STIMULUS AND EXCITATION
GLISSON, a Cambridge don and disciple of Harvey, wrote in 1677: "Every living body possesses the faculty of being stimulated by external influences. That is what we mean by irritability." This idea arises from the psychological concept of sensitivity. Psychological concepts at that time were but rudimentary and purely metaphysical but they were sufficient to embrace the idea that sensation might be the effect of the application of a stimulus or excitation in the corresponding sensorial organ. Thus also was it possible to justify the idea that movement likewise may result from excitation of an organ or muscle.
"Irritability is the property in muscular fibres of perceiving an irritation and reacting thereto. Thus irritation causes contraction and when such irritation ceases the muscle fibres relax. This is identical with natural perception through irritation of sensitive fibres . . . The power of movement is a general property of the organs and inherent to life itself . . . Thus irritability is the prime cause not only of muscular movement but also of life in general. The same thing becomes evident similarly in a state of sickness. Every part of the body suffering discomfort seeks to get rid of it. This effort may be called irritation, whilst the parts able to perceive the damage and react to it should be labelled as capable of being irritated."
Almost a century later, Albert Haller ( 1757) made a distinction between muscular and nervous excitability. "Incitability," he wrote, "is a property of the muscular fibres whilst sensitivity belongs exclusively to the nervous or muscular system containing nerve fibres. The muscles have the property of contracting quite independently from the nerves, but sensitivity depends definitely on the nervous system." Haller proved by his various experiments that muscle contraction can occur either by exciting it direct or by exciting its motor nerve. The nervous impulse provoked by excitation of the nerve descends through the latter and on reaching the muscle promotes in its turn excitation of the muscle itself and consequent contraction thereof.
"All living matter can be excited," was the axiom laid down by