Human Anatomy Vol 1

THORAX

Diaphragm

Fig. 12.7= Scheme to show how the size of the thoracic cavity
is reduced by the upward projection of the diaphragm, and by
the inward projection of the shoulders

encroached upon by the abdominal cavity due to the
upward convexity of the diaphragm.
In transverse section, the thorax is reniform (bean-
shaped, or kidney-shaped). The transverse diameter is
greater than the anteroposterior diameter. However,
in infants below the age of two years, it is circular. In
quadrupeds, the anteroposterior diameter is greater
than the transverse, as shown in Fig. 12.8.
In infants, the ribs are horizontal and as a result the
respiration is purely abdominal, by the action of the
diaphragm.
In adults, the thorax is oval. The ribs are oblique and
their movements alternately increase and decrease the

Human adult

Quadrupeds

Fig. 12.8: The shape of the thorax as seen in transverse section
in: Human adult, infants, and quadrupeds

diameters of the thorax. This results in the drawing in

of air into the thorax called inspiration and its expulsion

is called expiration. This is called thoracic respiration.In

the adult, we, therefore, have both abdominal and

thoracic respiration.

Diaphragm descends during inspiration to

increase the vertical diameter of thoracic cage.

Hiccups: These occur due to spasmodic involun-

tary contractions of the diaphragm accompanied

by closed glottis. These usually occur due to

gastric irritation. Hiccups may also be due to

phrenic nerve irritation, uraemia or hysteria.

The narrow upper end of the thorax, which is

continuous with the neck, is called the inlet of the thorax

(Fig.72.9).It is kidney-shaped. Its transverse diameter

is 10-12.5 cm. The anteroposterior diameter is about

5 cm.

Boundoiles

Anteriorly: Upper border of the manubrium sterni.

Posteriorly: Superior surface of the body of the first

thoracic vertebra.

an each side: First rib with its cartilage.

The plane of the inlet is directed downwards and

forwards with an obliquity of about 45 degrees. The

anterior part of the inlet lies 3.7 cm below the posterior

part, so that the upper border of the manubrium sterni

lies at the level of the upper border of the third thoracic

vertebra.

Vertebra T1

Vertebra T2

Vertebra T3

lntervertebral

disc

Vertebra T4

Manubrium sterni

Fig. 12.9: The plane of the inlet of the thorax