260 



THE CIRCULATION OF THE BLOOD 



other hand, by acting more directly on the heart so as to produce a fall in arterial and 

 a rise in venous pressure, causes at first a decided rise in cerebral pressure and later 

 a fall following the development of decided arterial hypotension. 



3. Amyl nitrite, injected into the jugular vein, causes at first a rise in venous 

 pressure and therefore in cerebral pressure. Later, however, marked arterial hypo- 

 tension develops, and the intracranial pressure declines. 



Fig. 80. To show simultaneous records of the arterial blood pressure (A), the venous pres- 

 sure (B), the intracranial pressure (C), the pressure in the venous sinuses (>). The fall in ar- 

 terial pressure produced by stimulation of the peripheral end of the vagus will be found to cause 

 a fall of intracranial and cerebral venous pressure, accompanying that in the arteries, but a rise 

 in that of the venous system. (From Leonard Hill.) 



4. During epileptic fits induced experimentally by excitation of the cortex, there 

 is a rise in venous pressure and correspondingly in intracranial pressure. In the more 

 violent convulsions produced by absinthe, there is very little change in systemic venous 

 pressure while the arterial pressure shows extreme variations, with which the intra- 

 cranial pressure runs parallel. With adrenalin, where both arterial and venous sys- 

 temic pressures rise enormously, there is of course a great rise in intracranial pressure 



