Human Anatomy Lesson 22
The arm is between the shoulder/axilla and forearm, and is composed of one bone, the humerus, and a small number of muscles on the anterior and posterior sides of the bone. In this lesson, we will follow the courses of various nerves, arteries, and veins that pass from the shoulder region to the forearm, and spend a little bit of time on the cubital fossa, which is the area just anterior to the elbow through which various structures pass to get into the forearm.
Learning Objectives - By the end of this lesson, you should be able to ...
- Describe various features and muscle attachments on the humerus and proximal radius and ulna
- List and describe the origin, attachment, innervation, and action of muscles of the arm
- List and describe arteries and veins of the arm
- List and describe the nerves that course through the arm
- Understand relationships in the cubital fossa
The humerus is the primary bone of the arm, but muscles originating in the arm also insert onto the radius and ulna.
- Humerus: there is a line on the posterior aspect of the humerus that runs in a superomedial to inferolateral direction, that is the attachment area of the lateral head of the triceps brachii muscle. The inferior-most extent of this line is continuous with the deltoid tuberosity approximately halfway down the lateral side of the humerus. The radial groove, which marks the course of the radial nerve and profunda brachii artery, lies just medial and parallel to the line of attachment for the triceps brachii. The attachment site of the coracobrachialis muscle is approximately halfway down the humerus on its medial border. Distally, the lateral supracondylar ridge and medial supracondylar ridge are flattened areas of bone that mark attachment sites for intermuscular septa that separate anterior and posterior compartment muscles. The condyle at the distal end of the humerus is made up of the capitulum, which articulates with the radius, and the trochlea, which articulates with the ulna. The capitulum looks like a ball or pestle, and it articulates with the mortar-looking head of the radius (does anybody know what a mortar and pestle look like anymore?). The trochlea looks like a spool, and it fits tightly into the trochlear notch of the ulna, which looks like a bird's beak. Two epicondyles are positioned laterally and slightly superiorly to either side of the condyle. Muscles in the anterior compartment of the forearm originate from the medial epicondyle, whereas muscles in the posterior compartment of the forearm originate from the lateral epicondyle. On the posterior surface of the humerus, the olecranon fossa marks the area where the olecranon process of the ulna locks into the elbow joint during maximum extension of the forearm relative to the arm.
- Radius: as promised, the head of the radius is about the size of a nickel and disc-shaped, with a shallow depression in the center sort of like the mortar of a mortar-and-pestle. The radial tuberosity is a thickening on the medial and anterior surface of the radius that marks the point of attachment for the biceps brachii muscle. The neck is between the head and radial tuberosity. We will discuss the distal end of the radius in the next lesson when we go over the forearm, wrist, and hand.
- Ulna: the olecranon process of the ulna is the most superior bone on the ulnar head, and inserts into the olecranon fossa of the distal humerus during full extension at the elbow joint. The coronoid process is a beak-like projection inferior to the olecranon, and the trochlear notch is the curved area in between the two. The lateral side of the coronoid process is adjacent to the radial notch, which marks the area where the head of the radius contacts the coronoid process of the ulna.
Muscles of the Arm
The three muscles of the anterior compartment - the coracobrachialis, biceps brachii, and brachialis - are innervated primarily by the musculocutaneous nerve, and the triceps brachii in the posterior compartment is innervated by the radial nerve.
- Coracobrachialis: runs from the coracoid process of the scapula to the medial side of the humerus halfway along its length. As noted in lesson #21, coracobrachialis passes through the axilla, and it crosses the shoulder joint, but not the elbow, so it flexes the arm at the shoulder joint. Coracobrachialis is innervated by C5, C6, and C7 fibers of the musculocutaneous nerve.
