276 Mr. A. G. Levy. A Research into tlte [Mar. 15, 



some knowledge of the elementary factors conditioning the physical 

 changes in the brain substance due to mechanical pressure. 



Grashey* has already definitely proved that the coefficient of com- 

 pressibility of the dead brain is slightly less than that of water, 

 which is equivalent to saying that under any ordinary pathological 

 conditions the brain tissue is incompressible. At the suggestion of 

 Professor V. Horsley, who devised an apparatus for the purpose, and 

 to whom I am indebted for advice and suggestions, an attempt was 

 made to determine the elasticity of the living brain, i.e., of the brain 

 mass with its full complement of circulating blood and lymph, and 

 further to test the truth of the generally accepted view that the 

 elasticity of the brain is proportional to the blood pressure. 



The brain was experimented upon in situ, the influence of the 

 cerebro-spinal fluid being excluded by the mere fact of the skull 

 being opened and the membranes partially reflected, thus releasing 

 the fluid from all tension. Thus we arrive at the properties of the 

 brain mass itself as it rests upon its osseous enclosure. 



There were, in the first place, considered the results of experiment 

 performed upon freshly-exposed brains, the blood pressure being 

 its normal height, in order to arrive at some conclusion as to tl 

 normal elasticity of the brain. The downward movement of a 

 plunger was measured after a given interval of time, and the extent 

 of this is termed the "excursion." The recovery of the brain 

 surface, to the exteut of which the term "recoil" is applied, was 

 measured until it failed to perceptibly develop further. The general 

 character of the excursion is that of a rapid plunge downwards 

 taking place within the space of one or two seconds, followed by a 

 much more gradual compression. Its extent varies within a con- 

 siderable range in different animals ; thus a weight of 50 grams 

 applied for one minute produces an excursion which varies in different 

 dogs from 4'5 to 7'3 mm. It also varies notably with the weight em- 

 ployed. There is no obvious relation between the depth of excursion 

 and the blood pressure. 



For convenience of expression and of comparison the ratio of the 

 excursion to the recoil is in the full paper expressed in the form of a 

 fraction, and this ratio is termed the " proportionate recoil ; " the 

 smaller this fraction the feebler the elastic reaction it denotes, and 

 vice-versa. A comparison of the ratios derived from experiments per- 

 formed under like conditions upon different animals brings out the 

 striking approximation which they bear to one another, whatever the 

 length of the excursion and whatever the existing blood pressure may 

 be. And such deviations from the average as the ratios present cannot 

 be found to have any relation to variations in either excursion or blood 



* G-nwhey, " Ueber Hirndruck und Hirncompression." ' Allg. Ztschr. f. Psychiat.,' 

 Berlin, 1887, vol. 43. 



