THE
BREAST: POSITION AND STRUCTURE
The breast is shown in its partially dissected state in the upper part of the plate and below in sagittal section. The size of the breast is variable, but in most instances it extends from the second through the sixth rib, and from the sternum to the anterior axillary line, with an axillary tail in the outer and upper portions, which can be palpated along the outer border of the pectoralis major muscle. The mammary tissue lies directly over the pectoralis major muscle and is separated from the outer fascia of this muscle by a layer of adipose tissue, which is continuous with the fatty stroma of the gland itself.
Fatty deposits surround and intermix
with the glandular elements and make up a significant portion of the breast
structure, providing much of its bulk and shape. The ratio of fatty to glandular
tissue varies among individuals and with the stage of life; with menopause, the
relative amount of fatty tissue increases as the glandular tissue declines. A
rich vascular and lymphatic network (discussed subsequently) supplies the
breasts.
The sensory innervation of the
breast follows the normal distribution of the dermatomes and is mainly derived
from the anterolateral and anteromedial branches of thoracic intercostal nerves
T3-T5. Supraclavicular nerves from the lower fibers of the cervical plexus also
provide innervation to the upper and lateral portions of the breast. Sensory
enervation of the nipple is from the lateral cutaneous branch of T4.
The center of the dome-shaped, fully
developed breast in the adult woman is marked by the areola mammae, a circular,
pigmented skin area from 1.5 to 2.5 cm in
diameter. The surface of the areola appears rough because of large, somewhat
modified sebaceous glands, the glands of Montgomery, which are located directly
beneath the skin in the thin subcutaneous tissue layer. The fatty secretion of
these glands is said to lubricate the nipple. Bundles of smooth muscles in the
areolar tissue serve to stiffen the nipple for a better grasp by the suckling
infant.
The nipple or mammary papilla is
elevated a few millimeters above the breast and contains 15 to 20 lactiferous
ducts surrounded by fibromuscular tissue and covered by wrinkled skin. Partly
within this compartment of the nipple and partly below its base, these ducts
expand to form the short sinus lactiferi or ampullae in which the milk may be
stored. These ampullae are the continuation of the mammary ducts, which extend
radially from the nipple toward the chest wall, and from them sprout variable
numbers of secondary tubules. These end in epithelial masses forming the
lobules or acinar structures of the breast. The number of tubules and the size of the acinar structures vary
greatly in different individuals and at different periods in life. In general,
the terminal tubules and acinar structures are most numerous during the
childbearing period and reach their full physiologic development only during
pregnancy and lactation. These epithelial structures constitute collectively
the parenchyma of the gland. The stroma is composed of a mixture of fibrous and
fatty tissue, and, in the absence of pregnancy and lactation, the relative amounts of fatty and
fibrous tissue determine the size and consistency of the breast.
The enveloping fascia of the breast
is continuous with the pectoral fascia. It subdivides the glands into lobules
and sends strands into the overlying skin, which, in the upper hemisphere, are
known as the suspensory ligaments of Cooper. Because these strands are not
taut, they allow for the natural motion of the breast, but result in breast
ptosis as these ligaments relax with age.
