Bones and Muscles in the Arm

The arm body part is the upper limb of vertebrates. Anatomy of the arm involves the detailed study of its bones, muscles, nerves, blood vessels, and connective tissues. Learning about the arm muscles and arm bones helps in understanding how movements are coordinated and how injuries can be prevented or treated.

In this article, we will cover the anatomy of the arm and read about the nerve supply and blood supply in the arm along with arm muscles and bones, in detail.

An arm is the upper limb or forelimb of vertebrates, such as humans and primates. It’s also known as the brachium in human anatomy. The arm is made up of three sections: the upper arm, forearm, and hand. It extends from the shoulder joint to the fingers and contains 30 bones, including the humerus in the upper arm and the radius and ulna in the forearm.

Major muscles, such as the biceps and triceps, enable flexion and extension. The brachial plexus provides the arm with motor and sensory innervation. Blood is supplied by the brachial, radial, and ulnar arteries and tendons and ligaments provide structural support and stability.

Also Read: Difference Between Ligaments and Tendons

The main muscles of the arm are:

Biceps Brachii

The biceps brachii, or biceps, is a large, thick muscle on the front of the upper arm between the shoulder and elbow.

• Origin: The biceps brachii has two heads:
  • Long head: Originates from the supraglenoid tubercle of the scapula.
  • Short head: Originates from the coracoid process of the scapula.
• Insertion: Both heads insert on the radial tuberosity and the bicipital aponeurosis into the deep fascia on the medial part of the forearm.
• Function: Flexes the elbow and supinates the forearm. It also assists in shoulder flexion due to its origin from the scapula.
• Innervation: Musculocutaneous nerve (C5-C7).

Brachialis

The brachialis is a muscle in the upper arm that flexes the elbow, allowing the forearm to move closer to the body.  It’s located in the anteroinferior area of the arm, deeper than the biceps brachii, and is part of the floor of the elbow pit.

• Origin: Distal half of the anterior surface of the humerus.
• Insertion: Coronoid process and the tuberosity of the ulna.
• Function: Major flexor of the elbow, particularly when the forearm is in a pronated position.
• Innervation: Musculocutaneous nerve (C5-C7) and, occasionally, contributions from the radial nerve (C5-C7).

Coracobrachialis

The coracobrachialis is a slender muscle in the upper arm’s front compartment that flexes and adducts the shoulder joint’s glenohumeral joint. It also helps rotate the arm internally and stabilizes the humeral head.

• Origin: Coracoid process of the scapula.
• Insertion: Medial surface of the humerus shaft.
• Function: Flexes and adducts the shoulder joint. It also stabilizes the humerus within the shoulder joint.
• Innervation: Musculocutaneous nerve (C5-C7).

Triceps Brachii

The triceps brachii, or triceps, is a large, thick muscle on the back of the upper arm that’s responsible for straightening the arm. The triceps is the only muscle that runs along the humerus, the main bone of the upper arm, between the shoulder and elbow.

• Origin: The triceps brachii has three heads:
  • Long head: Originates from the infraglenoid tubercle of the scapula.
  • Lateral head: Originates from the posterior surface of the humerus, superior to the radial groove.
  • Medial head: Originates from the posterior surface of the humerus, inferior to the radial groove.
• Insertion: All three heads converge into a single tendon that inserts on the olecranon process of the ulna.
• Function: It extends the elbow and the long head also assists in shoulder extension and adduction.
• Innervation: Radial nerve (C6-C8).

Other important muscles of the arm are:

• Deltoid: The deltoid covers the shoulder joint and helps in the abduction, flexion, and extension of the arm.
• Brachioradialis: Located on the lateral side of the forearm, the brachioradialis assists in flexing the elbow joint, particularly when the forearm is in a mid-position.
• Flexor Carpi Radialis: Located on the anterior side of the forearm.
• Flexor Carpi Ulnaris: Situated on the posterior side of the forearm.
• Extensor Carpi Radialis Longus and Brevis: These muscles, located on the posterior side of the forearm, extend and abduct the wrist.
• Extensor Carpi Ulnaris: Also situated on the posterior side of the forearm, this muscle extends and adducts the wrist.

