PROCEEDINGS

OF

THE ROYAL SOCIETY.

"A Research into the Elasticity of the Living Brain and the Conditions governing the Recovery of the Brain after Compression for short Periods."* By A. G. LEVY, M.B. (London). Communicated by Professor V. HORSLEY, F.R.S. Received February 21,-Read March 15, 1894. (From the Laboratory of the Pathological Department of University College, London.)

A. Object and Method of the Research.

1. Introduction.-This piece of work was undertaken at the suggestion of Professor V. Horsley, who devised the apparatus employed, and to whom I am indebted for advice and suggestions.

In view of the great frequency of compression of the brain as a pathological condition, it seems very advisable to attempt to obtain knowledge of some of the elementary factors conditioning the physical changes in the brain substance due to mechanical pressure.

The general plan adopted was (a) to compress the living brain (i.e., the brain substance plus lymph and circulating blood) by a given weight for a given time, to measure the amount of direct compression, and, after removal of the pressure, to measure the elastic recoil; (b) to investigate the influence upon the recoil of varying conditions of the vascular and respiratory systems.

Finding in an early stage of the investigation that the elasticity of the brain is very far from being perfect, I was further led to investigate the conditions, vascular, &c., which favoured more complete recovery of the brain surface to its normal level, especially after periods of prolonged compression.

The following, therefore, are records of an investigation of the

* Towards the expenses of this research a grant was made by the British Medical Association on the recommendation of the Scientific Grants Committee of the Association.

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physical properties of the brain substance, and deductions drawn from my observations must not be immediately applied to explain the changes in the brain seen in cases of meningeal hæmorrhage, tumours, &c., because, in order to ascertain the fundamental facts, the experiments in the present case have of necessity been made with the cranial cavity open, the dura mater partially reflected, and a certain quantity of the cerebro-spinal fluid consequently drained away. In pathological conditions the pressure is exerted in a closed, fluid-containing cavity, and hence any local compression must evoke a reaction from all parts of the cerebral environments, and all parts of the intracranial contents are at an equally exaggerated pressure. In my experiments, the cranial cavity being opened, the influence of the cranial walls is limited, and the pressure is directly exerted upon, and principally confined to, the part of the surface compressed and the mass below it.

I may mention that I started with the generally accepted premise that the elasticity of the brain is more or less proportional to the central blood pressure, and many of my earlier experiments were performed with the object of testing what at first sight seems an exceedingly reasonable view.

2. Historical Introduction.-I have throughout my experiments taken as an indisputable fact what was demonstrated by Grashey,* viz., that the cerebral tissue itself is practically incompressible (in fact, its coefficient of compressibility being rather less than that of water), and that any reduction in volume of the living brain must therefore be due to the squeezing out from its blood vessels, lymphatics, and ventricles, of blood, lymph, and cerebro-spinal fluid.

Grashey's experiments consisted of a careful estimation of the compressibility of the dead brain by hydraulic pressure in a delicate piezometer. His experiments conclusively showed what had indeed been the general teaching, but he was led to make this demonstration by reason of the extraordinary assertion of Adamkiewicz that the brain tissue itself bears the brunt of the diminution of the intracranial space.

The records on compression of the brain substance are numerous, and the methods employed by the investigators (Cooper, Duret, Kahler, Pick, Leyden, Adamkiewicz, Spencer and Horsley, Dean, Hill, &c.) are very various; but these experiments have been arranged from the clinical point of view, i.e., the effect of compression upon the functional activity of the nerve centres.

Some observers have noted the structural changes produced; thus,

Grashey, "Ueber Hirndruck und Hirncompression," 'Allg. Ztschr. f. Psychiat.,' Berlin, 1887, 43.

+ Adamkiewicz, "Die Lehre von Hirndruck und die Pathologie der Hirncompression," 'Sitzungsberichte d. K. Akademie, Wien,' Band 88, 1883.

Adamkiewicz investigated the structures microscopically, and Dean* by quantitative analysis, with special reference to oedema.

I can find, however, no systematic investigation into the immediate recovery of the brain after compression. Dean, in the course of his experiments, gave attention to the recovery after the compression by glass dises, but his observations were made by the unaided eye. Further, the majority of his experiments, in which the brain was examined several days after compression was removed, when secondary changes had taken place, fall into another category than those which I have carried out, for in no case were my observations prolonged for more than an hour after compression, and usually the period was much shorter. Moreover, contrary to the methods adopted by the just named observers, the influence of the cerebro-spinal fluid in the present research was excluded; the dura mater being opened, the fluid is released from all tension, and some of it escapes during the procedure described below, my object being to investigate the change in the brain mass alone.

