difference to the result. Distinct effects are 

 reported to be noticed by this method when adrenin in dilutions 

 of 1 : 500 millions is employed. 



The electrical response as an index of glandular activity has 

 been employed to study the activity of the thyroid gland. By 

 preliminary experiments with the submaxillary it appeared 

 that the electrical change is a manifestation of the secretory 

 process and not of anything else. The results of these 

 experiments will be described in the chapter on the thyroid 

 (p. 308). 



Abel has recently introduced a method which he calls 

 " vividiffusion," which seems likely to be of great service in 

 the study of the internal secretions. An artery or vein is 

 connected by a cannula to an apparatus made of celloidin in 

 the form of tubes immersed in a saline solution, and providing 

 for the return of the blood to the animal's body by another 

 cannula attached to a vein. The tubes and cannulse are filled 

 with saline solution. This is displaced into the body by the 

 inflow of blood when the circulation in the apparatus is estab- 

 lished. The blood leaving the artery flows through a perfectly 

 closed system and returns to the body within a minute or two, 

 while the diffusible substances which it contains can pass out 

 through the walls of the tubes. Coagulation of the blood is 

 prevented by means of hirudin. If, then, the hormones are 

 diffusible substances they may be separated from the circulating 

 blood by this method. 



The employment of cytoioxic sera as a means of investigating 

 the tissues concerned in internal secretion has so far not yielded 



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