The male reproductive system is adapted for internal fertilization. The penis is a structure that has evolved to deposit sperm safely within the female’s body. The tube that passes through the penis, the urethra, has two functions. It is the pathway for sperm out of the body as well as the pathway for urine to leave the body after having been stored in the uri- nary bladder. At the time of sexual excitement, a valve closes. This valve prevents the bladder from releasing urine. At the same time, nerve impulses from the brain cause blood vessels in the penis to relax. Blood rushes in and fills empty spaces.
