Anatomy of the arm

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Introduction0:00–0:50

Anatomically speaking, the arm is the part of the upper limb that is between the shoulder and the elbow joint. It has only one bone called the humerus, and an intricate network of muscles, vessels and nerves distributed around it.
The arm muscles are divided into two compartments separated by the humerus and the medial and lateral intermuscular septae.
The first one is the anterior compartment, and this one houses muscles responsible for elbow flexion. So it’s not a surprise that the anterior compartment is also called the flexor compartment.
Then there’s the posterior compartment. This one houses muscles responsible for elbow extension.
So it’s also called the extensor compartment. These two compartments mainly act on the elbow joint, but some muscles can also act on the glenohumeral joint.
Now, the anterior compartment contains three muscles: the biceps brachii or simply the biceps; the brachialis; and the coracobrachialis muscles.

Anterior compartment muscles0:50–1:01

Biceps brachii1:01–3:03

The biceps muscle usually has two heads, a short head and a long head. The short head attaches to the coracoid process of the scapula and descends anteromedially to the head of the humerus, while the long head attaches to the supraglenoid process of the scapula and descends in the intertubercular or bicipital groove.
It is held in the bicipital groove by the transverse humeral ligament, which extends from the lesser to the greater tubercle of the humerus to turn the groove into a canal for the tendon of the long head.
Both of these heads converge together and attach distally to the tuberosity of the radius, and blend with the fascia of the forearm via the bicipital aponeurosis.
The bicipital aponeurosis is a triangular membranous band that runs across the cubital fossa to blend with the deep fascia covering the flexor muscles on the medial side of the forearm.
The biceps is innervated by the musculocutaneous nerve, and acts on three different joints; the glenohumeral, the elbow joint, and the proximal radio-ulnar joint, though it primarily acts on the latter two.
On the glenohumeral joint, the biceps helps to prevent dislocation and stabilize the shoulder joint due to the tendon of the long head supporting the humeral head into the glenoid cavity.
The biceps also has a minor role in flexing and adducting the arm. Most importantly, the biceps muscle acts on the humero-ulnar joint by flexing the elbow joint.
A practical example of this is when you lift up a heavy object, like a bag full of groceries. The biceps muscle also acts on the proximal radioulnar joint.
Due to the orientation of the bicipital aponeurosis, flexion of the biceps pulls the aponeurosis and helps the supinator muscle to supinate the forearm, which is turning the palm upwards.
To remember the action of forearm supination, think about cupping your hands together to form a bowl and using it to drink a bowl of soup!

Coracobrachialis3:03–3:44

Alright, the next muscle in the anterior compartment is the coracobrachialis, which lies in the superior part of the arm towards the medial side.
Proximally, the coracobrachialis attaches to the coracoid process of the scapula, while distally, it attaches to the middle third of the anteromedial surface of the humeral midshaft.
So the coracobrachialis doesn't reach the elbow. As a member of the anterior compartment, it's also innervated by the musculocutaneous nerve.
The coracobrachialis mainly helps to flex and adduct the glenohumeral joint. It also plays an important role in stabilizing the shoulder joint by resisting downwards dislocation.

Brachialis3:44–4:23

Finally, the third muscle in the anterior compartment of the arm is the brachialis. Proximally, the brachialis muscle attaches to the distal half of the anterior surface of the humerus.
It then extends distally, crosses the elbow joint to attach to the coronoid process and tuberosity of the ulna. Unlike its neighbors in the anterior compartment which are only innervated by the musculocutaneous nerve, the brachialis muscle is also innervated by the radial nerve.
The brachialis only has one role: it flexes the forearm at the elbow joint, regardless of whether it’s pronated or supinated.

Posterior compartment muscles4:23–4:38

Now let’s switch gears and look at the two muscles in the posterior compartment of the arm: the triceps brachii or just the triceps, and the anconeus muscles, both of which are innervated by the radial nerve.

Triceps brachii4:38–5:32

With three bulky heads, the triceps is the main extensor of the forearm. Proximally, its long head attaches to the infraglenoid tubercle of the scapula, the lateral head to the posterior surface of the humerus just superior to the radial groove, and finally, the medial head attaches to the posterior surface of the humerus, immediately inferior to the radial groove.
As the muscle extends distally, all of its three heads fuse together, having a common distal attachment that is to the olecranon and the fascia of the forearm.
All three heads contribute to the extension of the forearm, however the medial head is the workhorse of forearm extension.The long head in particular also helps stabilize the shoulder joint and prevents dislocation by resisting downward forces of the arm.
It also helps extend and adduct the glenohumeral joint. Finally, let’s talk about the anconeus, which is this small triangular muscle at the posterior aspect of the elbow.