Also Read: Difference between Origin and Insertion Muscles

The bones of the arm consist of the humerus in the upper arm and the radius and ulna in the forearm. The arm’s bones enable movement in the shoulder, elbow, wrist, and fingers, which are essential for daily activities.

Forelimbs and Hindlimbs

• The human body has two pairs of limbs, forelimbs, and hindlimbs. Each pair comprises 30 bones.
• These limbs are bilateral pairs found in vertebrate animals, including humans.
• Forelimbs are adapted for functions such as grasping, manipulation, and support.
• Hindlimbs are specialized for locomotion, providing propulsion and support during movement.

Parts of the Arm

The arm is divided into several parts, including the upper arm, lower arm, forearm, and hand.

• Upper Arm: Also known as the brachium, it extends from the shoulder to the elbow joint.
• Lower Arm: It consists of the region between the elbow and the wrist and is commonly divided into the forearm and the hand.
• Forearm: This section includes the radius and ulna bones, extending from the elbow to the wrist.
• Hand: The terminal part of the upper limb, facilitates complex movements and fine motor skills.

Also Read: Difference Between Locomotion And Movement

Bones in the Arm

The human arm has three bones: the humerus, the radius, and the ulna. The humerus is the upper arm bone, and the radius and ulna are the forearm bones. The humerus connects the shoulder to the elbow, and the radius and ulna connect the elbow to the wrist. The three bones form a hinge joint called the elbow.

Humerus

• The humerus is the longest bone in the body, aside from the bones in the legs, and is located in the upper arm between the shoulder and elbow. 
• The head articulates with the scapula, forming the shoulder joint, while the distal end articulates with the radius and ulna.
• It’s responsible for supporting the shoulder and enabling a variety of arm movements. 
• The humerus also supports ligaments, tendons, muscles, and parts of the circulatory system.

Radius and Ulna

• The radius and ulna are the two bones in the forearm that work together to support the wrist and forearm, and allow the hand and wrist to rotate smoothly.
• The ulna is slightly longer and larger than the radius. 
• The radius pivots around the ulna to allow movement at the proximal and distal radio-ulnar joints.

Anatomy of Hand

The anatomy of the hand consists of 27 bones (including the carpals, metacarpals, and phalanges), muscle, tendons, ligaments, and nerves that allow complex movements and fine motor skills.

Wrist (Carpal Bones)

• Made up of 8 carpal bones (Trapezium, Trapezoid, Scaphoid, Lunate, Hamate, Capitate, Pisiform, Triquetrum).
• These eight small bones are arranged in two rows and articulate with the radius and ulna proximally and the metacarpal bones distally.
• They provide stability and flexibility to the wrist joint.

Palm (Metacarpal Bones)

• It contains 5 metacarpal bones.
• The metacarpals form the framework of the palm, connecting the wrist to the fingers.
• Each metacarpal bone corresponds to one of the five digits and provides support for grasping and manipulation.

Fingers (Phalanges)

• It comprises 14 phalanges bones.
• Each finger consists of three phalangeal bones (proximal, middle, and distal), except for the thumb, which has two.
• The phalanges allow for precise movements and gripping actions, essential for various tasks and activities.

Also Read: Arm Muscles Diagram and Anatomy

The blood supply to the arm mainly comes from the brachial artery, a major artery of the upper limb. Blood supply to the arm

Brachial Artery

The brachial artery arises from the axillary artery in the armpit and descends along the medial aspect of the arm. It provides oxygenated blood to the muscles and tissues of the arm and terminates into the radial and ulnar arteries at the elbow.

Branches of the Brachial Artery

• Deep Brachial Artery: Supplies blood to the muscles of the posterior compartment of the arm.
• Superior and Inferior Ulnar Collateral Arteries: Provide collateral circulation around the elbow joint.
• Nutrient Artery: Supplies the bone of the humerus.
• Anastomoses: The brachial artery forms an anastomosis with branches of the radial and ulnar arteries at various points. This network ensures adequate blood supply to the arm even if one of the arteries becomes obstructed.

