A penis is a sac on the penis. If it is too small or too big, the organ can get injured. Injuries can cause the penis to grow too far outside the vagina. The sac may be damaged or destroyed or not grow. A penile sac is the smallest part of the penis, the length of the shaft. The sac usually hangs under the skin or in a hollow of the skin. There is no ligament between the penis and the sac. The penis may move as it grows. When the penis grows, it pushes against the sac. It gets tighter and becomes more painful. There are two types of penile sacs. One type is made of blood vessels and it is called penile capillaries. When the penis gets bigger, the penile capillaries may get thicker and the blood vessels may be stretched, which causes the sac to be more sensitive. The other type of penile sac is made of connective tissue and is called the penis glans. It gets stretched too. It causes the penis to become harder, more swollen, and has some painful sensations as well. Here is an example of how the penis is made. It is made from several skin cells.