In many hoofed mammals (ungulates), the penis is mainly fibroelastic and relies less on increased blood flow; it straightens when retractor muscles relax rather than lengthening dramatically. These retractor muscles pull the organ back into the foreskin or a genital slit when not erect and are especially prominent in whales. Some mammals—such as rodents, seals, and most land carnivores—also possess a penis bone (baculum), which helps maintain erections during repeated mating.

In humans, the penis is the male organ for urination and sexual intercourse. It is made up of a base, shaft, glans, and foreskin, with spongy tissues (corpora cavernosa and corpus spongiosum), blood vessels, and nerves surrounding the urethra, which carries both urine and semen. Size, shape, and curvature vary widely and naturally among individuals.

The penis is the male organ for urination and sexual intercourse. It consists of a base, shaft, glans and foreskin, and contains spongy tissue, blood vessels and nerves. Inside, two corpora cavernosa and the corpus spongiosum fill with blood to produce an erection. The urethra runs through the penis, carrying both urine and semen. Variations in size, shape and curvature are normal, and the organ plays a central role in human reproduction and sexual function.

Eighteenth-Century Anatomical Illustration
The 18th-century plate from Diderot and d’Alembert’s Encyclopédie presents the penis in a precise, didactic manner typical of Enlightenment anatomy. It highlights the organ’s internal structure—erectile tissues, vessels, and the course of the urethra—treating male sexuality and reproduction as subjects for systematic scientific observation rather than myth or taboo.