Pressure on the Circulation of the Blood, 



479 



as clue to the diminution of the volume of the intestines — which results 

 from the compression of the intestinal gas — and the consequent in- 

 creased play of the intrathoracic organs. He was confirmed in this 

 opinion by the fact that the substitution of oxygen for air made no 

 difference in the results. 



A. Smith"^ attributes the symptoms which arise in caisson workers 

 partly to the effect of the increased pressure on the nervous tissue, 

 and partly to the congestion of the blood in the neural axis. He sup- 

 poses that the blood is driven by the compressed air from the periphery 

 to the cranio-vertebral cavity. This mechanical explanation of caisson 

 disease is contrary to the supposition which,, theoretically, seems cor- 

 rect, viz., that the atmospheric pressure is equally transmitted by the 

 blood to all parts. 



I have put the matter to the test of experiment. 



Method. — An anaesthetised dog or cat is placed at the bottom of an 

 autoclave after the insertion of a cannula in the carotid artery, and, in 

 some cases, of one . in the vena cava superior, has been carried out. 

 The cannulas are connected with Hg. monometers, and these, together 

 with a slow-movement recording drum, are also placed in the autoclave. 



The drum is then started and the monometers set to record the 

 arterial and venous pressures. The lid of the autoclave is next screwed 

 on, an oxygen bottle connected with the inlet tube and the pressure in 

 the autoclave rapidly raised to + two atmospheres (30 lbs.). The 

 outlet tap is finally opened, the pressure rapidly lowered, the lid taken 

 off, and the record observed. The times of increasing and diminishing 

 atmospheric pressure are noted with a watch. The whole operation 

 only takes two or three minutes. 



Besults. — Although exposed to this rapid change of atmospheric pres- 

 sure, the circulation of the blood scarcely varies. The arterial pressure 

 and venous pressures either continue at the same level or very slightly 

 fall. The respiratory oscillations are increased, and the pulse becomes 

 slightly less frequent. The gas injected contains, roughly, 80 per cent, 

 oxygen, and thus the oxygen tension is raised from 21 per cent, to about 

 190 per cent, of an atmosphere. The results are the same whether 

 compressed air or oxygen are employed. 



Condimon. — The mechanical congestion theory of caisson disease is 

 untenable. 



The expenses of this research have been met by a grant from the 

 Eoyal Society Government Grant. 



* Article " Caisson Disease," ' Allbutt's System of Medicine,' vol. 7, p. 88, 

 1899. 



