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Penis: the copulatory organ

The penis provides an outlet for both urine and the copulatory ejaculate (spermatozoa and semen). The histology and gross anatomy of the penis varies dramatically from species to species and from region to region within the same species. In general, the body of the penis consists of the urethra, erectile tissue (corpora cavernosa penis and corpora cavernosum urethra), smooth and skeletal muscle, touch and pressure receptors (Pacinian corpuscles) and a dense connective tissue capsule (tunica albuginea). 

Erectile tissue and the erectile mechanism. The erectile tissue* is composed of dense irregular connective tissue which contains numerous elastic fibers and sinuses. Under stimulation, the primary blood supply of the penis is directed through helicine arteries which open into the venous sinuses.   During erection, these vessels and the sinuses become engorged with blood, and the thin-walled veins beneath the tunica albuginea are effectively closed, further increasing the rigidity of the organ. Because the capsule around the erectile tissue of the corpus cavernosum urethra is not as thick as that around the corpus cavernosum penis, the urethra is not occluded during erection.  After ejaculation, the helicine arteries contract and regain their normal tone resulting in a relaxing of the pressure around the veins which leads to the restoration of normal blood flow to the region. 

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