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== A well-integrated organ ==
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.
== Simple sensations ==
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.
== Complex sensations ==
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.
== Dartos muscle ==
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.
== Why two layers? ==
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]].
== Summary ==