Anconeus5:32–5:59

Proximally, the anconeus attaches to the lateral epicondyle of the humerus. It extends distally to attach to the lateral surface of the olecranon and the proximal ulna.
The contraction of the anconeus assists the triceps in extending the forearm, and adds some stability to the elbow joint.

Quiz5:59–6:10

Alright!! Now let’s pause for a minute and see if you can identify the five muscles of the arm!

Nerves - Musculocutaneous nerve6:10–6:50

Ok then! Now let’s have a look at the nerves of the arm.
First, there is the musculocutaneous nerve.This nerve originates from the C5, C6, and C7 spinal nerve roots, and emerges as the terminal branch of the lateral cord of the brachial plexus, at the inferior border of the pectoralis minor muscle.
It pierces the coracobrachialis muscle, and continues distally, passing between the biceps and the brachialis. At this point, it sends branches that supply all of these three muscles, and then emerges lateral to the biceps with a new name: the lateral cutaneous nerve of the forearm where it supplies the skin of the lateral forearm.

Median nerve6:50–7:24

Next, there’s the median nerve, which originates from the C6, C7, C8, and T1 spinal roots, and arises from the medial and the lateral cords of the brachial plexus, and descends from the axilla to the arm.
In the arm, the median nerve initially runs on the lateral side of the brachial artery until it reaches the middle of the arm.
At this point, the median nerve crosses the brachial artery over to the medial side. This nerve keeps descending in the cubital fossa, passing deep to the bicipital aponeurosis and the median cubital vein.
Next, there’s the ulnar nerve. It originates from the C8 and T1 spinal roots, and emerges from the medial cord of the brachial plexus.

Ulnar nerve7:24–8:15

It then travels from the axilla to the arm, passing anterior to the teres major attachment on the humerus and to the long head of the triceps muscles, on the medial side of the brachial artery.
In the middle arm, the ulnar nerve pierces the medial intermuscular septum to enter the posterior compartment of the arm, descending distally between the medial head of the triceps muscle and the medial intermuscular septum.
In the distal arm, the ulnar nerve passes posterior to the medial epicondyle to enter the forearm. The median and the ulnar nerves only give small articular branches to the elbow.
Other nerves that pass through the arm include the medial cutaneous nerves of the arm, and the medial cutaneous nerve of the forearm.

Medial cutaneous nerves8:15–8:57

Both of these nerves carry fibers that originate from the C8 and T1 spinal roots, and they arise from the medial cord of the brachial plexus, providing sensory innervations.
The medial cutaneous nerve of the arm supplies the skin on the medial side of the arm as far as the medial epicondyle, and the median cutaneous nerve of the forearm supplies the skin on the medial side of the forearm as far as the distal wrist.
And let’s just briefly touch on the cutaneous innervation of the proximal and distal lateral portion of the arm, which are innervated by the axillary nerve, and the radial nerve, respectively.
Let’s wrap up on the nerves by looking at the radial nerve, which innervates muscles of the posterior compartment of the arm and forearm.

Radial nerve8:57–9:52

The radial nerve originates from the C5 to T1 spinal roots, and emerges from the posterior cord of the brachial plexus in the axilla.
After it exits the axilla, the radial nerve enters the posterior compartment of the arm behind the brachial artery and medial to the humerus.
It then runs inferolaterally in the posterior arm accompanied by the profunda brachii artery and passes around the shaft of the humerus in the radial groove.
When it passes through the radial groove and reaches the lateral border of the humerus, it pierces the lateral intermuscular septum to enter the anterior compartment of the arm, then coursing around to the lateral epicondyle to enter the cubital fossa, where it divides into deep and superficial branches.

Quiz9:52–10:07

OK, so before we continue, feel free to press pause and see if you can identify the median nerve, the radial nerve, and medial cutaneous nerve of the arm.

