Craving is a sign of physiological need. It is a sensory phenomenon, of which, however, explicit awareness cannot always be discovered. It is conspicuously noticed in cases of disturbance of the body secretions, such as occurs in over-function of the thyroid gland. It is regarded as a crude body-consciousness that something is the matter. In motorial organisms it causes visible reaction: this expresses itself in what is termed restlessness. But the unrest may show itself by a fixation more particularly in the muscles of emotional expression, although the manifestation is not confined to these; shallow respirations and restricted amplitude of movement in limbs and trunk may be observed also. In cerebrate animals the reaction of the individual is under the guidance of preceding impressions stored in the pallium and known as memories; whereas in the animals without a pallium all reaction is accomplished through stable mechanisms known as instincts. Both of these types of reaction are tropisms merely; but the former are labile, conditionable; whereas the latter cannot be modified. The science of conditionable reactions of cerebrate animals is called psychology, and the means by which the reactions are influenced are called psychogenetic, whether these are healthy or diseased. It must not be forgotten, however, that the genesis of a psychological disturbance may be purely somatic, although the manner in which the reaction shows itself is contingent mainly upon the features of the individual which have been derived from previous sensory impressions and their resultant motor reactions commonly known as experience. It is the influence of these upon the hereditary dispositions of the individual which constitute what is known as "make-up" or character; and it is this which determines the form which reaction to stimulus must take, whether the stimulus is purely psychological or somatic. Now physiological discomfort is an experience universal at one time of life or another; but the reaction to it is infinite in variety; and while part of it depends upon the congenital dispositions which are the common property of humanity, a larger part is contingent upon the psychogenetic factors which have stamped the individual. II