8 Mr. A. G. Levy. On the Elasticity of the Living 



i. Excursion. The character of the excursion evoked by a compres- 

 sion of 50 grams for one minute is as follows : 



Firstly, there is a rapid fall of the index in the first few seconds ; 

 this being followed by a very much more gradual compression 

 (fig. 3). So that three-fourths of the whole excursion may take 

 place in the first two seconds. 



The absolute extent of the excursion varies in different dogs very 

 considerably (the time of compression being equal) ; thus the amount 

 in one minute varied in two separate cases from 4'5 mm. to 7'3 mm. 

 Variation in the absolute amount of depression of the brain surface is 

 to be expected where differently shaped heads were experimented 

 upon. 



There appeared to be some relation between the size of the dog and 

 the extent of the excursion, probably because there is in a large brain 

 a greater depth of collapsible mass beneath the plunger. On the 

 other hand, it was very noticeable that the extent bears no relation 

 to the height of the blood pressure ; in fact, it is curious that often 

 the largest excursions accompanied a normally high blood pressure 

 (i.e., a well-filled, highly collapsible brain). 



Observations were also made with a very short time of compres- 

 sion, such as of two seconds' duration. Under these circumstances 

 the absolute excursion is notably less, viz., 2'4 to 3'5 mm. I have 

 already mentioned the rapidity of the development of the first part of 

 such a curve. The actual curve is shown in fig. 5. 



ii. Recoil. The character of the recoil after one minute compres- 

 sion with 50 grams is well shown in fig. 3. As a rule it is nearly 

 completed in a few seconds, but develops further during one to two 

 minutes, and then usually stops. After this point the process un- 

 doubtedly continues in many cases, but it is too gradual to record. 

 After half to three-quarters of an hour, the brain in most cases will 

 be found to have resumed its normal level. The recoil was measured 

 at the point at which it became apparently stationary. 



For convenience of expression, to denote the relation of the recoil 

 to the compression, I employ a fraction, the numerator of which 

 is the numerical value of the recoil expressed as unity, and the 

 denominator is the value of the excursion compared to the recoil. 

 Thus, where the excursion is twice the recoil the fraction = 1/2 ; and 

 to the ratio I apply the term " proportionate recoil." The smaller 

 the fraction the less the elasticity; the nearer it approaches unity the 

 more perfect the elasticity. 



We are now in a position to see the degree to which the brain 

 recovers itself after a compression of 50 grams for one minute. The 

 proportionate recoil is very constant, i.e., nearly 1/2, in many cases 

 almost exactly so, but varying within the extreme limits of 1/1"8 to 

 1/2-3 (e.g., fig. 3 = 1/1-9, fig. 4 = 1/1-8). 



