We have thus far found that sexual excitation arises :
(a) as a reproduction of satisfaction experienced in connection with other organic processes
(b) through appropriate peripheral stimulation of erotogenic zones
(c) as an expression of certain 'instincts' the origin of which remains unclear.
Mechanical Excitations
Freud adds to this the production of sexual excitation by rhythmic mechanical (i.e. passive) agitation of the body, operating on three ways: via vestibular nerves, the skin and deeper parts (muscles etc.)
Muscular Activity
Freud addresses the point that children need a lot of (active) muscular exercise, and derive satisfaction from it. Does this connect to sexual satisfaction? In many young people the connection between romping and sexual excitement is among the determinants of the direction subsequently taken by their sexual instincts.
Affective Processes
Freud proposes that all intense affective/emotional processes touch upon sexuality. Hence dread of exams in pathogenic cases etc. and the erotogenic effect attached to pain.
Intellectual work
Freud proposed as fact that intellectual work produces sexual excitation in young people (and some adults).
Varieties of Sexual Constitution
Freud proposes that just as innate sexual constitutions derive from variety in the development of the erotogenic zones, this is also reflected in the indirect sources of sexual excitation. Since these indirect / external sources will be of greatly varying strength from person to person, further light may be shed on the problem of differentiating sexual constitutions from investigating this further.
Pathways of Mutual Influence
Freud proposes that the 'sources' of sexual excitation in fact represent pathways and the effects can therefore travel in the reverse direction too (i.e. be mutual) this could explain disorders of nutrition if the erotogenic functions of the zone corresponding to the lips is disturbed. Furthermore, if intellectual work produces sexual excitation, so could sexual excitation produce more attentive behaviour. Freud concludes this essay with a note that whilst these pathways therefore probably carry out some role in sexual attraction in normal people, as well as explain various pathologies, very little about them is known with certainty, except their existence and the likelihood of being bidirectionally traversable.