The Heart and Mediastinum

Question 1

[12-minute video]: external features of the heart

Answer 1

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Question 2

[11-minute video]: Coronary Circulation

Answer 2

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Question 3

[6-minute video]: cadaveric demonstration of the Coronary Arteries and their branches

Answer 3

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Question 4

[4-minute video]: cadaveric demonstration of the Cardiac Veins

Answer 4

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Question 5

Outline the divisions of the mediastinum.

Answer 5

The mediastinum is artificially divided into two parts:
(a) superior mediastinum and (b) inferior mediastinum by an imaginary plane (transverse thoracic plane) passing through the sternal angle anteriorly, and lower border of the body of the fourth thoracic (T4) vertebra/intervertebral disc T4 and T5 vertebrae posteriorly.

The inferior mediastinum is further subdivided into three parts by the pericardium (enclosing heart). The part in front of the pericardium is called anterior mediastinum, and the part behind the pericardium is called posterior mediastinum. The pericardium and its contents (heart and roots of its great vessels) constitute the middle mediastinum.

Question 6

State the boundaries of the mediastinum.

Answer 6

Anterior: Sternum
Posterior: Vertebral column (bodies of thoracic vertebrae and intervening intervertebral discs)
Superior: Superior thoracic aperture
Inferior: Diaphragm
On each side: Mediastinal pleura

Question 7

State the boundaries of the superior mediastinum.

Answer 7

Anterior: Manubrium sterni
Posterior: Bodies of upper four thoracic vertebrae
Superior: Plane of superior thoracic aperture (thoracic inlet)
Inferior: An imaginary plane passing through the sternal angle in front and lower border of the body of fourth thoracic vertebra behind (transverse thoracic plane)
On each side (lateral): Mediastinal pleura

Question 8

State the contents of the superior mediastinum in terms of:
(a) arteries
(b) veins

Answer 8

(a) arteries
☛ Arch of aorta [and three branches:]
☛ Brachiocephalic artery
☛ Left common carotid artery
☛ Left subclavian artery

(b) veins
☛ Right and left brachiocephalic veins
☛ Upper half of the superior vena cava (SVC)
☛ Left superior intercostal vein
[Diagram]

Question 9

State the contents of the superior mediastinum in terms of:
(a) nerves
(b) lymphoid organs and lymphatics

Answer 9

(a) nerves
☛ right and left phrenic nerves
☛ right and left vagus nerves
☛ left recurrent laryngeal nerve
☛ sympathetic trunks and cardiac nerves
[Diagram]

(b) lymphoid organs and lymphatics
☛ lymph nodes
☛ thoracic duct
☛ thymus

Question 10

State the contents of the superior mediastinum in terms of:
(a) viscera
(b) muscles

Answer 10

(a) viscera
☛ trachea
☛ oesophagus
[Diagram]

(b) muscles
☛ sternohyoid
☛ sternothyroid
☛ longus colli

Question 11

State the contents of the anterior mediastinum.

Answer 11

☛ loose areolar tissue
☛ superior and inferior sternopericardial ligaments stretching between the sternum and pericardium
☛ three or four lymph nodes
☛ mediastinal branches of internal thoracic (mammary) arteries
☛ lower portion of thymus (in children)

Question 12

The middle mediastinum contains the heart and pericardium and other structures. Name the arteries and veins present within the middle mediastinum.

Answer 12

Arteries:
☛ Ascending aorta
☛ Pulmonary trunk dividing into two pulmonary arteries
☛ Pericardiacophrenic arteries

Veins:
☛ Superior vena cava (lower part)
☛ Upper part of inferior vena cava
☛ Azygos vein (terminal part)
☛ Pulmonary veins (right and left)

Question 13

Name the nerves, lymph nodes and viscera of the middle mediastinum.

Answer 13

Nerves:
☛ phrenic nerves
☛ deep cardiac plexus

Lymph nodes:
☛ Tracheobronchial lymph nodes

Viscera:
☛ Bifurcation of the trachea
☛ Right and left principal bronchi

Question 14

State 7 contents of the posterior mediastinum.

Answer 14

☛ esophagus
☛ thoracic duct
☛ descending thoracic aorta and its branches
☛ azygos, hemiazygos and accessory hemiazygos veins
☛ vagus nerves
☛ sympathetic trunks and splanchnic nerves
☛ posterior mediastinal lymph nodes

Question 15

List the events that occur at the sternal angle.

