Brain and the Conditions of Recovery after Compression. 15 



tion required. The presence of vase-motor fibres to the cerebral 

 arteries has been denied, or, at any rate, they have never been demon- 

 strated ; but Roy and Sherrington* have shown that the calibre of 

 the cerebral vessels may be altered by certain substances, notably 

 acids and alkalies, and they consider this a direct action on the vessel 

 walls. It occurred to me to try the effect of amyl nitrite, and I 

 found that I could obtain a good fall of blood pressure, whilst the 

 brain surface would remain at the normal level, or, if previouf'y de- 

 pressed, it would rise. 



It is an interesting fact, and one which has a bearing on the vaso- 

 motor question just referred to, that the subsequent recovery of blood 

 pressure is accompanied by a/a/Z of the brain surface. This is well 

 shown in fig. 6. 



It is obvious from my experiments that the brain is, tinder the 

 effect of the nitrite, undoubtedly flushed with blood to an extent 

 sufficient to neutralise its tendency to collapse; whether by vaso- 

 motor or local action on the vessel walls it matters little, as far as 

 the purposes of my experiment go. I see no reason to believe that 

 the flushing is a result of increased venous pressure, this being the 

 alternative explanation usually suggested. I administered the drug 

 by means of clinical capsules, one or two at a time, crushed, and 

 placed under the ether funnel. I thus obtained differences in the 

 general blood pressure of from 20 to 30 mm. of mercury, and yet the 

 brain surface remained at a constant, or nearly constant, level. 



The chief difficulty in this method lay in the short time during 

 which the fall lasted, thus necessitating very prompt action so as to 

 apply the compression at the right moment. 



The results of four experiments are tabulated below (p. 16). 



It is seen that in each case we find that with the lowered blood 

 pressure the elasticity is increased, markedly so in Nos. 17, 18, 

 and 20, thus still further emphasising the fact that the elasticity 

 does not, or at any rate need not, depend on central blood pressure. 

 It is also seen in the experiments performed under the influence 

 of the nitrite that the excursion is less. Thus, then, we find a true 

 increase of recoil under the vascular conditions induced by this 

 drug. 



D. Comparison of the Results of Experiments in which the Effect of 

 Increased Venous Pressure on the Recoil of the Brain was Measured. 



(a.) Anatomical Considerations, These experiments were under- 

 taken in view of the marked influence which variations in venous 

 pressure are held to exert on cerebral vascular conditions. 



* Roy and Sherrington, " Eegulation of the Blood Supply to the Brain," 

 ' Journal of Physiology,' vol. 11. 



