The Probert Encyclopaedia of Medicine

PENIS

The penis is the male genital organ which carries the duct for the emission of sperm, and ejection of urine. In mammals it is comprised mainly of erectile tissue, many mammals (including most primates, rodents and others) except humans having in addition a bone (known as the baculum bacula or os Penis) in the penis to allow instant arousal, or erection, to facilitate penetration of the female for the transference of the sperm, humans lacking this bone require to be aroused in order to be able to copulate.
Research Penis