"This elevation of temperature is especially wellmarked in the region of the occiput."

These experiments, as we see, apply only to the appreciation of temperature externally estimated, on the skin of the cranium. The brain was not directly investigated.

Schiff has supplied this omission, he has entered the cranium, and by means of thermoscopic instruments of extreme sensibility has succeeded in directly examining the cerebral substance at the moment when it came in contact with external excitations, and thus determining what degree of elevation of temperature the brain is susceptible of attaining in its operations.

This ingenious physiologist has therefore succeeded in defining not only what regions of the cerebral cortex are isolatedly called into play by such or such kinds of sensorial impressions, and demonstrating experimentally that there are isolated circumscribed spots reserved for such or such kinds of sensorial impressions (as has already been described on the authority of anatomy); but also that the arrival of these impressions resolves itself into a local development of heat in the special area where it disseminates itself; and that the heat thus developed is a dynamic phenomenon independent of the circulatory activity, a true vital reaction of the sensorium-that, in a word, it is the direct result of the participation of the psychic element on the arrival of the sensorial excitation.

Thus," he says, "the psychical activity, independently of the sensorial impressions which call it into play, is connected with a production of heat in the nervous centres, a greater amount of heat than that which simple sen

sorial impressions engender. This conclusion is justified by the decrease of the calorific effect of a strong and always identical sensorial impression, which animals have been made to experience many times in succession. Let us take a pullet," he adds, "whose sight or hearing we assail by appropriate means. The first impression which the unprepared animal receives will excite in it more intense psychical reflex actions than the succeeding excitations of the same nature, since it insensibly becomes habituated to them." Thus, by eliminating gradually the part played by psychical action in sensorial absorption, he arrives at an estimate of the heat evoked by the arrival of simple sensorial impressions, and that which proceeds from the direct participation of the psychical activity at the beginning of the experiment.*

We thus understand, after this series of experiments, how prolonged efforts of the mind, and moral emotions of all sorts, from the very fact of their awaking the activity of the sensorium, are calculated to have an immediate effect upon the essential phenomena of the nutrition of the brain.

They show us, indeed, on the one hand, that sustained intellectual work is accompanied by a loss of phosphorized substance on the part of the cerebral cell in vibration; that it uses it up like an ignited pile which is burning away its own essential constituents; and that, * Schiff, 1.c., "Archives de Physiologie," 1870, p. 451.

+ Louyer-Villermay cites the example of a celebrated lawyer who lost his memory in consequence of too long-continued intellectual work; and Moreau de la Sarthe reports a similar case which occurred in a German savant, after an intense concentration of mind. We also know of a great number of musicians who have become deaf by the immoderate exercise of the organ of hearing. "Journal d'hygiène," 1875. (De la surdité chez les Musiciens, par Dr. Prat.)

on the other hand, all moral emotion perceived through the sensorium, all effective participation of this same sensorium in an excitation from the external world, becomes at the same time the occasion of a local development of heat.

These facts are destined to have a direct effect upon our knowledge of the essential conditions of the integrity of the cerebral functions, and to formulate absolute hygienic principles with regard to them.

It stands to reason, indeed, that if the cerebral cell expend its reserve material during its diurnal activity, it is absolutely necessary, to enable it to continue alive and in health, that it shall repose and sleep regularly. Sleep is to the brain, what needful repose is to our fatigued limbs, the necessary condition of its health. Every one knows, indeed, how great is the number of individuals who have sown the seeds of a cerebral disease by a prolonged infraction of these simple laws of hygiene, and who through reiterated vigils and exaggerated expenditures of activity, have thus passed the physiological limit of the resources at their disposal, and incurred expenditure above their receipts.

On the other hand this development of heat, which is produced in certain circumscribed localities of the brain when an emotion or sensorial impression is reverberating through the plexuses of the sensorium, further shows us with what circumspection we should manage this kind of excitation in individuals whose brain is in a painful condition, either from a recent congestion, or from former congestions grafted one upon another.

We all know from more or less personal experience, that when we have a headache, and our sensorium is

in a state of hyperæsthesia, the smallest noises, the slightest external incidents, produce in us painful shocks, and that the absolute incapacity for work is most painful.

All doctors know how often, in persons excited by the occurrence of repeated cerebral congestions, paralytics, maniacs, and even patients with certain forms of melancholia, the unexpected calling up of an old emotion, the sight of a relative, may have a sad effect upon their cerebral condition. We see, indeed, their faces redden and grow pale, and very often the effect of an emotion inopportunely provoked, is but the prelude to the return of more and more serious congestive accidents.

PART II.

GENERAL PROPERTIES OF THE NERVOUS

ELEMENTS.

THE phenomena of the life of the nervous centres, spite of their apparent complexity, are nevertheless regulated by laws which are in general simplecommon principles, which indisputably give them an air of near relationship. These common principles are, moreover, themselves reducible to elementary vital properties, which form the basis of each of them in particular, and constitute, in a manner, the simple primordial principles which we constantly find underlying every combination of nervous activity, however complicated it may be. These fundamental properties, which thus serve as elementary materials for every dynamic action of the system, may at the present day be thus epitomized, under three principal heads :

1. Sensibility, by which the nerve-cells feel excitation from without, and react in consequence, by virtue of the excitement of their natural affinities.

2. Organic Phosphorescence, by which the nervous elements, like bodies which have received the vibrations of light, preserve for a prolonged period traces of the

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