Answer 15

1. The tracheal bifurcation
2. The intervertebral disk between T4 and T5 lies at this angle.
3. The pulmonary trunk divides into the right and left pulmonary arteries at this level.
4. Tracheobronchial nodes surround the tracheal bifurcation.
5. The azygos vein joins the superior vena cava from behind near the sternal angle.
6. The ligamentum arteriosum connects the aortic arch to the left pulmonary artery.
7. The thoracic duct crosses from the right to the left of the midline as it ascends along the vertebral column.
8. The alar fascia fuses with the buccopharyngeal fascia

Question 16

State the common causes of mediastinal syndrome.

Answer 16

☞ bronchogenic carcinoma
☞ aneurysm of aorta
☞ enlargement of mediastinal lymph nodes in Hodgkin syndrome

Question 17

State the clinical features of mediastinal syndrome.

Answer 17

☞ enlargement of veins in the upper half of the body; due to obstruction of superior vena cava
☞ dyspnea (difficulty in breathing); due to compression of trachea
☞ dysphagia (difficulty in swallowing); due to compression of esophagus
☞ dysphonia; due to compression of left recurrent laryngeal nerve
☞ erosion of bodies of thoracic vertebrae; due to pressure on the vertebral column

Question 18

(a) State the commonest cause of widening of inferior mediastinum.
(b) Why is mediastinum a dynamic, pliable, and movable septum?
(c) State the common cause of mediastinal shift to the diseased side.
(d) State the common cause of mediastinal shift to the healthy side.

Answer 18

(a) Hypertrophy of the heart
(b) Because structures forming it are hollow, fluid- or air-filled and bound together by loose connective tissue
(c) Collapse of lung and atelectasis
(d) Pneumothorax or hydrothorax

Question 19

The pericardium is a fibroserous sac which encloses the heart and the roots of its great blood vessels. State the functions of the pericardium.

Answer 19

(a) restricts excessive movements of the heart,
(b) serves as a lubricated container in which heart can contract and relax smoothly, and
(c) limits the cardiac distension.

Question 20

State the two components of the pericardium.

Answer 20

(a) an outer single layered fibrous sac called fibrous pericardium, and
(b) inner double layered serous sac called serous pericardium.

☞ The serous pericardium has a parietal layer and a visceral layer (epicardium)
[Diagram]

Question 21

State the differences between the parietal and visceral pericardium.

Answer 21

☞ Parietal pericardium is adherent to the fibrous pericardium while visceral pericardium is adherent to the myocardium of the heart.
☞ Parietal pericardium develops from somatopleuric mesoderm while visceral pericardium develops from splanchnopleuric mesoderm.
☞ Parietal pericardium is innervated by the somatic nerve fibers while visceral pericardium is innervated by the autonomic nerve fibers.
☞ Parietal pericardium is sensitive to pain while visceral pericardium is insensitive to pain.

Question 22

State the contents of the pericardium.

Answer 22

1. Heart with its vessels and nerves
2. Ascending aorta
3. Pulmonary trunk
4. Lower half of superior vena cava
5. Terminal part of inferior vena cava
6. Terminal parts of pulmonary veins

Question 23

Name the two sinuses between the parietal and visceral layers of serous pericardium, and state why they are formed.

Answer 23

1. Transverse sinus
2. Oblique sinus

A reflection of pericardium having the upper margin of left atrium separating the two sinuses

☞ They are formed due to the reflection of visceral layer of serous pericardium around great vessels of the heart.
[Diagram 1] [Diagram 2] [Diagram 3] [Cadaveric image]

Question 24

The epicardium at the roots of great blood vessels is arranged into tubes: (a) arterial tube and (b) venous tube. What vessels does each tube enclose?

Answer 24

(a) arterial tube; ascending aorta and pulmonary trunk
(b) venous tube; superior vena cava, inferior vena cava and four pulmonary veins

Question 25

What is the transverse sinus of the pericardium?

