168 THE DUCTLESS GLANDS 



(see Fig. 42). In some cases Oliver and Schafer noticed in 

 organs enclosed by a plethysmograph an apparent struggle 

 between the diminution in size resulting from contraction of 

 the arterioles and the expansion due to swelling of the larger 

 bloodvessels, and some of their curves show a passive dilation 

 at the beginning of the effect of an injection, followed by a 

 prolonged diminution in size due to a more marked contraction 

 of smaller arteries having supervened. The medium -sized 

 arteries also participate in the dilation. 



The contraction of the arterioles occurs in a frog with its 

 nervous system destroyed, as stated above. It also occurs 

 after section of the spinal cord and after section of the nerves 

 going to the limb. Therefore the contraction must be due to 

 the direct action of the active principle of the adrenal medulla 

 upon the muscular tissue of the bloodvessels. This question 

 as to the precise tissues upon which adrenin acts will be referred 

 to again later on. 



The rise of blood-pressure occurs after a certain interval of 

 latency (twenty seconds in the dog) occupied by the passage 

 of the extract from the vein into the arteries. The rise takes 

 place, whether the vagi be cut or not, and whether atropin has 

 been injected or not. But it is much greater after section of 

 the vagi or after injection of atropin, because of the concomi- 

 tant effect of the injection upon the cardiac inhibitory centre 

 in the medulla (see Fig. 41). The rise is rapid, but only lasts 

 a few minutes. During the rise the Traube-Hering curves are 

 abolished, and in the cat and the rabbit the effect of stimula- 

 tion of the depressor nerve is in abeyance. Sometimes a rise 

 followed by a fall is obtained, and sometimes a pure fall. 

 These effects will be referred to again later. 



But the rise of blood -pressure is due not only to constriction of 

 arterioles, but also to increased rate and energy of the heartbeat. 



Another striking phenomenon noticed by Oliver and Schafer 

 was cardiac inhibition through the vagi. Sometimes in the 

 earlier stages of the action of the extract, the heart, instead of 

 being augmented and accelerated, is strongly inhibited. When 

 the vagi are cut or atropin administered, this effect is abolished, 

 and the constriction of the arterioles, combined with the aug- 

 mentation of the heart, produces an enormous rise of the blood- 

 pressure. The cardiac inhibition is of central origin, but the 

 augmentation is due to the direct action on the heart. Some- 



