308 THE CIRCULATION OF THE BLOOD 



circulating blood, by means of the vital red method of Keith, Rowntree and 

 Geraghty, 52 described elsewhere (page 86). 



Although the oligemia is due in great part to dilatation of the capillaries 

 and venules of the intestine, as can be shown by inspection, it is also partly 

 dependent upon dilatation of vessels elsewhere, since histamine shock can 

 be induced in animals from which all of the intestines have been removed. 

 The vessels of the skeletal muscles are probably the chief extraabdominal 

 vessels affected, for although no dilatation of these can ordinarily be seen 

 in histamine shock, it becomes quite evident in animals which have been 

 transfused before being shocked. The capillaries (and venules) in these 

 areas evidently lose their tone so that they become too roomy for the 

 available blood. As a matter of fact Dale and Richards 53 have shown that 

 histamine abolishes the tone of capillaries at the same time that it in- 

 creases the permeability of the walls and so permits the plasma to leak 

 through. It is on account of this latter action that histamine when it is 

 rubbed on the scarified skin soon causes the formation of a wheal like that 

 following the lash of a whip. 54 



When histamine is given to unanesthetized animals about ten times 

 as much can be withstood as in those that are anesthetized with ether. 38 

 At first sight this result might seem to discount the observations on ether- 

 ized animals, but on the 'contrary they greatly enhance their importance. 

 They indicate that whereas the normal animal is able to combat the toxic 

 action of histamine, ether greatly depresses this power, an observation 

 which agrees remarkably with the clinical experience that administration 

 of ether is most dangerous in persons who are threatened with shock. The 

 poisoning effect of ether persists for some time after the anesthetic is re- 

 moved, and it is no doubt dependent upon a toxic action on the endothe- 

 lium of the capillaries, for it is particularly in such animals that concen- 

 tration of the blood is evident after histamine. It is of great significance 

 that histamine did not readily produce shock in nitrous oxide anesthesia. 



Hemorrhage also greatly predisposes to histamine shock, but in this case 

 the blood is not nearly so concentrated as ordinarily because of the passage 

 of plasma from the tissue spaces into the vessels, which, it will be remem- 

 bered, is the natural reaction of an animal to hemorrhage alone. The 

 cause of shock in such animals is mainly the opening up of the vessels. 



Many bacterial toxins, both when applied to scarified skin and when in- 

 jected intravenously, have effects very like those of histamine. It is also 

 well known that shock is peculiarly common after injuries in which there 

 has been extensive destruction of tissue. The facts warrant the suggestion 

 that shock may be due to liberation from damaged tissues, particularly 

 the muscles and the viscera, of toxic substances acting like histamine. This 

 conforms with the fact that shock is most common when there has been 



