The bladder, again, is an internal organ which makes its functional needs felt only at intervals, and thus, when those needs occur during sleep, they become conscious in imagery which easily recalls the source of the stimulus. It may, indeed, be said that vesical dreams are full of instruction in the light they throw on the psychology of dreaming. This has long been well known to writers on dreams. Thus Scherner, many years ago, insisted on the interest and importance of vesical dreams. In women, especially, he regarded them as very frequent and developed, most dream stories of women, he considered, containing symbolic representations of this organic irritation. Water, in some form or another, is naturally the commonest symbol. In Scherner's opinion, also, all dreams of fish playing in the water are vesical dreams.[69] In its simplest form the vesical dream is what Freud would term a wish-dream of infantile type, frequently in the magnified form common in dreams, and sometimes transferred from the dreamer himself to become objectified in another person, or even an inanimate object.[70] There is, however, a very important difference according to whether these dreams take place in an adult or in a young child. In the adult it almost invariably happens that the dream act remains merely a dream act, and no corresponding motor impulse is transmitted to the bladder. But when such dreams occur to very young children, in whose brains the motor inhibitory mechanism is not yet fully established, it not infrequently happens that the motor impulse is transmitted and the expulsive action of the bladder is set up in sympathy with the imagery of the dream; thus is established the condition known as nocturnal enuresis. As the young brain develops, and inhibition becomes more perfect, these vesical dreams cease to exert any actual effect on the bladder, even when, as sometimes happens, they continue to occur at intervals in adult life.[71] Occasionally, both in those who have and those who have not suffered from nocturnal enuresis in childhood (especially women), vesical dreams of this character may occur without even any real distension of the bladder. In some of these cases the dream can be shown to be due to a reminiscence or suggestion from the waking life of the previous day. Dreams stimulated by organic sensations from within are thus found to resemble those proceeding from sensory sensations from without in that they are both exactly simulated by dreams which are mainly of central origin.