The visible penis consists of two sections - the penis head or glans penis and the penile shaft. However, the penis continues as a hidden structure laying under the scrotum and reaching further backward in the perineal area towards the prostate. When the penis is erect, also the perineum, the area between the legs will be swollen.
The urinary tube (urethra) goes from the bladder, then through the prostate, then enters this penile structure just beneath the prostate and then goes forward all the way to the tip of the penis.
THE ERECTILE BODIES
In the visible penis and in the structure extending backwards there are three bodies that swell during erection, erectile bodies. The bodies are mostly made of a tight mesh of fine blood vessels with very elastic walls. There are two bodies along the upper part of the penis (corpora cavernosa) that extend backwards under the scrotum. Then there is one body around the urethra (corpus spongiosum) that continues backwards to just under the prostate.
The arteries leading blood to the erectile bodies and the veins leading blood from these bodies have muscles in their walls that can close off or relax to regulate the blood flow to and from these bodies.
When the muscles at the entrance relax and those at the outlet constrict, blood is filled into the mesh by the blood pressure. The erectile bodies will then engorge as a balloon and get straight. The swelling and straightening of the erectile bodies will inflate the whole penis and make it erect. This activity is regulated by nerve impulses and the substance nitrogen oxide working as a local tissue hormone.
All the inner structures of the penis are surrounded by a robust sheet of elastic connective tissue, the fibrous envelope. In the area between the legs there is a sheet of muscles between the fibrous envelope and the perineal skin, the two bolbospongiosus muscles. These muscles contract during ejaculation and help to pump out the semen.
THE PENILE SKIN
The penile skin is thick and robust. Between the skin and the fibrous envelope, there is a thin area consisting of a soft and extremely elastic connective tissue. Therefore the skin can easily be moved up and down along the penile shaft.
The penis head is covered by a thin skin which is extremely sensitive by means of a rich innervation. Still this thin skin is both strong and elastic. In addition the skin in an uncircumcised penis is covered by a hood that is not stuck to the surface of the head after early infancy, the foreskin.
The foreskin is a continuation of the skin covering the shaft that goes to the tip of the penis and is then folded back inwards, making a double layer, where the inner fold is fastened just under the penis head. Between the two folds there are an extremely soft and elastic tissue.
The foreskin can easily be retracted to show the bare penis head, and it uses to retract itself when the penis is erect. When the penis is not erect, the foreskin protects the penis head.
In early infancy the foreskin is fixed to the penis head, but will gradually loosen, usually during early infancy, but in a preteen boy there can still be some adhesions left around the base of the head.
In some cultures it is custom to remove the foreskin for various reasons by an operation called circumcision., making the penis head permanently exposed and visible. When circumcision is done in early infancy, the procedure also involves tearing the fixed foreskin loose from the surface of the head.
The skin of the penis head produces a white creamy substance called smegma which helps to lubricate the penis during intercource. Smegma is a blending of oils secreted through the skin on the glans, cells loosened from the skin surface and substances produced by sebaceous glands on the inside of the foreskin. Whith improper hygiene too much smegma can accumulate and give excessive bacterial growth and bad smell.
THE INNERVATION AND BLOOD SUPPLY OF THE PENIS
The main nerves controlling the penile function and bringing sensual stimuli from the penis are the dorsal penile nerves, situated on both sides in the connective tissue of the upper side of the penis and branching out to all parts of the penis. Erection is controlled by sensual stimuli going to the spinal cord and control impulses coming back, both through this nerve. Ejaculation is controlled by other nerves going to the muscle sheet in the perineum and to all other structures involved in bringing the semen out.
The main blood supply of the penis comes from the dorsal penile arteries going in the connective tissue at the upper side of the penis and branching off to all parts of the penile skin and inner penile structures.