of a gratification which has been experienced in conjunction with other
organic processes; (_b_) through the appropriate peripheral stimulation of
erogenous zones; (_c_) and as an expression of some "impulse," like the
looking and cruelty impulses, the origin of which we do not yet fully
understand. The psychoanalytic investigation of later life which leads
back to childhood and the contemporary observation of the child itself
cooeperate to reveal to us still other regularly-flowing sources of the
sexual excitement. The observation of childhood has the disadvantage of
treating easily misunderstood material, while psychoanalysis is made
difficult by the fact that it can reach its objects and conclusions only
by great detours; still the united efforts of both methods achieve a
sufficient degree of positive understanding.

In investigating the erogenous zones we have already found that these
skin regions merely show the special exaggeration of a form of
sensitiveness which is to a certain degree found over the whole surface
of the skin. It will therefore not surprise us to learn that certain
forms of general sensitiveness in the skin can be ascribed to very
distinct erogenous action. Among these we will above all mention the
temperature sensitiveness; this will perhaps prepare us for the
understanding of the therapeutic effects of warm baths.

*Mechanical Excitation.*--We must, moreover, describe here the
production of sexual excitation by means of rhythmic mechanical shaking
of the body. There are three kinds of exciting influences: those acting
on the sensory apparatus of the vestibular nerves, those acting on the
skin, and those acting on the deep parts, such as the muscles and
joints. The sexual excitation produced by these influences seems to be
of a pleasurable nature--it is worth emphasizing that for some time we
shall continue to use indiscriminately the terms "sexual excitement" and
"gratification" leaving the search for an explanation of the terms to a
later time--and that the pleasure is produced by mechanical stimulation
is proved by the fact that children are so fond of play involving
passive motion, like swinging or flying in the air, and repeatedly
demand its repetition.[20] As we know, rocking is regularly used in
putting restless children to sleep. The shaking sensation experienced in
wagons and railroad trains exerts such a fascinating influence on older
children, that all boys, at least at one time in t