deep skull a greater depth of brain substance comes directly under compression than in the case of a flat skull, but this and the foregoing conditions have but a subsidiary influence upon the effects observed, the main factor which influences the more immediate recoil of the brain surface, in experiments performed upon the organ in situ, being the rigidity of the bony floor it rests upon.

(b.) Moulding.-After a compression of thirty seconds or so there frequently results a faint ringed impression of the disc-this usually disappearing in about half an hour. After several experiments have been performed upon the same spot, a considerable saucer-like depression in some cases ensues.

(c.) Edema.-On several occasions a curious phenomenon was observed, which most probably has its origin in local oedema. It is more especially marked in experimenting with small weights, 20 to 50 grams, applied for a couple of seconds only. When the weight, after compression, has been removed, and the brain surface, in recovering, has ceased to push the disc up farther, then if the passively resting disc be removed from the surface, the latter will start a further rise almost immediately, so that in a few minutes it may even be at a higher level than that at which it stood when the experiment was started. After more severe compression this is not observed.

(d.) Powerful respiratory movements, as will be shown directly, are liable to vitiate results by aiding recovery after compression, through the medium of increased venous pressure. Hence the necessity already mentioned of maintaining a fairly deep narcosis.

(e.) The direct effect of compression upon the blood pressure and respiration.

I found that the local compression by any weights that I employed was, in view of the cranium being freely opened, insufficient to produce any effect, either upon the blood pressure or respiration, this being so far fortunate as then could be no indirect influence from this cause upon the elasticity of the brain. The minimum weight which can affect respiration, even slightly, was 200 grams.

B. Results of Compression Experiments on the uninjured Brain, the Blood Pressure being normal.

(a.) Definitions.—I employ the term "uninjured brain" to denote that no previous experiment has been performed on the same brain ; the effect of previous compression in destroying the value of subsequent experiments is discussed later.

The term "excursion" I have applied to the extent to which the plunger enters into the cranial cavity (by a reason of sinking of the brain) on a weight being applied. Fig. 3 (ab).

1min.

FIG. 3.

"

2 min: Time in

"

"

0 15 30 45 60′′ 15 30 45 60 Seconds.

By the term "recoil" I mean the amount of recovery of the brain as measured by the instrument. Fig. 3 (bc).

(b.) Results of the Experiments.-In this section, unless otherwise stated, the weight used was 50 grams and the compressing disc 1 cm. in diameter.

i. Excursion.-The character of the excursion evoked by a compression 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 73 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 compression, such as of two seconds' duration. Under these circumstances the absolute excursion is notably less, viz., 24 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 compression 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 undoubtedly 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 1/19, fig. 4 = 1/18).

=

If, however, the pressure be kept up for only two to five seconds the recoil is more perfect. In fact, the immediate recoil which takes place in the first two or three seconds may give a ratio of 1/2 or even 1/1·5; after an interval of thirty seconds or so it may be 1/14 or even nearly 1. E.g., Experiment 12a = 1/12 (fig. 5), Experiment 14a = 1/15 (immediate).

If the pressure be prolonged, say, for six minutes, the fraction becomes smaller, and this is shown in fig. 6, where the ratio = 1/2.74, and the surface does not further recover after a long interval—more than half an hour.

Thus we see that although the elastic reaction of the brain is very fair when the compression lasts only a few seconds, longer compression affects the brain in such a fashion that it does not recover so readily.

Thus, also, we see that for the same weight and same length of compression, the elasticity is very much the same in different brains; i.e., with a large excursion there is a proportionately large recoil, and vice versa.

iii. Results of Experiments with varying Weights.-A few experiments performed with a weight of 20 grams show a comparatively small excursion, but the ratio is similar to that obtained with a 50 grams weight. Weights such as 100 grams and over seem to be