THE CHEST.
The chest or thorax occupies the upper part of the trunk in front and
is a dome-shaped cavity containing and protecting the heart and lungs.
Its walls are formed by the dorsal vertebræ at the back, the ribs at
either side, and the sternum and costal cartilages in front, all well
covered with muscles. The floor is formed by the diaphragm. Through the
upper opening of the chest pass the trachea, the esophagus, and many
important vessels and nerves.
[Illustration: FIG. 36.--Thorax (anterior view.) (Ingals.)]
The shape of the chest may vary in disease. Thus, in rickets there is
the prominent “pigeon” breast and the rosary, that is, a bead at the
juncture of each rib with the costal cartilage, while in emphysema the
chest is enlarged in all directions and barrel-shaped. In severe cases
of lateral curvature it is distorted but may be improved by exercises.
=The Sternum.=--The sternum or breast-bone is a long narrow bone and
has three parts, the _manubrium_ or handle above, the _gladiolus_ or
sword, and the _ensiform cartilage_ at the lower end. On either side
are notches for the costal cartilages; for the first seven ribs as
well as the clavicle articulate with it. Except for some muscles along
the edges it lies directly under the skin and the ridge between the
manubrium and the gladiolus can be felt in the living, a fact which
assists in determining the position of the different ribs in cases of
fracture, as the second rib articulates at this point.
[Illustration: FIG. 37.--A and B, typical ribs; C, first rib; D,
twelfth rib. 1, head; 2, neck; 3, tuberosity; 4, grooved edge; 5,
shaft; 6, oval depression for costa cartilage.]
=The Ribs.=--The ribs are twenty-four in number, twelve on each
side, of which the upper seven, which articulate with the sternum
by individual cartilages, are called _true ribs_, the other five
_false ribs_. Of the false ribs the upper three articulate indirectly
with the sternum through the seventh cartilage, with which their
cartilages unite, while the other two have their anterior extremities
free and are called _floating ribs_. All the ribs slope down toward
the front and are by nature more freely movable in women than in
men. Most of the ribs have a head divided by a little ridge into two
facets for articulation with the dorsal vertebræ, a flattened neck,
a tuberosity at the base of the neck with a facet for articulation
with the transverse process of the vertebra below, an angle, and a
shaft, which is externally convex and is grooved on its lower edge
for the intercostal vessels and nerve. The first and second, eleventh
and twelfth ribs, however, are somewhat peculiar, the first two being
shorter, flatter and rather broader than the rest and the first having
only one facet on the head, while the last two have only one facet on
the head and no neck or tuberosity.
The _costal cartilages_ serve to prolong the ribs and greatly increase
the elasticity of the chest wall. They grow longer down to the seventh
and then decrease again in length.
The ribs, except the first and second, which are protected by the
clavicle, are frequently _broken_. Such a break causes pain in
breathing and sometimes the end of a rib pierces the lung tissue
and swelling all over the body results, due to the presence of air.
_Caries_ or death of the rib is also frequent. _Fracture of the
sternum_ occurs occasionally, generally from direct force, as from a
blow with the knee in foot-ball, and there may be dislocation between
the manubrium and gladiolus.
=Muscles of the Chest.=--The spaces between the ribs, from the
tubercle of the rib behind to the cartilage in front, are filled by
the _external intercostal muscles_, which pass downward and forward
from the lower border of one rib to the upper border of the one below.
There are, therefore, eleven pairs of these muscles. There are also
eleven pairs of the _internal intercostals_, which commence at the
sternum and extend back to the angle of the rib. These extend downward
and backward. The external intercostals raise and evert the ribs in
inspiration, the internal depress and invert them in expiration.
The chief respiratory muscle, however, is the _diaphragm_, a somewhat
fan-shaped muscle that forms the floor of the chest cavity. It takes
its origin from the ensiform cartilage, the six or seven lower ribs and
their cartilages, and from the upper three or four lumbar vertebræ,
that is, from the whole of the internal circumference of the thorax,
and is inserted into the central cordiform tendon. It has several large
and several small openings for the aorta, the esophagus, the venæ cavæ,
the thoracic duct, and various nerves, and its surfaces are covered by
serous membranes, by the two pleuræ and the pericardium above and by
the peritoneum below. It partially supports the heart and lungs. Convex
toward the chest, it becomes flattened in contraction and so increases
the capacity of the chest. It aids in all expulsive acts, as sneezing,
coughing, laughing, urinating, defecating, vomiting, and childbirth.
_Hiccough_ is spasm of the diaphragm.
[Illustration: FIG. 38.--Interior view of the diaphragm. (Leidy.)
1-3, The three lobes of the central tendon, surrounded by the fleshy
fasciculi derived from the inferior margin of the thorax; 4, 5, the
crura; 6, 7, the arcuate ligaments; 8, aortic orifice; 9, esophageal
orifice; 10, quadrate foramen; 11, psoas muscle; 12, quadrate lumbar
muscle.]
The _arteries_ of the chest are the intercostal branches of the
subclavian and the thoracic aorta, the phrenic, mediastinal, and
intercostal branches of the internal mammary, and the thoracic branches
of the axillary.
The _nerves_ are the intercostals and phrenics.
=Mammary Glands.=--On the outside of the chest walls, lodged in the
fascia of the pectoral muscles, are the mammary glands, accessory
organs of the generative system. They exist in both sexes but are
only rudimentary in the male. In the female they are small before
puberty but enlarge as the generative organs become more completely
developed, forming two hemispherical eminences, one on either side,
between the third and seventh ribs. During pregnancy they increase once
more in size preparatory to the secretion of the milk, and in old age
they atrophy. From the middle projects a small pinkish-brown conical
eminence, the _nipple_, surrounded by a paler area, the _areola_. After
the second month of pregnancy both nipple and areola become darker in
color, a point of great diagnostic value in early pregnancy.
The mammary glands themselves consist of lobules of gland tissue with
a central lactiferous tubule, the lobules being gathered into lobes
with fatty tissue between. From the juncture of these tubules result
fifteen or twenty excretory ducts, the _tubuli lactiferi_, which
converge toward the areola. Beneath the nipple they dilate, forming the
_ampullæ_, and then contract again to pass out through the nipple as
straight tubes.
_Breast abscess_ occurs most commonly in nursing mothers, as where
a part is most active there is most danger of abscess. Many benign
_tumors_ of the breast, as the fibrous tumors, occur and are especially
common in young women. If a fibrous tumor is allowed to develop it may
become cancerous. _Cancer_, however, generally occurs after the age of
forty and is usually due to some irritation, as to a blow from a ball.
The _arteries_ of the breasts are the thoracic branches of the
axillary, the intercostal, and the internal mammary.
The _nerves_ are from the thoracic cutaneous.