Breast and Pectoral Region

#2 - Pectoral Region

After turning the cadaver to the supine position make the following skin incisions (N174) (once again exact placement is not significant):
1. From the suprasternal notch (N174) to the xiphoid process (67/N174).
2. From the tip of the xiphoid process laterally one-half way around the body.
3. From the suprasternal notch along the clavicle over the shoulder and down the arm to a point halfway between the shoulder and elbow.
4. Carry the last incision medially one-half way around the arm.
Reflect the skin and superficial fascia on the thoracic wall, axilla and arm. Abduction of the arm will facilitate access to the axilla.
As superficial fascia is removed, anterior and lateral cutaneous branches of intercostal nerves (77/N182) accompanied by perforating and lateral branches of intercostal arteries may be encountered.
If your cadaver is a female, make a sagittal section through the nipple and the mammary gland (72-74/N175). Locate several lactiferous sinuses and lactiferous ducts. (The mammary glands atrophy with age: in most cadavers glandular tissue is not evident.) Please demonstrate your findings to neighboring dissection teams with male cadavers.
Clean the pectoralis major muscle (685/N182). Note that it forms the anterior wall of the axilla.
Locate the cephalic vein (684/N182) coursing between the pectoralis major and deltoid muscles.
Cut across the abdominal and sternal parts of the pectoralis muscle about 2 inches from their attachments to ribs and rectus sheath and the clavicular part one-half inch from the clavicle. Before completing the detachment of the clavicular origin isolate the lateral pectoral nerve (709/N182) and branches of the thoracoacromial artery (699/N182) by probing the deltopectoral triangle. As the pectoralis major is reflected laterally, the medial pectoral nerve (712/N182) piercing the pectoralis minor before entering the deep surface of pectoralis major will be encountered, as well as pectoral branches of the thoracoacromial a. Save as many of these structures as possible; some may require cutting to complete the muscle reflection. These two nerves have not been named for their positions in the body, but rather for their origins from cords of the brachial plexus.
Note the pectoralis minor (689/N183).
The clavipectoral fascia stretches between the clavicle and the pectoralis minor muscle. Structures that pass through this fascia are:
1. Lateral pectoral nerve.
2. Cephalic vein.
3. Thoracoacromial branch of the axillary artery (note its branches to surrounding structures).
Slit the clavipectoral fascia at a point close to the clavicle and expose the subclavius muscle (689/N183), which is encased by the tough fascial layer.
With the arm abducted, considerable fat will be noted in the axilla. The fat must be removed with blunt dissection; fingers are effective dissection tools in this area. Another approach is to clean the nerves and blood vessels rather than picking out the fat. For exposure of nerves and blood vessels a hemostat is a particularly useful tool. Lymph nodes may be encountered, as well as the intercostobrachial nerve (87/N183), the lateral cutaneous branch of the second intercostal nerve, that traverses the fat. Lymph nodes tend to be found at branch points of major blood vessels. Lymphatic vessels (capillaries) usually carry lymph that moves retrograde to arterial flow. As you encounter lymph nodes you should investigate in the text book the structures or areas they drain because they will be the site for metastases from malignant tumors. After removal of the bulk of the fat, identify all of the structures forming the boundaries of the axilla (N411).
1. Anterior wall
  1. Pectoralis major muscle
  2. Pectoralis minor muscle
2. Posterior wall
  1. Subscapularis muscle
  2. Latissimus dorsi muscle
  3. Teres major muscle
3. Lateral wall
  1. Humerus
  2. Flexor muscles in the arm
4. Medial wall
  1. Serratus anterior muscle
Palpate the axillary sheath of fascia (699/N411) containing the axillary vessels and parts of the brachial plexus. The axillary sheath is a sleeve-like extension of the fascia in the neck which surrounds the nerves and vessels as they pass from neck to axilla behind the clavicle.
Note that the pectoralis minor passes anterior to the axillary sheath and arbitrarily divides the axillary artery (N412) into three parts for descriptive purposes (699). Detach the pectoralis minor from its attachment to ribs and reflect superiorly.
