Page 77 - 86 human physiology part-2

Chapter 15

Pubic hair in boys

Pubic hair often appears on a boy shortly after the genitalia begin to grow. As in girls, the first
appearance of pubic hair is termed pubarche and the pubic hairs are usually first visible at the dorsal
(abdominal) base of the penis. The first few hairs are described as stage 2. Stage 3 is usually reached
within another 6 to 12 months, when the hairs are too numerous to count. By stage 4, the pubic hairs
densely fill the "pubic triangle." Stage 5 refers to spread of pubic hair to the inner thighs and upward
towards the umbilicus as part of the developing abdominal hair.

Body and facial hair in boys

In the months and years following the appearance of pubic hair, other areas of skin which respond
to androgens (see glossary) develop heavier hair (androgenic hair) in roughly the following sequence:
underarm (axillary) hair, perianal hair, upper lip hair, sideburn (preauricular) hair, periareolar hair, and
the rest of the beard area. Arm, leg, chest, abdominal, and back hair become heavier more gradually.
There is a large range in amount of body hair among adult men, and significant differences in timing
and quantity of hair growth among different ethnic groups.

Voice change

Under the influence of androgens, the voice box, or larynx, grows in both genders. This growth is
far more prominent in boys, causing the male voice to drop, rather abruptly, usually about one octave,
because the larger vocal folds have a lower fundamental frequency. Occasionally, this is accompanied
by cracking and breaking sounds in the early stages. Most of the voice change happens during stage 4
of male puberty around the time of peak growth. However, it usually precedes the development of
significant facial hair by several months to years. The time it takes for the voice to drop varies
according to how far it has to go (tenor, baritone, or bass).

Height growth in boys

Compared to girls' early growth spurt, growth accelerates more slowly in boys and lasts longer,
resulting in a taller adult stature among males than females (on average about 10 cm or 4 inches). The
difference is attributed to the much greater potency of estradiol compared to testosterone in promoting
bone growth, maturation, and epiphyseal closure. In boys, growth begins to accelerate about 9 months
after the first signs of testicular enlargement and the peak year of the growth spurt occurs about 2 years
after the onset of puberty, reaching a peak velocity of about 8.5-12 cm or 3.5-5 inches per year. The
feet and hands experience their growth spurt first, followed by the limbs, and finally ending in the
trunk. Epiphyseal closure and adult height are reached more slowly, at an average age of about 17.5
years. As in girls, this last growth primarily involves the spine rather than the limbs.

Male musculature and body shape

By the end of puberty, adult men have heavier bones and nearly twice as much skeletal muscle.
Some of the bone growth (e.g., shoulder width and jaw) is disproportionately greater, resulting in
noticeably different male and female skeletal shapes. The average adult male has about 150% of the

292 | Human Physiology