Answer 25

It is a transverse recess behind the ascending aorta and pulmonary trunk and in front of superior vena cava and superior pulmonary veins. It is a horizontal passage between the two pericardial tubes. On each side it communicates with the general pericardial cavity.
☞ It develops due to degeneration of dorsal mesocardium.
[4-minute video]: sinuses of the heart

Question 26

What is the oblique sinus? Why does it develop? What is its role?

Answer 26

☞ It is a recess of serous pericardium behind the base of the heart (actually left atrium). It is enclosed by ‘J-shaped’ sheath of visceral layer of serous pericardium enclosing six veins (i.e., 2 vena cavae and 4 pulmonary veins).
☞ The oblique sinus is akin to lesser sac behind the stomach and develops as a result of absorption of four pulmonary veins into the left atrium.
☞ The oblique sinus permits the distension of left atrium during return of oxygenated blood in it from the lungs.

Question 27

Outline the boundaries of the oblique sinus of pericardium.

Answer 27

Anteriorly: left atrium
Posteriorly: parietal pericardium
On right side: reflection of visceral pericardium along the right pulmonary veins and inferior vena cava
On the left side: reflection of visceral pericardium along the left pulmonary veins
Superiorly: reflection of visceral pericardium along the right and left superior pulmonary veins
Inferiorly: it is open
[4-minute video]: sinuses of the heart

Question 28

State the surgical significance of transverse pericardial sinus.

Answer 28

☞ During cardiac surgery, after the pericardial sac is opened anteriorly, a finger is passed through the transverse sinus of pericardium, posterior to the aorta and pulmonary trunk. [Illustration1] [Illustration2]
☞ A temporary ligature is passed through the transverse sinus around the aorta and pulmonary trunk. The tubes of heart-lung machine are inserted into these vessels and ligature is tightened.

Question 29

State the arterial supply to the pericardium.

Answer 29

To fibrous pericardium and parietal layer of serous pericardium:
1. Internal thoracic artery.
2. Musculophrenic arteries.
3. Descending thoracic aorta.

To visceral layer of serous pericardium:
Coronary arteries.

Question 30

State the nerve supply of the pericardium.

Answer 30

Fibrous pericardium and parietal layer of the serous pericardium: phrenic nerves
Visceral layer of serous pericardium: branches of sympathetic trunks and vagus nerves

Pain of pericarditis originates from ____________ pericardium (parietal or visceral?).

Question 31

Discuss pericarditis and cardiac tamponade.

Answer 31

The inflammation of the serous pericardium is called pericarditis which causes accumulation of serous fluid in the pericardial cavity, the pericardial effusion. The excessive accumulation of serous fluid in the pericardial cavity may compress the thin-walled atria and interfere with the filling of the heart during diastole and consequently the cardiac output is diminished. This condition is clinically termed cardiac tamponade.

Question 32

The heart is ____________ in shape and placed obliquely behind the sternum and adjoining parts of costal cartilages so that one-third of the heart is to the right of median plane and two-third of the heart is to the left of the median plane.

Answer 32

pyramidal

Question 33

The heart consists of four chambers. On the surface the atria are separated from the ventricles by the ____(a)____ and ventricles from each other by ____(b)____.

Answer 33

(a) atrioventricular groove/coronary sulcus
(b) interventricular grooves
[Diagram]; chambers of the heart as viewed from the sternocostal surface
[Diagram]; posterior aspect of the heart

Question 34

The heart presents the following external features: _______________.

Answer 34

1. apex
2. base
3. three surfaces [sternocostal, diaphragmatic, and left]
4. four borders (right, left, upper and inferior)

Question 35

What forms the apex of the heart?

Answer 35

Left ventricle

Further notes:
☞ The apex is directed downwards and forwards, and to the left. It lies at the level of the 5th intercostal space.

Question 36

What forms the base of the heart/posterior surface of the heart?

Answer 36

The two atria, mainly by the left atrium. [2/3 by the posterior surface of the left atrium, 1/3 by the posterior surface of the right atrium]

Note that clinically, base is the upper border of the heart where great blood vessels (superior vena cava, ascending aorta, and pulmonary trunk) are attached.

Question 37

State the characteristic features of the base of the heart.

Answer 37

1. It lies opposite to the apex.
2. It lies in front of the middle four thoracic vertebrae (i.e., T5–T8) in the lying-down position and descends one vertebra in the erect posture (T6–T9).
3. The base is separated from vertebral column by the oblique pericardial sinus, esophagus, and aorta.

