The expulsion of SEMEN from the PENIS during ORGASM. The stimulation of sexual intercourse (COITUS) or masturbation produces a spinal reflex action that causes ejaculation. As well as containing spermatozoa (male germ cells), the semen comprises several constituents arising from COWPER'S GLANDS, the PROSTATE GLAND, the TESTICLES, and the SEMINAL VESICLES and these are discharged in sequence. (See also PREMATURE EJACULATION.)