As a male becomes sexually excited and ready to mount his female, his sexual equipment is operable and semen is poised for discharge. But wait! There is yet another all-important dimension to be considered. Before excitement kicks in, the penis is soft, flexible and altogether floppy (flaccid) in many species. This is no good for intromission. First, it must stiffen, else it is likely to bend on impact. This hardening and enlargement of the penis is erection, and it is induced by the sight of a female in heat, smell, or the anticipation of coitus. As we shall see later, in man, it can occur spontaneously or through imagination or fantasy.

ERECTION

Messages run from the brain, through the spinal cord, to be carried in nerves near to the end of the cord at the level of the pelvis. These pelvic nerves (nervi erigentes) end in the arterial blood vessels of the penis and, when stimulated, cause them to dilate. Unlike most arteries, which end in capillaries, these end in large cavernous spaces in the shaft of the penis, which become filled with blood and endow the organ with a size and stiffness, the like of which, if for the first time, it has not previously experienced. The muscles in the walls of the blood vessels, which cause them to dilate, respond involuntarily, so erection is classed as being an involuntary or parasympathetic reflex. The blood vessels are quite long and are thus seen to be variably coiled (helicine arteries of the penis).