Question 38

What forms the sternocostal/anterior surface of the heart?

Answer 38

Mainly by right atrium and right ventricle (separated from each other by anterior atrioventricular groove), partly by left auricle and left ventricle.

Note:
☞ The left atrium is hidden on the front by the ascending aorta and pulmonary trunk.
☞ The part of sternocostal surface is uncovered by the left lung (cardiac notch) forming an area of superficial cardiac dullness.
☞ Auricle ≠ atrium; auricle is outpouching from atrium

Question 39

The diaphragmatic/inferior surface is flat and rests on the central tendon of the diaphragm. What forms it?

Answer 39

The left and right ventricles which are separated from each other by the posterior interventricular groove.

The left ventricles form left two-third of this surface and right ventricle forms only right one-third of this surface.

Question 40

What forms the left surface of the heart?

Answer 40

Mainly by the left ventricle and partly by the left atrium and auricle. It is directed upwards, backwards, and to the left.

Question 41

Discuss the morphology of the right border of the heart.

Answer 41

It is more or less vertical and is formed by the right atrium. It extends from the right side of the opening of SVC to that of IVC and separates the base from the sternocostal surface.

Question 42

Discuss the morphology of the left border of the heart.

Answer 42

It is curved and oblique. It is formed mainly by the left ventricle and partly by the left auricle. It extends from left auricle to the apex of the heart and separates sternocostal and left surfaces.

Question 43

Discuss the morphology of the inferior border of the heart.

Answer 43

It is nearly horizontal and extends from the opening of IVC to the apex of the heart. It is formed by the right ventricle. The right atrium also forms a part of this border. The inferior border separates the sternocostal surface from the diaphragmatic surface. Near the apex it presents a notch called incisura apicis cordis.

Note:
The anterior interventricular sulcus and posterior interventricular sulcus extend from the base of the ventricular portion to a notch, the notch of cardiac apex, (or incisura apicis cordis) on the acute margin of the heart just to the right of the apex.

Question 44

Discuss the morphology of the upper border of the heart.

Answer 44

It is slightly oblique and is formed by the right and left atria, mainly by the latter. The upper border is obscured from the view on the sternocostal surface because ascending aorta and pulmonary trunk lie in front of it. On the surface of the body it can be marked by a line joining a point on the lower border of the 2nd left costal cartilage, 1.5 in from the median plane to a point on the upper border of 3rd right costal cartilage, 1 inch away from the median plane.

Question 45

Name three sulci that are used to demarkate the heart chambers.

Answer 45

1. Coronary sulcus (atrioventricular groove)
2. Anterior interventricular sulcus
3. Posterior interventricular sulcus
4. [5-minute video]

Question 46

Outline the circulation of blood in the heart.

Answer 46

☛ The right atrium receives deoxygenated blood from the whole body through superior and inferior venae cavae.
☛ The blood flows from right atrium into right ventricle through right atrioventricular orifice.
☛ The blood is prevented from regurgitating back to the atrium by the right atrioventricular valve.
☛ The right ventricle contracts and propels the blood into the pulmonary trunk, pulmonary arteries, and finally into the lung where blood is oxygenated (pulmonary circulation).
☛ The left atrium receives the oxygenated blood from lungs through four pulmonary veins.
☛ The blood from left atrium flows into left ventricle through left atrioventricular orifice.
☛ The blood is prevented from regurgitating back to the atrium by means of left atrioventricular valve.
☛ The left ventricle strongly contracts and propels the blood into the ascending aorta and then into the systemic circulation.

Question 47

The Right Atrium
The right atrium has both smooth and trabeculated surfaces known as ____(a)____ and ____(b)____ respectively. The two are separated by the ____(c)____. The trabeculated part has the right atrial appendage [right auricle]. The smooth surface presents two features a fossa and its slightly elevated margin: ____(d)____.

Answer 47

(a) sinus venarum
(b) musculi pectinati [the pectinate muscles]
(c) crista terminalis
(d) fossa ovalis and limbus fossa ovalis/annulus ovalis

Further notes:
The crista terminalis is a C-shaped ridge located in the endocardial aspect of the right atrium. It extends along the posterolateral wall of the right atrium, from the orifice of the superior vena cava to the orifice of the inferior vena cava. Inferiorly, the crista terminalis merges with the valve of the inferior vena cava. It corresponds to the sulcus terminalis on the external surface of the heart.
The sulcus terminalis is a groove on the outer surface of the right atrium of the heart.