- Biceps brachii: this muscle has two heads - a short head, which runs from the coracoid process through the axilla along with coracobrachialis, and a long head, which originates on a tendon from the supraglenoid tubercle of the scapula and runs through the glenohumeral joint and intertubercular sulcus. The two heads of biceps brachii converge to insert onto the radial tuberosity. The tendon also gives off a bicipital aponeurosis which blends with deep fascia over the ulna, thereby extending the attachment area. Biceps brachi is a weak flexor of the arm at the shoulder joint, a strong flexor of the forearm at the elbow joint, and the most powerful supinator of the forearm at the flexed elbow joint. Biceps brachii is innervated by C5 and C6 fibers of the musculocutaneous nerve.
- Brachialis: runs from the distal half of anterior side of the humerus and intermuscular septa and attaches to the tuberosity of ulna inferior to the coronoid process. The brachialis muscle is superficial to the coracobrachialis at midshaft, but is the deepest muscle in the distal half of the arm, and the biceps brachii muscle lies on top of it on its entire course down the arm. Brachialis flexes the forearm at the elbow joint, and is innervated by C5 and C6 fibers of the musculocutaneous nerve.
* Note: a small part of the lateral part of brachialis is innervated by the radial nerve.
- Triceps brachii: as per its name, this muscle has three heads - a long head, which originates on the infraglenoid tubercle of the scapula, a medial head, which originates on the posterior surface of the humerus inferior to the radial groove, and a lateral head, which originates superior to the radial groove on the posterior surface of the humerus. The three heads converge to form a tendon which inserts on the olecranon process of the ulna. Triceps brachii extends the forearm at the elbow joint, although the long head crosses the shoulder joint and so can weakly extend and AD-duct the arm at the shoulder joint. Triceps brachii is innervated by C6, C7, and C8 fibers of the radial nerve.
Arteries and Veins
There are two main arteries of the arm - the brachial artery, which runs on the anterior side of the humerus, and the deep artery of the arm (a.k.a. profunda brachii artery), which is an offshoot of the brachial artery that runs on the posterior side of the humerus.
- Brachial artery: this artery is the continuation of the axillary artery and the subclavian artery before that - it changes name at the lower border of the teres major muscle, and divides into the radial artery and ulnar artery just distal to the elbow. The brachial artery is positioned medial to the arm muscles at the inferior border of teres major, and ends up just medial to the tendon of biceps brachii at the elbow, between the trochlea and capitulum of the distal humerus.
- Deep artery of the arm (profunda brachii artery): this artery branches off the brachial artery approximately 1/3 of the way down the humerus. It enters the posterior compartment with the radial nerve and passes through the triangular interval (made up by the humeral shaft, inferior margin of teres major, and lateral margin of the long head of triceps), running along the radial groove deep to the lateral head of triceps brachii. The deep artery of the arm gives off a superior branch which anastomoses with the posterior circumflex humeral artery and splits inferiorly into the radial collateral artery and middle collateral artery, which contribute to anastomoses at the elbow joint.
- Basilic vein: this large cutaneous vein runs on the medial side of the arm parallel to the short head of biceps brachii in a position medial to the brachial artery. It forms from the basilic vein and median cubital vein in the cubital fossa, runs superiorly and medially, and penetrates the deep fascia approximately 1/3 the way up the humerus, where its name changes to the axillary vein in the axilla.
- Cephalic vein: this vein begins on the lateral side of the forearm, and runs through the cubital fossa into the arm, where it maintains a course parallel to the lateral portion of the long head of biceps brachii. In the shoulder, it runs between the deltoid and pectoralis major muscles over the origin of pectoralis minor, and enters the axillary vein between pectoralis minor and the first rib.
- Paired brachial veins: these two deep veins accompany the brachial artery through the cubital fossa and arm to drain into the basilic vein halfway up the humerus.
The main nerves in the arm are terminal branches of the brachial plexus, which we discussed in lesson #21. Here I provide information about the course of these nerves in the arm and their position as they cross the elbow joint.
