The foreskin occupies a prominent position on an important organ. The foreskin’s location and structure indicate that it is the most important sensory tissue of the penis. Its persistence over millions of years suggests that it has played a role in the propagation of the species.
Structurally, the penis is highly integrated. The glans, foreskin and skin of the penile shaft function as a single unit, not as a collection of separate parts with entirely different functions. The functions of the glans and foreskin are similar, and overlapping, but come fully into their own at different times during intercourse.
The outer surface of the foreskin is specialized to detect feather-light touch and other sensations, including painful ones. The infamous “zipper injury” is an extreme example of the sort of damage the outer skin was designed to detect and prevent, long before the zipper posed a threat to the uninitiated.
Compared with the true (outer) skin of the foreskin, the glans is only feebly sensitive to light touch, pain, heat and cold. This is part of the reason we call the foreskin the primary sensory tissue of the penis. Without the foreskin, the end of the penis is numb to a host of sensations that tell the owner whether one of his most prized organs is in good company, or should move to safety.
Thanks to its ridged band, the inner lining of the foreskin is specialized sexual tissue. The ridged band readily expands and contracts and is obviously designed to detect stretching forces. When penile shaft skin tugs on the ridged band, special “genital corpuscles” in the peaks of the ridges detect movement and trigger ejaculation. Stretching of the ridged band may also trigger and sustain erection.
Electrical stimulation of the glans triggers nerve impulses that pass to the spinal cord and then to the muscle of ejaculation. Clearly the glans has much in common with the foreskin. Where foreskin and glans part company, functionally as well as physically, is in their sensitivity to light touch, pain and heat and cold. Contrary to common opinion, the glans is not highly sensitive to a broad range of stimuli.
It is unclear whether the ridged band simply plays “backup” for the glans, or whether the two have different functions. The location of the retracted ridged band on the erect penile shaft suggests that the difference is one of timing. Possibly, the foreskin and its ridged band are designed to ensure that sexual reflexes are triggered when, and only when, these structures are stretched during intercourse. The biological importance of the ridged band to conception is self-evident, but there is still a major gap in our understanding of the relation between form and function of the penis.
Penile skin has two important characteristics, apparent only on erection. Firstly penile skin tenses, stiffens and shortens, firming up the connection between shaft skin and ridged band. This change allows for the transmission of movement from the base of the erect penis to the ridged band.
Secondly, penile skin undergoes a marked frictional change, brought about by stiff, forward-pointing skin folds. The mechanism is similar to that which raises goosebumps.
The changes in penile skin are brought about by contraction of the Dartos muscle. Between them, stiffening and frictionality ensure that the ridged band is instantly “alerted” to changes in position of the penis within the vagina.
The double-layering of the foreskin allows the delicate ridged band, which normally is safely hidden from view, to be deployed on the upper surface of the penile shaft during erection. There it stands a better chance of being activated. Double-layering also eases vaginal entry by offsetting the frictional resistance of “erect” shaft skin.
The various parts of the penis, including the foreskin, form a functional whole. The foreskin is the primary sensory tissue of the penis. The ridged band of the foreskin is built to trigger orgasm and ejaculation.
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