3. Description of the Apparatus for Measuring the Compression and Recovery therefrom.-An apparatus was designed for the purpose of applying known weights vertically to the brain surface, so that the amount of depression of the surface could be accurately measured, and also the recoil after the removal of the weight.

This apparatus is shown in front and back view in figs. 1 and 2. It consists essentially of two supports for screwing into the skull, a plunger through the medium of which to apply the weights, and a dial to indicate the excursion of the plunger.

To consider the parts in detail: the supporting pillars consist of three portions, see fig. 1 (a, b, and c). The foot (a) screws into the bone, and (b) is a cap which screws on to it. The two when fitted together form a socket to receive the globular end of (c) the pillar, which thus acts in a ball-and-socket joint, and can be firmly fixed in any position by screwing down (b). By this means the pillar may be made to assume a perpendicular position when the screw has to be inserted into a portion of the skull which is not horizontal.

The pillars support the dial through the medium of the slots (d, d), and are fixed by screws (e, e), the slots themselves being firmly fixed in position by the thumb screws (f, f). These slots allow, among other movements, of lateral shifting of the dial. These arrangements conveniently give great range of movement of the dial, so that the plunger may be placed into a position of accurate perpendicularity.

The plunger consists of a straight rod, carrying above a weight pan (g), and, passing through the body of the dial, it has attached to it by a special arrangement a thread which, looping round the axis *H, P. Dean, " Cerebro-spinal Pressure," Journal of Pathology and Bacteriology,' 1892.

of the dial, rotates this in its excursions up and down. An index hand attached to the axis indicates tenths of millimetres on the dial.

Below, the plunger has screw arrangements (h), by which discs (i) of varying sizes can be applied. In all experiments a disc of 1 cm. diameter was used unless otherwise mentioned, and it was by the screw attachment correctly adjusted to the surface of the brain. The weight of the plunger was accurately counterbalanced by a weight which acts over a pulley (j), so that the disc remains at rest in any position, e.g., on the brain surface, without exerting any pressure when there is no weight in the pan.

This suspended counterbalancing weight also acted as a plumb line by which the perpendicularity of the plunger was secured, this position being essential to avoid all friction. The figures show the apparatus applied to a horizontal surface.

4. Method of Experiment.-The animal, invariably a dog, was kept under ether through the medium of a glass cannula, inserted into the trachea and attached by india-rubber tubing to a funnel, this latter being placed over cotton-wool saturated with ether. The narcosis must be maintained equably throughout the experiment, to avoid irregularity in respiratory movements. The head was firmly sup

ported in the usual holder. The skull was exposed on one side, and the median and fronto-temporal ridges chiselled away as far as necessary, so as to give a flat surface for inserting the screws. The skull was trephined close to, but carefully avoiding, the longitudinal sinus. found hæmorrhage a very troublesome accident, and that it was essential to stop it by means of wax and amadou before proceeding with the experiment, as the presence of even a thin layer of blood on the cortex marred the result. The object of applying the compression as near as possible to the middle line was to arrange the line of pressure perpendicularly downwards, so as to avoid any elastic reaction from the falx cerebri; this seemed to be more important in using large weights, which evoke larger excursions. I enlarged the opening in the skull with bone forceps, so as to allow of easy insertion of the plunger, and give sufficient margin to observe the condition of the brain surface. The next step was to reflect the dura mater, insert the screws (using a guarded bradawl to make preliminary holes), and then to erect the apparatus so that the disc of the plunger rested flat on the surface of the brain, and the plunger itself was absolutely perpendicular.

The blood pressure was recorded by means of a mercurial manometer connected with the femoral artery. The respirations were traced by means of a Paul Bert receiver and a Marey recording tambour.

In performing an experiment I noted the moment of applying the weight to the pan, and then recorded the figures the index pointed to at intervals of so many seconds during and after compression, and also read off the blood pressure at stated intervals.

5. Considerations Affecting the Plan of Experiments.-A few remarks on special conditions observed during my experiments may now be briefly noted.

(a.) Various conditions of the cranium and its contents, affecting the apparent elasticity of the brain, and requiring attention, are as follows:

Adhesions of the pia mater to the dura mater, as, for instance, by vessels running from one to the other.

The Falx Cerebri.-This is not large as a rule in the dog, often not more than 5 or 6 mm. broad in a small dog, and is not likely to have much influence, but large weights applied in a direction fairly perpendicularly to its surface might reasonably be expected to be limited in effect by its elasticity.

The Tentorium.-This is generally almost totally ossified in dogs, but in some dogs there exists sufficient membrane to prove a factor in causing some elastic reaction from the base of the skull.

The Shape of the Cranium.-It seems probable that the vertical depth of the skull cavity should modify results somewhat, as with a