TBL5 - Pectoral Muscles & Breasts

Question 1

Where are the pectoralis major and minor located?

Answer 1

1) The pectoralis major is a large, fan-shaped muscle that covers the superior part of the thorax
2) The pectoralis minor lies in the anterior wall of the axilla where it is almost completely covered by the much larger pectoralis major

Question 2

What are the proximal and distal attachments of the pectoralis major?

Answer 2

1) Proximal attachment:
a) Clavicular head: anterior surface of medial half of clavicle
b) Sternocostal head: anterior surface of sternum, superior six costal cartilages, aponeurosis of external oblique muscle
2) Distal attachment: Lateral lip of intertubercular sulcus of humerus

Question 3

What is the function of the pectoralis major?

Answer 3

1) Adducts and medially rotates humerus; draws scapula anteriorly and inferiorly
2) Acting alone, clavicular head flexes humerus and sternocostal head extends it from the flexed position

Question 4

What innervates the pectoralis major?

Answer 4

1) Sternocostal head (C7, C8, T1) - Lateral & Medial cutaneous nerve
2) Clavicular head (C5, C6) - Lateral cutaneous nerve

Question 5

What innervates the pectoralis minor?

Answer 5

Medial pectoral nerve (C8, T1)

Question 6

What are the proximal and distal attachments of the pectoralis minor?

Answer 6

1) Proximal: 3rd–5th ribs near their costal
cartilages
2) Distal: Medial border and superior surface of coracoid process of scapula

Question 7

What is the function of the pectoralis minor?

Answer 7

Stabilizes scapula by drawing it inferiorly and anteriorly against thoracic wall

Question 8

What are the proximal and distal attachments of the serratus anterior?

Answer 8

1) Proximal: External surfaces of lateral parts
of 1st–8th ribs
2) Distal: Anterior surface of medial border of scapula

Question 9

What is the function of the serratus anterior?

Answer 9

1) Protracts scapula and holds it against thoracic wall

2) Rotates scapula

Question 10

What innervates the serratus anterior?

Answer 10

Long thoracic nerve (C5, C6, C7)

Question 11

What allows for upward rotation of the scapula?

Answer 11

The serratus anterior works synergistically with the superior and inferior parts of the trapezius to rotate the scapula upward thereby enabling the deltoid to fully abduct the arm above the horizontal plane

Question 12

Why is the long thoracic nerve vulnerable to injury and how are abduction and rotation at the glenohumeral joint hindered after its injury? How does the winged scapula deformation occur?

Answer 12

1) When the serratus anterior is paralyzed owing to injury to the long thoracic nerve, the medial border of the scapula moves laterally and posteriorly away from the thoracic wall, giving the scapula the appearance of a wing, especially when the person leans on a hand or presses the upper limb against a wall
2) When the arm is raised, the medial border and inferior angle of the scapula pull markedly away from the posterior thoracic wall, a deformation known as a winged scapula
3) In addition, the upper limb may not be able to be abducted above the horizontal position because the serratus anterior is unable to rotate the glenoid cavity superiorly to allow complete abduction of the limb
4) Remember, the trapezius also helps raise the arm above the horizontal. Although protected when the limbs are at one’s sides, the long thoracic nerve is exceptional in that it courses on the superficial aspect of the serratus anterior, which it supplies. Thus when the limbs are elevated, as in a knife fight, the nerve is especially vulnerable. Weapons, including bullets directed toward the thorax, are a common source of injury

Question 13

What are the pectoral muscles derived from?

Answer 13

The pectoral muscles are derivatives of myoblasts in the parietal layer of lateral plate mesoderm

Question 14

What is the Poland sequence?

Answer 14

1) A Poland sequence is characterized by absence of the pectoralis minor and partial loss of the pectoralis major (usually the sternal head) muscles
2) The nipple and areola are absent or displaced, and there are often digital defects (syndactyly [fused digits] and brachydactyly [short digits]) on the affected side
3) Can be problematic in females due to breast development

Question 15

How do mammary glands and nipples form?

Answer 15

1) Paired mammary glands are modified apocrine sweat glands with a cutaneous origin. Present in both males and females, they consist of parenchyma, which is formed from ducts, and connective tissue stroma. Parenchyma derives embryonically from surface ectoderm; stroma arises from surrounding mesenchyme
2) The 6-week embryo has two ventral ridge-like thickenings of epidermis, the mammary (milk) lines, extending from axillae to the inguinal area. The major part of each ridge disappears almost immediately, but one pair remains in the pectoral area and penetrates the mesenchyme
3) Then, 15-25 solid epithelial cords develop from each and are later canalized to form future lactiferous ducts. Mesenchyme gives rise to loose connective tissue around each duct. Denser connective tissue forms septa between them to divide the gland into lobes

Question 16

What occurs to the breasts through childhood and puberty?

Answer 16

1) In childhood, the breasts consist of 15-25 rudimentary lactiferous ducts that open onto the surface of the nipple
2) During puberty, ovarian estrogen and progesterone induce budding of multiple, small terminal ducts from the distal end of each lactiferous duct. The ovarian hormones also induce white fat accumulation between the budding terminal ducts

Question 17

What occurs to the breasts during pregnancy?

Answer 17

1) During pregnancy, the ovarian hormones and prolactin from the pituitary gland stimulate secretory acini formation from distal ends of the terminal ducts
2) The hormones also induce milk secretion into the lumens of the acini during lactation

Question 18

What occurs to the breasts after breast feeding? What occurs to the breasts after menopause?

Answer 18

1) After breast-feeding, the breasts return to their resting state
2) After menopause, acini and terminal ducts regress but remaining lactiferous ducts can transform into fluid-filled cysts

Question 19

On what muscles do they breasts lie on?

Answer 19

Two-thirds of each breast lies on the pectoralis major muscle and one-third rests on the serratus anterior muscle

Question 20

What firmly attaches to the breasts to the overlying dermis of the skin?

Answer 20

1) The mammary glands are firmly attached to the dermis of the overlying skin, especially by substantial skin ligaments
(L. retinacula cutis), the suspensory ligaments
2) These condensations of fibrous connective tissue, particularly well developed in the superior part of the gland, help support the lobes and lobules of the mammary gland

Question 21

Where does lymph from the lateral and medial quadrants of the breasts drain to?

Answer 21

1) Most lymph formed in lateral quadrants of the breasts drains initially into the pectoral lymph nodes, a constituent group of the axillary nodes
2) Lymph from the medial quadrants drains into the parasternal lymph nodes that, like the axillary nodes, drain mainly into the supraclavicular lymph nodes

Question 22

Where do breast carcinomas typically arise and what is the most common site for metastasis?

Answer 22

1) Understanding the lymphatic drainage of the breasts is of practical importance in predicting the metastases (dispersal) of cancer cells from a carcinoma of the breast (breast cancer)
2) Carcinomas of the breast are malignant tumors, usually adenocarcinomas (glandular cancer) arising from the epithelial cells of the lactiferous ducts in the mammary gland lobules
3) Metastatic cancer cells that enter a lymphatic vessel usually pass through two or three groups of lymph nodes before entering the venous system
4) Breast cancer typically spreads by means of lymphatic vessels (lymphogenic metastasis), which carry cancer cells from the breast to the lymph nodes, chiefly those in the axilla. The cells lodge in the nodes, producing nests of tumor cells (metastases)
5) Because most of lymphatic drainage of the breast is to the axillary lymph nodes, they are the most common site of metastasis from a breast cancer