Venous Drainage

• Veins accompany the arteries and carry deoxygenated blood back to the heart.
• The major veins of the arm include the basilic, cephalic, and brachial veins, which merge to form the axillary vein.

The nerve supply to the arm is provided by branches of the brachial plexus, a network of nerves originating from the spinal cord in the neck. The nerve supply to the arm in detail is given below:

Brachial Plexus

• Formed by the anterior rami (nerve roots) of spinal nerves C5 to T1.
• Located in the neck and axilla.
• Gives rise to several nerves that innervate the muscles and skin of the arm.

Major Nerves of the Brachial Plexus

1. Musculocutaneous Nerve (C5-C7)
   • Innervates the muscles in the anterior compartment of the arm, including the biceps brachii and brachialis.
   • Supplies sensation to the lateral aspect of the forearm.
2. Median Nerve (C6-T1)
   • Innervates muscles in the anterior compartment of the forearm (except flexor carpi ulnaris and part of the flexor digitorum profundus).
   • Provides sensation to the lateral palm and digits 1-3 and the lateral half of digit 4 (thumb, index, middle, and half of the ring finger).
3. Ulnar Nerve (C8-T1)
   • Innervates muscles in the anterior and posterior compartments of the forearm.
   • Supplies sensation to the medial aspect of the hand and digits 4 and 5.
4. Radial Nerve (C5-T1)
   • Innervates muscles in the posterior compartment of the arm and forearm.
   • Provides sensation to the posterior arm, forearm, and hand (except digits 4 and 5).

Other Nerves

• Axillary Nerve: Branch of the posterior cord of the brachial plexus; innervates the deltoid and teres minor muscles.
• Medial and Lateral Pectoral Nerves: Innervate the pectoralis major muscles.
• Dermatomes: Each spinal nerve root contributes to a specific dermatome, an area of skin innervated by sensory fibers from that nerve root. Dermatomes from C5 to T1 cover various regions of the arm and forearm, providing sensory input.

Functions of the Arm include:

• The arm provides the strength and dexterity needed to lift and carry objects of varying sizes and weights.
• It allows for precise movements for grasping objects, whether close or far away.
• The arm muscles allow pushing objects away from the body and pulling them closer, helping in activities such as opening doors and moving furniture.
• The hand and fingers, controlled by the arm muscles, enable the manipulation of objects for tasks like typing, writing, and handling tools.
• Arms play an important role in non-verbal communication through gestures, expressing emotions, emphasizing speech, and conveying information.
• In activities requiring balance or stabilization, such as during sports or physical exertion, the arm muscles contribute to maintaining equilibrium.
• The arms assist in supporting and stabilizing body weight during activities such as climbing, lifting oneself from a seated position, or performing exercises like push-ups.

The arm, or upper extremity, is a functional unit of the upper body that consists of three sections: the upper arm, forearm, and hand. It contains 30 bones, nerves, blood vessels, and muscles. The arm’s muscles allow for fine motor movements, such as wiggling fingers or fastening a button, as well as larger movements, such as straightening the elbow or raising the arms above the head.

Also Read:

• List of 206 Bones in Human Body with Types & Diagram
• Vertebrae – Bones, Diagram and Features
• What Are The 22 Bones Of The Skull?
• Skeletal System: Anatomy and Function, Diagram
• Difference Between Bone And Cartilage
• Difference Between Compact And Spongy Bones

Why is it Called an Arm?

It’s called an “arm” due to its function as a limb extending from the shoulder to the hand, enabling manipulation, movement, and support.

How Many Bones are in an Arm?

The arm consists of three bones: the humerus in the upper arm and the radius and ulna in the forearm.

What is the Longest Bone in the Arm?

The humerus is the longest bone in the arm, extending from the shoulder to the elbow.

How Many Joints are in the Arm?

There are three joints in the arm: the shoulder (glenohumeral) joint, the elbow joint, and the radioulnar joints.

What is the Name of the Muscle in the Arm?

The biceps brachii is a prominent muscle in the arm responsible for flexing the elbow and supinating the forearm.

What is the Lower Arm called?

The lower arm is called the forearm, consisting of the radius and ulna bones.