Testes
The male gonad is the testis (pl. testes), and the existence of the testes is the primary male sex characteristic (fig. 1-47). The testes produce and secrete the male hormone testosterone, which influences the development and maintenance of the male accessory sex organs and the secondary sex characteristics of the male. The male accessory sex organs include two groups of organs: the internal sex organs and the external sex organs. See section titled "Male Reproductive System" for more information on the male accessory sex organs.

Male Secondary Sex Characteristics

Male secondary sex characteristics influenced by the hormone testosterone are as follows:

Increased growth of hair, particularly in the areas of the face, chest, axilla, and pubic region.

Enlargement of the larynx (Adam's apple) and thickening of the vocal cords, which produces a lower-pitched voice.

Thickening of the skin.
Increased muscle growth, broadening of the shoulder and narrowing of the waist.

Thickening and strengthening of the bones.

Ovaries
The female gonads, the ovaries, produce the hormones estrogen and progesterone (fig. 1-47). Estrogen influences the development and maintenance of the female accessory sex organs and the secondary sex characteristics, and promotes changes in the mucous lining of the uterus (endometrium) during the menstrual cycle. Progesterone prepares the uterus for the reception and development of the fertilized ovum and maintains the lining during pregnancy.

Today, progesterone and estrogen hormones (naturally derived) are incorporated into oral contraceptives or birth control pills. The combination of hormones released through this monthly series of pills fools the body into not preparing (building-up of uterine lining) for implantation of an embryo. Because the uterus has not prepared for implantation, pregnancy cannot occur.

Female accessory sex organs are also divided into internal and external accessory sex organs. See section titled "Female Reproductive System" for more information on the female accessory sex organs.

Female Secondary Sex Characteristics
Female secondary sex characteristics influenced by the hormone estrogen are listed below.

Development of the breasts and the ductile system of the mammary glands within the breasts.

Increased quantities of fatty (or adipose) tissue in the subcutaneous layer, especially in the breasts, thighs, and buttocks.

Increased vascularization of the skin.