Note that the axillary artery (N412) is surrounded by parts of the brachial plexus. Being careful not to remove or cut any branches of the plexus, dissect the axillary artery and as many of the following branches as possible (tributaries of the axillary vein must be removed in order to expose more important structures, but attempt to preserve the axillary vein itself, unless it is so large that it obscures major structures).
1. First part.
  1. Superior thoracic. ( Unimportant branch)
2. Second part.
  1. Thoracoacromial and its 4 branches or areas (N410).
  2. Lateral thoracic (N410).
3. Third part.
  1. Subscapular (N410), its circumflex scapular branch (N410), and its continuation as the thoracodorsal artery (N410).
  2. Posterior humeral circumflex (N410) - accompanies axillary nerve.
  3. Anterior humeral circumflex (N410) - small, not as important as posterior.
With blunt dissection separate the cords of the brachial plexus (709-713/N413) from the axillary artery (only the infraclavicular portion of the brachial plexus will be seen at this time; the remainder will be exposed when the root of the neck is dissected). Locate all the following branches (See Netter plates 412& 413):
1. From the lateral cord.
  1. Lateral pectoral (N412 & 413) - to pectoralis major muscle.
  2. Musculocutaneous (N412 & 413) - follow to coracobrachialis muscle at this time.
  3. Lateral root of the median (N412 & 413) (no branches in the arm).
2. From the medial cord
  1. Medial pectoral (N412 & 413) - to pectoralis minor and major muscles
  2. Medial brachial cutaneous (N412 & 413) (Medial cutaneous of the arm) a slender nerve which lies medial to axillary vein. It will hang free because of skin removal.
  3. Medial antebrachial cutaneous (N412 & 413) (medial cutaneous of the forearm)
  4. Ulnar (no branches in the arm) (N412 & 413).
  5. Medial root of the median (N412 & 413).
3. From the posterior cord
  1. Upper subscapular (N412 & 413) - follow to subscapularis muscle
  2. Thoracodorsal (N412 & 413) - follow to latissimus dorsi muscle.
  3. Lower subscapular (N412 & 413) - follow to subscapularis muscle and teres major muscle.
  4. Axillary (N412 & 413) - follow to deltoid and teres minor muscle.
  5. Radial (N412 & 413) - follow until it disappears deep to triceps muscle fibers. Note its branches to long and lateral heads of the triceps prior to this point.
4. Direct branches.
  1. The long thoracic nerve (N412 & 413) will be found on the lateral surface of the serratus anterior (N412). It is an unusual position. Most muscles are innervated from their deep surfaces. Its origin from anterior rami will be discovered in the root of the neck.
  2. The intercostobrachial nerve (N412) from T2 (not a branch of the brachial plexus) emerges from the second intercostal space, supplies skin on the inner part of the arm after traversing the fat of the axilla.

The musculocutaneous, median, and ulnar nerves form the letter "M" anterior to the axillary artery. However, there are variations in pattern. Ask your lab instructor to clarify any departures from the usual textbook pattern.

During the course of your dissection you should find many instances in which the anatomic structure in the body being dissected varies from text book description. Muscles may be doubled or missing, branching of blood vessels may be discrepant from the usual pattern, embryological remnants may persist, etc. You are asked to record any variations you discover on forms posted on the bulletin board in the lab so that all class members may benefit from your finding. It should be obvious to you that variations from normal text on atlas anatomy can become very important for examining clinicians and for surgeons. One of the great benefits of cadaver dissection is the understanding that there is variation and that one should approach every patient with awareness that not everyone is exactly alike.

One of the major end points of study of nerves is to be able to predict the deficits which would occur when the nerve is lesioned, i.e., severed or crushed. To do this it is necessary to know what structures the nerve supplies, what function those structures have, and what residual functions persist. The effects of nerve lesion may be described in the position assumed by the limb because of unopposed action of antagonist muscles, or the inability or weakness about which a patient might complain.