Brain and the Conditions of Recovery after Compression. 13 



sure of 125 mm. down to one of 20 mm.). But, under these circum- 

 stances, the fall is due to a draining of blood from all parts of the 

 body, the brain included, and hence the latter begins to collapse soon 

 after the bleeding has been started. This collapse, however, is not 

 always great. Thns, in one experiment (fig. 4), after allowing the 

 brain to recover from the trial experiment, and then bleeding 150 c.c., 

 the fall of blood pressure of 60 mm. thus produced only caused the 

 brain to collapse so that its vertical thickness was reduced by not 

 more than 2'4 mm. 



Of course, this collapse produces a new set of intracranial condi- 

 tions, e.g., the brain contains less fluid, and hence its texture is altered ; 

 it does not fit its surrounding's so closely its vertical depth is less. 

 These new conditions act in diverse ways as regards its elasticity. 



But we cannot disregard the fact that the fall of blood pressure 

 measured in the femoral artery is very great, and, were there a,ny 

 measurable direct relation between the elastic reaction and the blood 

 pressure, this fall, especially when the brain collapse was not great, 

 would make itself evident in spite of the altered conditions under 

 which the second experiment ha<? to be performed. 



The following table gives the results of experiments of this 

 kind (p. 14). 



Summary. In Nos. 4 and 8 there is a distinct diminution in recoil. 

 Tn No. 6 the recoil is exactly the same in both experiments, and these 

 being conducted under comparatively favourable circumstances as 

 regards the fall of brain surface, this result is of more value than the 

 others. 



In No. 7 there is an actual increase in " elasticitv." 



In every case the excursion is diminished after bleeding. 



It will thus be seen that there is no constant relation between the 

 fall of blood pressure and the proportionate recoil, and that in those 

 cases in which a lowered pressure is accompanied by a lowered elas- 

 ticitv, the diminution is very small, and in no way commensurate 

 with the fall. A series of bleeding experiments, performed without 

 registering the exact fall of blood pressure, gave results similarly 

 divergent. 



(c.) Comparison of the "Results of Experiments in which the Blond 

 Pressure was Varied by means of Amyl Nitrite. It was now evident 

 that, if possible, a method should be found of reducing or raising the 

 central arterial pressure without causing an alteration in the level of 

 the brain surface. In order to do this, the blood supply to the brain 

 must be kept up or diminished according as the central blood pres- 

 sure falls or rises, i.e., when the pressure falls the cerebral vessels 

 should dilate, and rice versa. 



Now, supposing a fall could be produced by a general vaso-motor 

 action, brain arteries being included, here we should have the condi- 



