"Some persons," says GLISSON, "may doubt whether there really exists a natural perception of irritation in the fibres; but we have elsewhere asserted in general the reality of natural perception, to wit, in my work, de Vita Naturae; and whoever has known it, will readily admit this quality in fibres imbued with inherent, influent, and vital spirits. We do not expect, in this place, to establish it as a general principle; but if any proof, derived from a knowledge of the structure, uses, and actions of the fibres, can be adduced, it may be here attempted." "It is indubitable that the fibres are alternately at rest and in motion; for, during sleep, they are all relaxed, with the exception of such as subserve the functions of respiration and circulation, and even these are by turns quiet and active. During waking again, they are all in a state of moderate tonic motion; and moreover, during all movements of the limbs, the antagonist muscles yield spontaneously, the abductors being active, while the adductors are relaxed, and vice versa. Hence it is manifest, that the fibres are alternately quiescent and active: but, since they are not _principal_ or sui arbitrii agents, it is necessary, in order to the new movement, that they should be irritated from some source: for, it is impossible that a fibre in repose, can be set in action without an irritating cause; nor can we conceive of a part being irritated without _perceiving_ the irritation. It is like speaking to a deaf man, or trying to awaken a dead one." "If you say, fibres are possessed of sensibility, and can be excited by virtue thereof, I confess that they are sensible parts, and may thereby perceive some, not all irritating causes; but whether sensation excites them immediately, or rather, is transmitted to the brain, and irritates the animal appetency; and further, whether the animal appetence, effects a movement in them directly, and to what sort of perception this irritation may be properly and immediately ascribed, is detailed in order below, when we come to explain sensitive perception, and perception a phantasia imperata."