492 SCIENCE PROGRESS. 



lies in the fact that the nerves supplying them (vagi and 

 splanchnics) exert a powerful influence on the vascular 

 system — an influence which might presumably affect or 

 check altogether any secreto-motor functions that these 

 nerves might possess. 



Pawlow has succeeded in avoiding both these sources of 

 error, and in proving conclusively the existence of definite 

 secretory nerves to both stomach and pancreas. I will deal 

 first with his researches on the pancreas. 



Claude Bernard, in 1856, showed that a secretion of 

 pancreatic juice might be evoked by introduction of ether 

 into the stomach, and that a secretion already in progress 

 was checked if vomiting occurred, both facts pointing to the 

 existence of a distinct reflex nervous mechanism presiding 

 over the secretion. This conclusion was further strengthened 

 by the experiments of Bernstein (1869), who showed that 

 the pancreas might be reflexly affected through the vagus 

 (afferent) in two ways, and therefore concluded that this 

 gland was provided with two sets of nerves, excitatory and 

 inhibitory. Attempts, however, to find the efferent channels 

 of these impulses failed. 



In 1875 Heidenhain pointed out that direct excitation of 

 the medulla, under certain circumstances, might call forth a 

 pancreatic secretion, but failed, like his predecessors, in dis- 

 tinguishing the efferent nerves carrying the secreto-motor 

 impulses. 



Later on (1877) Pawlow and Afanassiew showed that 

 excitation of the central end of all sensory nerves, and not 

 only of the vagus, inhibited an already existing flow of pan- 

 creatic juice. 



A consideration of these various experiments enables us 

 to perceive certain precautions which would have to be taken 

 in any subsequent experiments on the subject. There must 

 be no excitation of sensory nerves in carrying out the pre- 

 liminary part of the experiment. On the other hand, it is 

 not permissible to make use of drugs or anaesthetics to 

 depress the sensibility. In exciting the vagus steps must 

 be taken to prevent simultaneous excitation of the cardio- 

 inhibitory fibres. 