Vessels - Brachial artery10:07–12:17

Moving on, let’s have a look at the arteries in the arm . The main artery supplying blood to the structures of the arm is the brachial artery, which continues from the axillary artery after it passes the inferior border of the teres major muscle.
In the arm, the brachial artery initially lies medial to the humerus, and it can be palpated here by pressing it against the medial aspect of the humerus.
Then, the brachial artery travels inferolaterally in the arm, accompanied by the median nerve, which crosses anterior to it.
When the brachial artery arrives in the mid-arm region, it gives off its biggest branch called the profunda brachii, also called the deep artery of the arm.
But “profunda brachii” sounds fancier, doesn’t it? The profunda brachii courses posterolaterally to the humerus between the long head and lateral head of the triceps, traveling along the radial nerve in the radial groove.
It terminates at the level of the elbow, by dividing into the middle and radial collateral arteries that contribute to the periarticular arterial anastomosis around the elbow.
Right after giving off the profunda brachii in the mid-arm region, the brachial artery gives off its second branch, the superior ulnar collateral artery.
After it emerges, the superior ulnar collateral artery accompanies the ulnar nerve in the posterior aspect of the arm as it runs posterior to the medial epicondyle of the humerus.
Here, the superior ulnar collateral artery anastomoses with the posterior ulnar recurrent artery, and the inferior ulnar collateral artery, which all participate in the articular anastomoses around the elbow joint.
The third branch of the brachial artery is the inferior ulnar collateral artery. It also originates medially in the distal arm, immediately superior to the medial epicondyle.
Then it passes anterior to the medial epicondyle and terminates by anastomosing with the anterior ulnar recurrent artery.
The brachial artery eventually terminates in the cubital fossa and has two main terminal branches called the ulnar and radial arteries which go on to supply the hand and forearm.
And, you know it, after the arteries, come the veins. The arm has both superficial and deep veins, which freely anastomose with each other: the superficial veins, which course through the subcutaneous tissue; and the deep veins, which are usually buried deep in muscles and accompany major arteries.

Cephalic vein12:17–13:09

In the arm, we have two major superficial veins: The cephalic vein which typically runs on the lateral aspect of the arm, and the basilic vein which runs on the medial aspect.
As the cephalic vein ascends from the anterolateral aspect of the forearm, it courses superiorly in the arm on the lateral side between the deltoid and pectoralis major muscles in the deltopectoral groove through the deltopectoral triangle.
At this level, the cephalic vein empties into the terminal part of the axillary vein after it pierces the costocoracoid membrane of the clavipectoral fascia.
On the medial aspect of the arm, the basilic vein also ascends from the medial side of the forearm, and when it gets in the middle of the distal third of the arm, the basilic vein pierces the brachial fascia to pass deep to it, running superiorly parallel to the brachial artery, eventually merging with the deep veins accompanying the axillary artery to drain into the axillary vein.

Basilic vein13:09–13:30

Brachial vein13:30–13:57

In the arm, there is only one important deep vein, and that’s the brachial vein, or rather the paired veins running with the brachial artery known collectively as the brachial vein.
The brachial vein begins at the elbow and results from the junction of the ulnar and the radial veins in the cubital fossa.
As it ascends in the arm, the brachial vein accompanies the brachial artery, joins with the basilic vein to form the axillary vein.
Finally, let’s look at the lymphatic drainage of the arm. Just like the veins, we have superficial and deep lymphatic vessels.

Lymphatic drainage13:57–14:37

Superficial lymphatic vessels arise in the arm from the cubital lymph nodes located proximal to the medial epicondyle and medial to the basilic vein, and then ascend in the arm terminating in the lateral or humeral axillary lymph nodes.
The most superficial lymph vessels accompany the cephalic vein, and enter the apical axillary lymph nodes or the more deltopectoral lymph nodes.
The deep lymphatic vessels follow the course of the brachial vein, terminating into the humeral or lateral axillary lymph nodes.
One last pause before we recap!! Do you still remember the major branches of the brachial artery in the arm?

Quiz14:37–14:50

Here’s a hint - there’s three of them! Alright, as a quick recap.

Review14:50–16:02

The anterior compartment of the arm contains three muscles: the biceps brachii, the coracobrachialis, and the brachialis.
All three of these muscles are innervated by the musculocutaneous nerve, and the brachialis also receives innervation from the radial nerve.
The posterior compartment of the arm only has two muscles which are both innervated by the radial nerve; the three-headed triceps, and the little anconeus.
The major nerves in the arm are the median nerve, the ulnar nerve, the medial cutaneous nerve the radial nerve, and the musculocutaneous nerve, all of which derive from the brachial plexus.
The main artery of the arm is the brachial artery, and it gives its three main branches in the medial aspect of the arm: the profunda brachii, and the superior and inferior ulnar collateral arteries.
The superficial veins of the arm are the cephalic vein on the lateral aspect of the arm, which empties into the axillary vein; and the basilic vein located in the medial aspect of the arm, which merges with the brachial vein to form the axillary