Question 48

List the openings into the right atrium and name the valves involved where present.

Answer 48

(a) opening of the superior vena cava
The SVC opens at the upper end of the right atrium and has no valve. It returns the blood to the heart from the upper half of the body.

(b) opening of the inferior vena cava
The IVC opens at the lower end of the right atrium close to the interatrial septum. It is guarded by a rudimentary non-functioning semilunar valve called valve of the inferior vena cava/Eustachian valve.

(c) opening of the coronary sinus
The coronary sinus, which drains most of the blood from the heart, opens into the right atrium between the openings of IVC and right atrioventricular orifice. It is also guarded by a rudimentary non-functioning valve, Thebesian valve.

(d) right atrioventricular orifice [largest opening]
It communicates the right atrial chamber with the right ventricular chamber. It lies anterior to the opening of IVC and is guarded by the tricuspid valve.

(e) many small orifices of small veins
These are the opening of venae cordis minimae (Thebesian veins) and anterior cardiac veins.

Question 49

The Right Ventricle
It has an upper smooth part termed the ____(a)____ which continues with the main pulmonary artery via the ____(b)____.
The lower part is trabeculated, and displays two types of trabecular systems: ____(c)____ and ____(d)____.
The ____(e)____ is a trabecula that houses the right bundle branch.

Answer 49

(a) infundibulum/conus arteriosus
(b) pulmonary valve
(c) trabeculae carneae and (d) papillary muscles
(e) septomarginal (moderator band)

Further notes:
The papillary muscles anchor the cusps of the tricuspid valve.
There are two types of trabeculae: ridges and bridges.

Question 50

State the internal features of the Left Atrium.

Answer 50

1. The interior of left atrium is smooth, but the left auricle possesses muscular ridges in the form of reticulum [is trabeculated].
2. The anterior wall of left atrial cavity presents fossa lunata, which corresponds to the fossa ovalis of the right atrium.

Question 51

State three openings of the left atrium.

Answer 51

(a) Openings of four pulmonary veins in its posterior wall, two on each side. They have no valves.
(b) Left atrioventricular orifice. It is guarded by the mitral valve.

Question 52

The Left Ventricle
The interior of the left ventricle is divided into two parts. State those parts and the differences between them.

Answer 52

The two parts:
(a) a large lower rough inflowing part
(b) a small upper smooth outflowing part known as the aortic vestibule

Differences:
☛ The inflowing part develops from the primitive ventricle; the outflowing part develops from the bulbus cordis.
☛ The inflowing part lies below the aortic vestibule; the outflowing part lies between the membranous part of the interventricular septum and anterior cusp of the mitral valve.
☛ The inflowing part is rough due to presence of trabeculae carneae and forms most of the left ventricular chamber; the outflowing part is smooth and forms smooth small upper part—the aortic vestibule, which gives rise to the ascending aorta.

Question 53

State the openings of the left ventricle.

Answer 53

1. Left atrioventricular orifice
2. Aortic orifice

Question 54

State five differences between the right and left ventricles.
[Hints: blood, wall, papillary muscles, moderator band, cavity shape]

Answer 54

☛ The right ventricle receives deoxygenated blood from right atrium and pumps it to the lungs through pulmonary trunk; The left ventricle receives oxygenated blood from left atrium and pumps it to the whole body through aorta.
☛ The wall of the right ventricle is thinner than that of the left (ratio 1:3).
☛ The right ventricle possesses three papillary muscles [anterior, posterior and septal]; The left ventricle possesses two [anterior and posterior].
☛ The moderator band is present in the right ventricle and absent in the left.
☛ Cavity of right ventricle is crescentic in shape in cross section; Cavity of left ventricle is circular in shape in cross section.

Question 55

Briefly outline the origin and course of the right coronary artery.

