304 THE CIRCULATION OF THE BLOOD 



Experimental Investigations of Shock 



In the investigation of a problem of this nature little real progress 

 can be made unless it is possible to reproduce the condition by experi- 

 mental means on laboratory animals. The various factors which con- 

 tribute to bring about the shock can then be investigated under con- 

 trolled conditions and a rational therapy evolved. It was only after 

 attacking the problem of shock in this manner that it became possible 

 to treat the condition in man with any measure of success. For inducing 

 shock experimentally several methods have been employed, of which the 

 following are important; 1, rough manipulation of the abdominal vis- 

 cera; 2, repeated electrical stimulation of large afferent nerves; 

 3, applying a clamp, off and on, for a little over two hours to the 

 inferior vena cava just above the liver, or to the aorta, to such a degree 

 that the arterial blood pressure is kept at about 40 mm. Hg. 31 ; 4, mas- 

 sive injections of adrenaline. Since the experiments are usually per- 

 formed on anesthetized animals, the effect of the anesthetic is a contrib- 

 utory factor in producing the shock. 



Having induced a condition of shock, the first step in an investiga- 

 tion into its cause consists in a differentiation of the symptoms into pri- 

 mary and secondary. 



The earlier investigators were naturally attracted to the pronounced 

 fall in blood pressure as the most outstanding symptom in shock; and 

 attention was directed to its cause. These might be either a lowering of 

 peripheral resistance or a diminished output of blood from the left ventri- 

 cle. It was believed by Crile that the former was the cause, and that it 

 developed because of a universal dilatation of the arterioles brought about 

 by exhaustion of the tone of the vasoconstrictor center. It has been 

 clearly shown, however, that the tone of this center is practically normal 

 in shock, and that the arterioles are maintained not in a dilated but in a 

 contracted state, indicating clearly therefore that the low blood pressure 

 must be dependent upon inadequate output of blood from the heart. The 

 evidence for this conclusion is as follows: (1) W. T. Porter 26 and his col- 

 laborators have shown that both pressor and depressor reflexes (page 243) 

 are perfectly normal in a rabbit that is in a condition of extreme shock. It 

 is particularly important that depressor effects are still obtained in shock, 

 since this indicates that tonic activity of the center must still be present. 

 (2) Morrison and Hooker 29 found that the outflow of blood from the or- 

 gans of a shocked animal, when these are perfused through their blood 

 vessels with the organ in situ, is less than that from the same organs under 

 normal conditions. Furthermore, severing of the nerve of such an organ 

 has the usual effect of causing an increased outflow. (3) This same fact has 

 been shown by Seelig and Joseph, 27 who cut the vasomotor nerve proceeding 