- Musculocutaneous nerve: after leaving the axilla, this nerve passes through coracobrachialis and runs inferiorly and laterally between biceps brachii and brachialis, giving off branches to all three muscles. At the elbow it surfaces through the deep fascia, at which point it is called the lateral cutaneous nerve of forearm, which provides sensory innervation to skin on the lateral surface of the forearm.
- Median nerve: after leaving the axilla, this nerve runs lateral to the brachial artery, but crosses it over in the distal portion of the arm, so that it lies medial to the brachial artery in the cubital fossa. The median nerve supplies the muscles in the anterior compartment of the forearm and part of the hand, but does not give off any branches in the arm.
- Ulnar nerve: after leaving the axilla, the ulnar nerve runs on the medial side of the brachial artery down to the distal 1/3 of the arm, at which point it continues its medial path, while the brachial artery takes a more medial course. At this point, the ulnar nerve penetrates the medial intermuscular septum to enter the posterior compartment, where it lies anterior to the medial head of triceps brachii. It passes posterior to the medial epicondyle, which it "wraps around" to enter the anterior compartment of the forearm, where it provides innervation to 1 1/2 flexor muscles in the forearm and the majority of intrinsic muscles of the hand, in addition to skin over the medial 1 1/2 digits.
- Radial nerve: after leaving the axilla, this nerve lies posterior to the brachial artery, and runs with the profunda brachii artery along the radial groove of the humerus, entering the posterior compartment by passing through the triangular interval. It passes from medial to lateral, entering the anterior compartment of the arm through the lateral intermuscular septum between the brachialis and brachioradialis muscles. The radial nerve innervates triceps brachii and part of brachialis in the arm and brachioradialis and extensor carpi radialis longus in the forearm, as well as providing cutaneous sensory innervation to the arm and forearm.
Ligaments of the Elbow Joint
The elbow joint is reinforced by three named ligaments that tie the humerus tightly to the radius and ulna, and encircle the head of the radius and bind it to the lateral side of the ulna. We did not go over these ligaments in fall 2019.
- Annular ligament of radius: thickened band that encircles the head of the radius from the olecranon posteriorly to the trochlea anteriorly.
- Radial collateral ligament: thickened fibrous membrane of the joint capsule which runs from the lateral epicondyle to the head of the radius.
- Ulnar collateral ligament: thickened fibrous membrane of the joint capsule which runs from the medial epicondyle to the lateral aspect of the trochlea of the ulna. If you are a baseball fan, then you might recognize that R.A. Dickey, a famous knuckle-baller who spent time with the Mets and Toronto Blue Jays, is congenitally missing the ulnar collateral ligament. This is often cited as a reason why he is not able to throw very hard (his pitches top out in the low- to mid-80's, and his knuckle-balls have been clocked between 56 and the upper-70s m.p.h.).
Relationships in the Cubital Fossa
The cubital fossa is located anterior to the elbow joint between two muscles: brachioradialis, which is a lateral muscle of the forearm that originates from the supraepicondylar ridge of the humerus; and pronator teres, which is an anterior forearm muscle that arises from the medial epicondyle (we will cover these two muscles in the next lesson). The "floor" of the fossa is made up of the brachialis muscle. There are a few important relationships in the cubital fossa:
- From lateral to medial run the tendon of biceps brachii, brachial artery, and the median nerve
- The brachial artery splits into the radial and ulnar artery at the apex of the cubital fossa, around the level of the radial head or neck
- The median nerves, as noted above, lies immediately medial to the brachial artery and leaves the fossa by passing between the two heads of the pronator teres muscle
- The radial nerve lies just under the brachioradialis muscle, and splits into a superficial branch which continues into the anterior compartment of the forearm and a deep branch which passes between the two heads of supinator to enter the posterior compartment
- The ulnar nerve does not pass through the cubital fossa, but instead passes posterior to the medial epicondyle, so that it does not pass through the carpal tunnel in the wrist
© 2015 Robert McCarthy