Answer 55

origin: anterior aortic sinus of the ascending aorta, immediately above the aortic valve
course:
☛ after its origin from the ascending aorta, it runs forward between the pulmonary trunk and the right auricle
☛ it then descends almost vertically in the right atrioventricular groove up to the junction of the right and the inferior borders of the heart.
☛ At the inferior border of the heart, it turns posteriorly and runs in the posterior atrioventricular groove (right posterior coronary sulcus) up to the posterior interventricular groove where it terminates by anastomosing with the left coronary artery.
[9-minute video]

Question 56

List the branches and distribution of the right coronary artery.

Answer 56

[The arteries in bold are the major branches of the right coronary artery.]

1. Right conus artery: It supplies the anterior surface of the pulmonary conus (infundibulum of the right ventricle).
2. Atrial branches: They supply the atria. One of the atrial branches - the sinoatrial nodal artery supplies the SA node in 60% cases. In 40% of individuals it arises from the left coronary artery.
3. Anterior ventricular branches: They are two or three and supply the anterior surface of the right ventricle. The marginal branch is the largest and runs along the lower margin of the sternocostal surface to reach the apex.
4. Posterior ventricular branches: They are usually two and supply the diaphragmatic surface of the right ventricle.
5. Posterior interventricular artery: It runs in the posterior interventricular groove up to the apex and supplies the:
   ☛ posterior part of the interventricular septum
   ☛ atrioventricular node in 60% of the cases
   ☛ right and left ventricles
   [9-minute video]

Question 57

Briefly describe the origin and course of the left coronary artery.

Answer 57

origin: The left coronary artery arises from the left posterior aortic sinus of the ascending aorta, immediately above the aortic valve.
course:
☛ After arising from the ascending aorta, the left coronary artery runs forwards and to the left between the pulmonary trunk and the left auricle.
☛ It then divides into an anterior interventricular and circumflex artery.
☛ The anterior interventricular artery runs downwards in the interventricular groove to the apex of the heart.
☛ It then passes posteriorly around the apex of the heart to enter the posterior interventricular groove to terminate by anastomosing with the posterior interventricular artery - a branch of the right coronary artery.
☛ The circumflex artery winds around the left margin of the heart and continues in the left posterior coronary sulcus up to the posterior interventricular groove where it terminates by anastomosing with the right coronary artery.
[Video]: left coronary artery starts at 2:29

Question 58

List the branches and distribution of the left coronary artery.

Answer 58

[The arteries in bold are the major branches of the left coronary artery.]

(a) Anterior interventricular artery/left anterior descending artery: It supplies:
☛ anterior part of the interventricular septum
☛ the greater part of the left ventricle and part of the right ventricle
☛ a part of left bundle branch (of His)

(b) Circumflex artery
It gives a left marginal artery that supplies the left margin of the left ventricle up to the apex of the heart.

(c) Diagonal artery
It may arise directly from the trunk of the left coronary artery.

(d) Conus artery: it supplies the pulmonary conus

(e) Atrial branches: They supply the left atrium.

[Video]: left coronary artery starts at 2:29

Question 59

What stands out about anterior cardiac veins in comparison to other veins of the heart?

Answer 59

Unlike the majority of cardiac veins that empty into the coronary sinus, the anterior cardiac veins drain directly into the right atrium.

Question 60

Briefly discuss angina pectoris.

Answer 60

☛ narrowing of coronary arteries causes the blood supply of the cardiac muscles to be reduced
☛ on exertion, the patient feels moderately severe pain in the region of the left pericardium that may last as long as 20 minutes
☛ the pain is often referred to the left shoulder and medial side of the arm and forearm
☛ pain occurs on extertion and is relieved by rest. [This is because the coronary arteries are so narrowed that the ischemia of cardiac muscle occurs only on exertion.]

Question 61

Briefly discuss myocardial infarction.

Answer 61

☛ A sudden block of one of the larger branches of either coronary artery usually leads to myocardial ischemia followed by the myocardial necrosis [myocardial infarction].
☛ The part of the heart suffering from MI stops functioning and often causes death. This condition is termed heart attack or coronary attack.
☛ The clinical features of MI include:
(a) a sensation of pressure/sinking and pain in the chest that lasts longer than 30 mins
(b) nausea or vomiting, sweating, shortness of breath, and tachycardia.
(c) pain radiates to the medial side of the arm, forearm, and hand. Sometimes, it may be referred to jaw or neck.