S

scrotum The saclike structure, a thin layer of
MUSCLEand SKIN, suspended from the base of a
man’s pelvis that contains the TESTICLES. A pair of
ligaments, the spermatic cords, extend from the
lower abdomen to support the scrotum. The sper-
matic cords also serve as the conduits for the
BLOODvessels and nerves that supply the testicles.
The abdominal and scrotal muscles contract or
relax to raise or lower the scrotum, maintaining
the appropriate temperature for spermatogenesis
(SPERMproduction). Sperm production, which is a
key function of the testicles, requires a tempera-
ture of 96ºF to 96.5ºF, about 2 degrees lower than
body temperature. Pubic HAIRcovers the outside of
the scrotum after PUBERTY.
See also HERNIA; SPERMATOCELE; VARICOCELE.

secondary sexual characteristics The physical
changes that distinguish the genders from each
other. Secondary sexual characteristics emerge
with PUBERTYand establish sexual and reproduc-
tive maturity: males produce viable SPERMcapable
of causing PREGNANCY, and females produce
ripened OVA (eggs) capable of fertilization that
results in pregnancy.

FEMALE SECONDARY SEXUAL CHARACTERISTICS
growth of pubic and axillary (underarm) HAIR
thickened and coarse or dark leg hair
enlarged breasts and broadened hips

MALE SECONDARY SEXUAL CHARACTERISTICS

growth of pubic, axillary, chest and facial hair
thickened, darkened, and coarse arm and leg hair
broadened shoulders and chest
increased MUSCLEmass and definition
deepened voice and prominent Adam’s apple in the THROAT
enlarged TESTICLES, enlarged and elongated PENIS

See also CONCEPTION; CONTRACEPTION; FERTILITY;

MENSTRUAL CYCLE; MENSTRUATION; PREGNANCY.

semen The fluid of a man’s EJACULATION. In a fer-

tile man about 5 percent of the semen content is

SPERM; in a man who has had a VASECTOMYsemen

does not contain sperm. The seminal vesicles and

the PROSTATE GLAND produce the milky fluid of

semen, which is primarily a water base that con-

tains proteins, sugars (notably fructose and some

GLUCOSE), lipids (fatty acids), electrolytes, and

PROSTAGLANDINS. The bulbourethral glands, also

called Cowper’s glands, add a gelatinous secretion

to the semen that thickens it. Semen may flow

back into the BLADDERrather than out of the PENIS

(RETROGRADE EJACULATION) in a man with a PROSTA-

TECTOMY(surgery to remove the prostate gland).

The electrolytes protect sperm on their journey

through the VAGINAand into the UTERUS. The sug-

ars, particularly fructose, and lipids provide nutri-

tion for the sperm. Vaginal secretions are highly

acidic and deadly to sperm. The thickness of

semen helps contain and insulate sperm as they

travel through the VAGINA, though the semen thins

by the time it reaches the UTERUSto release the

sperm. The electrolytes in semen make it highly

alkaline, helping neutralize the vaginal environ-

ment to improve sperm survival. Prostaglandins

help suppress the IMMUNE RESPONSE, the natural

reaction of the woman’s IMMUNE SYSTEMto the

presence of the sperm. The semen also has the

ability to carry various pathogens such as viruses

and BACTERIAthat can spread SEXUALLY TRANSMITTED

DISEASES(STDS).

Semen analysis is a laboratory examination of a

semen sample to measure the concentrations of its

ingredients and the number and characteristics of

the sperm. ALCOHOLconsumption, cigarette smok-

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