Experiments 25c and 266 show that a venous congestion of brain (as indicated by its expansion) is not necessarily accompanied by an increase of elasticity.

The failure to obtain changes in elasticity in every case must, I think, be attributed to the variation in calibre of the various channels of outflow, chiefly, probably, as regards the communications with the vertebral plexus; the common occurrence of variations in the size of the superior cerebral is patent after a few dissections.

E. Experiments illustrating the Influence which varying Vascular Conditions have in Restoring the Volume of a Brain which has undergone Prolonged or Severe Compression.

(a.) Influence of a Rise of the General Blood Pressure.—After the brain has been compressed as described for six minutes, the elasticity, as before mentioned, is considerably reduced, and the brain surface continues depressed for a long period. If now the etherisation be diminished, the blood pressure may be caused to rise gradually, but considerably, without anything more than a comparatively insignificant rise in the level of the brain surface accompanying it (see fig. 10, in which the blood pressure rose 35 mm. in the space of 12 minutes). Thus we find a notable difference in the reactions of normal and compressed brains to increase of blood pressure. As is well known, in the uninjured brain the surface rises and falls concurrently with variations in the blood pressure.

(b.) The Influence of Venous Congestion produced by Asphyxia.-In cases of prolonged or severe compression the brain surface may nearly always be made rapidly to recover by inducing asphyxia In an experiment (fig. 11) the brain had been severely compressed by repeated experiments on the same spot, so that the recoil was small and had become stationary in one minute. A rise of blood pressure of 30 mm., induced by diminishing the degree of etherisation, had no appreciable effect, but total recovery rapidly supervened with the development of asphyxia. This recovery cannot be ascribed to the rise in the general blood pressure induced by the asphyxia (in this case only 10 mm., which contrasts with the 30 mm. produced by diminishing the ether supply). This is also borne out by the fact that the recovery of the surface may be obtained when there is no rise of the blood pressure, as sometimes occurs during asphyxia, or the expansion may actually progress whilst the blood pressure is in its stage of fall.

Frequently the surface commences to recover when the asphyxial gasps are at their strongest, but recovery will go on when the respirations are small or even when they cease, as they will sometimes, directly the trachea is clamped.

Index figures.

FIG. 10.

1.0

On 2 3 4 5 6 7 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

The extent of recovery varies in different cases; rarely it fails to take place at all.

It is important to notice that not only does the general surface of the hemisphere rise, but that the saucer-shaped depression due to compression is obliterated.

To obtain a definite idea of the extent to which the pressure in the cerebral veins rises during asphyxia, I took the venous pressure during asphyxia from the superior cerebral in two dogs. In one case in which the normal pressure was very low, about 15 of water, it quickly rose to 8 cm., and in the other in which the normal was 6.5 cm. water, after the usual preliminary fall, it rose again to 7.5 cm. The asphyxial gasps when strong may cause a variation in pressure amounting to 1 cm. water.

min.

FIG. 11.

Time in minutes.

On30" 2′ 3′ 4′ 5′ 6' 7' 8' 9' 10' 11' 12' 13' 14'

12.0

11.5

11.0

10.5

10.0

9.5

9.0

8.5

8.0

7.5

7.0

6.5

6.0

(c.) Venous Pressure Varied by Artificial Respiration.-By this means the surface may often be made to recover completely, or nearly so.

Conclusion.

The foregoing experiments show clearly that the elastic recoil of the living brain is due primarily to the elasticity of the brain substance, and is not due to the pressure in the arterial system (general).

The recoil, however, varies notably with changes in the pressure in the venous system (cerebral).

Blood pressure.

DESCRIPTION OF DIAGRAMS.

The continuous lines represent the curves of the variations in level of the brain surface produced by pressure and otherwise. The dotted lines represent the height of the blood pressure.

The times in which compression and recoil are measured are noted above the diagrams in seconds. The enumeration of seconds always commences afresh at the point at which the weight is removed.

Fig. 3. 50 grams applied for 1 minute. This diagram shows typically the characters of the excursion and recoil.

Fig. 4. 50 grams applied for 1 minute. Between experiments a and b the brain was allowed to entirely recover its volume, and the animal was then bled to the extent of 150 c.c. This procedure caused a fall of the brain surface to 3.4.

Fig. 5. Shows a typical curve when 50 grams are applied for 4 seconds only. Fig. 6. The curves as far as show the concurrent variations of brain surface and blood pressure under the influence of amyl nitrite. At* experiment a was performed. Between experiments a and b the effect of the amyl nitrite was allowed to wear off completely. 20 grams was the weight employed.

Fig. 7. (a) was an experiment upon the normal brain.

(b) was performed after administration of amyl nitrite. Weight 50 grams.

Fig. 8. (a) performed with a cannula in one superior cerebral vein.

(b) performed with the opposite superior cerebral also blocked. Weight

Fig. 9. (a) performed on the normal brain.

(b) performed with both external and both internal jugulars clamped.

Fig. 10. 50 grams applied for 6 minutes. Etherisation diminished at * with a consequent rise of blood pressure.

Fig. 11. 50 grams applied for 30 seconds to a brain which had undergone considerable previous compression. Etherisation diminished at *. Trachea clamped at O.

"On the Temperature of the Carbons of the Electric Arc; with a Note on the Temperature of the Sun. Experiments made at Daramona, Streete, Co. Westmeath." By W. E. WILSON, M.R.I.A., and P. L. GRAY, B.Sc., A.R.C.S. Communicated by G. JOHNSTONE STONEY, F.R.S. Received November 14,-Read November 22, 1894.

The temperature of the positive pole of the electric arc, which is now generally believed to be the boiling point of carbon, is usually taken, on the authority of Violle,* as approximately equal to 3500° C. Violle's method of determining it was as follows:-The carbons of the arc were placed horizontally, and the positive pole was so arranged

* Violle, 'Jour. de Phys.,' 3rd Series, vol. 2, 1893, p. 545.

that pieces of its substance could be detached while the arc was passing; these white-hot pieces fell into a calorimeter, and from the amount of heat given up, the temperature was calculated, assuming the specific heat of carbon at this point to have its theoretical value. The method does not seem at first sight a very reliable one, and Violle states that the result is only to be regarded as an approximation.

The method adopted by the authors of this paper is exactly the same as that which they employed last year in their "Experimental Investigations on the Effective Temperature of the Sun,"* in the account of which full descriptions of the apparatus used, &c., are given.

A Brockie-Pell arc lamp was employed in the experiments, the current being obtained from a dynamo worked by a gas engine. It would have been preferable for some reasons to have worked the arc off the 26 Epstein accumulators which we had at our disposal, but the current from these was used in heating the platinum strip, and we did not wish to run the cells off too quickly. Platinoid resistances were inserted in circuit with the arc until it burnt steadily.

The general arrangement of the apparatus is shown diagrammatically in Fig. 1.

P is the platinum-strip radiator (our modification of Joly's meldometer), the dotted line representing the water-jacket which is placed over the strip. R is the radio-micrometer; A and B are tubes through which radiation can pass to fall on the receiving surfaces within. The diameter of the aperture at A is accurately known; as also is its distance from the receiving-surface, so that the apparent area of platinum, as seen from the latter, may be calculated.

L is the lamp, which is placed inside a wooden box, lined internally with tin-plate, both wood and metal being pierced with small holes opposite to the arc.

A screen, S, hangs in front of the box, and contains a small, carefully-measured hole, which can be adjusted until the brightest (or any) part of the glowing carbons shines directly into the tube B. * Wilson and Gray, Phil. Trans.,' A, vol. 185, 1894, p. 361.