This is that new state of cerebral ischmia, opposed to the phase of congestive activity, which as an alter native fact of the general order that exists in the brain of all living beings, inevitably reveals itself whenever their cerebral cells, having exhausted their accumulated nervous forces, become fatigued by exercise and fall into the physiological collapse of sleep.* Where the life of the nervous elements is stilled, a stillness also takes place in the most minute currents of the circulation, and these two phenomena, which act and react on one another in the ascending phases of activity, similarly affect each other in its descending phases. When the vital movement becomes slack, and histological sensibility dull, the demand upon the blood is less imperious.
2.—If, from a chemical point of view, the phenomena of cerebral activity are characterized and gauged in a precise manner by the real loss of brain-substance, and the passage of ,a certain quantity of phosphorized matters in the urine, from a physical point of view they present characters which are no less significant, and no less important to recognize.
The authors who have already occupied themselves with the question, as to what appreciable physical modifications are presented by the brain-substance while in activity, have noted in a precise manner that this inward labour reveals itself by sensible signs, in the form of a more intense disengagement of heat ; and that the brain, like a muscle in action, manifests its dynamic power by a local increase of heat, appreciable by the instruments of the physical laboratory.
Thus, Lombard (of Boston), who was the first to institute experiments in this direction, arrived at the following results, by means of very exact thermoelectric apparatuses :— " In the condition of cerebral repose," he says, "during wakefulness, the temperature of the head varies very rapidly. The variations are very slight, not attaining of a degree centigrade, but they are not the less worthy of attention, for this reason—that they are confined to the head.
" The variations of temperature appear to be con nected with different degrees of cerebral activity. During active brain-work it never exceeds of a degree centigrade.
" Every cause that attracts the attention—a noise, or the sight of an object or a person—produces elevation of temperature.
"An elevation of temperature also occurs under the influence of an emotion, or during an interesting reading aloud.
" This elevation of temperature is especially well marked 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
Schiff has supplied this omission, he has entered the cranium, and by means of thermoscopic instruments of extreme sensibility has succeeded in directly exam ining 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 con nected 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 succeed ing excitations of the same nature, since it insensibly becomes habituated to them." Thus, by eliminating gradually the part played by psychical action in sen sorial 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 experi ment.* 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 sensorial, are calculated to have an immediate effect upon the essential phenomena of the nutrition of the brain.