(3) Erection. These three tissue masses are erectile (capable of erection)
and contain blood sinuses (channels). When sexually stimulated, the arteries of the
penis dilate. Large quantities of blood enter the blood sinuses. Expansion of these
spaces compresses the veins, draining the penis and causing most blood entering to be
retained. An erection is caused by these vascular changes, the erection being a
parasympathetic reflex. When the arteries constrict and the pressure on the veins is
relieved, the penis returns to its flaccid (soft, limp) state.
Figure 1-4. Internal structure of the penis.