essay.[9]

*Thumbsucking.*--Thumbsucking, which manifests itself in the nursing
baby and which may be continued till maturity or throughout life,
consists in a rhythmic repetition of sucking contact with the mouth (the
lips), wherein the purpose of taking nourishment is excluded. A part of
the lip itself, the tongue, which is another preferable skin region
within reach, and even the big toe--may be taken as objects for sucking.
Simultaneously, there is also a desire to grasp things, which manifests
itself in a rhythmical pulling of the ear lobe and which may cause the
child to grasp a part of another person (generally the ear) for the same
purpose. The pleasure-sucking is connected with an entire exhaustion of
attention and leads to sleep or even to a motor reaction in the form of
an orgasm.[10] Pleasure-sucking is often combined with a rubbing contact
with certain sensitive parts of the body, such as the breast and
external genitals. It is by this road that many children go from
thumb-sucking to masturbation.

Lindner himself has recognized the sexual nature of this action and
openly emphasized it. In the nursery thumbsucking is often treated in
the same way as any other sexual "naughtiness" of the child. A very
strong objection was raised against this view by many pediatrists and
neurologists which in part is certainly due to the confusion of the
terms "sexual" and "genital." This contradiction raises the difficult
question, which cannot be rejected, namely, in what general traits do we
wish to recognize the sexual manifestations of the child. I believe that
the association of the manifestations into which we gained an insight
through psychoanalytic investigation justify us in claiming thumbsucking
as a sexual activity and in studying through it the essential features
of the infantile sexual activity.