In a previous communication,1 two of us discussed the mechanism of headache produced experimentally by the intravenous injection of histamine, emphasizing the significance of the secondary rise in arterial pressure in the production of the headache. It was found that in patients suffering from post-traumatic headaches such an injection usually "reproduced" the posttraumatic headache (i. e., was followed by a headache which was the same in character and location as the usual post-traumatic one). The conclusion was drawn that the injection of histamine activated a physiologic mechanism similar to that concerned in the production of the majority of post-traumatic headaches.
In the present paper, we wish to report the results of further studies bearing on the mechanism of headaches produced by the intravenous injection of histamine and on the relation between such headaches and various other types of headache encountered clinically. From the facts observed certain conclusions can be