Page:Popular Science Monthly Volume 33.djvu/349

Rh One of the most delicate questions of the theory of muscular sensations is that of defining tactual sensations. Having abstracted from touch all that relates to the sense of effort, what is left to constitute touch proper? Sensations of temperature, and what we call sensations of contact. But can there be sensations of contact without there being more or less of pressure, traction, etc.? Is simple contact felt, otherwise than as heat or cold, when we abstract all muscular sensation? Might we not simply revert from it, as did Biran, to the distinction between passive touch and active touch, the latter including the effort? But there appear to be pathological cases where the touch persists while the muscular sense is abolished, as, for example, where the patient with his eyes closed can not tell where his limbs are, whether his arm is raised up or lying down, etc.; but these cases relate to the localization of sensations, another of the most complex questions, and to that of the perception of our body, which is no less so.

There is left the physiological question proper, that of the seat of muscular sensation, on which there are two theories. According to what is called the centrifugal theory, the feeling of muscular effort is connected with the outgoing current of the motor influx. According to the other, the centripetal theory, it is produced by the sensations returning from the member in motion to the centers. Both of these theories find points of support in experiments made upon hysteric patients, who have in these days become veritable analytical machines for the use of psychology. On the one side are hysterics who, having lost the muscular sense, and shut their eyes, have no knowledge of the passive movements that are impressed on their limbs; and yet this loss of muscular sense takes away none of the precision of the motions which the subject executes; an observation which is interpreted by some authors as favoring the centrifugal theory; because, centripetal sensations being abolished with these patients, there must exist some condition of consciousness regulating their movements, and that condition of consciousness can be determined only by the outgoing current of the motor influx. There are, on the contrary, other hysterics who, losing consciousness of the passive movements, lose also that of active motions, and become incapable of executing a single act with shut eyes, which is interpreted as meaning that voluntary motions are impossible when centripetal sensations are abolished. This interpretation would indicate that there exists no feeling allied with the motor discharge and competent to regulate motions in the absence of centripetal sensations. It is apparent that physiology has yet very far to go before it can pretend to have solved these questions. But, as facts, the experiments in question are very interesting; and it happens