The excitability of the brain may be altered either by conditions which modify the nerve-cells or the circulation. A deficient circulation greatly depresses the excitability, and it is very low when much haemorrhage has occurred.

One method of investigating the action of drugs on the excitability of the brain consists in trephining so as to expose the cortical substance and then stimulating it by a faradaic current before and after the administration of a drug either by inhalation or injection. Another method has been employed by Albertoni, who first trephines on one side, and having estimated the strength of current sufficient to produce an epileptic convulsion when applied to a motor centre, he allows the wound to heal, and then gives for a length of time the drug on which he wishes to experiment. He then exposes the corresponding motor area on the other side and observes whether the strength of current required to produce an epileptic convulsion is greater or less than before.

The excitability of the motor centres is greatly lowered by anaesthetics, so that as anaesthesia becomes deeper, irritation of the motor centres has less and less effect, and when anaesthesia is very profound, such irritation has no action whatever.1 The motor centres, however, are less affected than the sensory ones by anaesthetics, so that they will still react to faradaic irritation when the sensation of pain has been completely abolished.

Alcohol also diminishes the excitability of the motor centres, so that the epileptic convulsions which usually follow the application of strong currents to the cortex are less readily produced after its administration, as well as after ether and chloroform.2 Chloral for a time diminishes the excitability of the brain, lengthening the latent period, so that stronger currents or more numerous stimuli must be used to produce a result: it will temporarily abolish the excitability. Cold (not freezing) greatly lowers or destroys excitability, and this may be followed by a period of increased excitability with a shorter latent period.3

Bromide of potassium, according to Albertoni, when given for several weeks together, greatly diminishes the excitability of the motor centres, so that when dogs are thoroughly under its influence it is almost impossible to produce epileptic convulsions by irritation of the cortical substance. Atropine in small doses increases the excitability of the brain in monkeys, but in large doses paralyses it. It greatly increases the tendency to epileptic convulsions in dogs, so that they can be produced by very much slighter stimuli than usual, and strychnine, absinthe, and canna-bin have a similar action in this respect.1 Physostigmine appears to increase the excitability of motor centres in the brain; for when guinea-pigs have been rendered epileptic by section of a sciatic nerve, the administration of physostigmine greatly increases the number of fits.

1 This was observed in the case of ether by Hitzig, Untersuchungen uber das Gehirn, Berlin, 1874. I have had several opportunities of observing the same thing in regard to chloroform when assisting my friend Dr. Ferrier in experiments on the brain.

2 Francois-Franck and Pitres, op. cit.

3 De Varigny, Recherches experimentales sur l'excitabilite electrique des circon-volutions cerebrales et sur la periode d'excitation latente du cerveau. Paris, 1884, p. 138.