The infraspinatus muscle is a thick, triangular muscle and one of the four muscles that forms the rotator cuff. It  is found in the posterior aspect of the scapula, in its infraspinous fossa, inferior to the scapular spine. The muscle is covered on its posterior aspect by a thick fascia, the infraspinatus fascia. This fascia separates the infraspinatus muscle from the teres minor and teres major muscles.

The muscle originates from the infraspinous fossa and from the deep aspect of the infraspinatus fascia. The muscular fibers converge superolaterally for form a tendon that inserts into the the greater tubercle of the head of the humerus. The tendon hugs the glenohumeral joint capsule and is separated from it by a small bursa. Some of the tendon fibers insert into the joint capsule.

The infraspinatus is the main external rotator of the shoulder. When the arm is fixed, it adducts the inferior angle of the scapula.

It receives innervation by way of the suprascapular nerve (C5, C6), which arises from the superior trunk of the brachial plexus.

Note: The side image modified from the original by Henry VanDyke Carter, MD. Public domain. Animated image below by Wikimedia Commons - Anatomography [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)]

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015

The levator scapulae muscle (levator anguli scapulæ) is a triangular multipennate muscle which extends between the cervical spine and the scapula. This muscle is deep to the sternocleidomastoid and trapezius muscle.

It is formed by discrete muscular slips that originate from the first four transverse processes (C1-C4). It can have an extra slip from C5 (as shown in the side image).

These muscular slips pass posteroinferiorly, joining, and inserting in the superior scapular angle and the scapular medial border between the superior scapular angle and the medial origin of the scapular spine. It may attach to the scapular spine.

There are other anatomical variations including muscular slips that may extend to the occipital bone or mastoid process, to the trapezius, scalene, or serratus anterior magnus muscles, or to the first or second rib.

It receives nerve supply from the fourth and fifth cervical nerves and by a branch from the dorsal scapular nerve. The dorsal scapular nerve arises from the C5 root of the brachial plexus.

It receives its blood supply from the dorsal scapular artery.

The function of this muscle depends on which bony element is fixed, the scapula or the cervical spine. When the spine is fixed, the levator scapulae elevates the scapula and pulls the superior portion of the medial scapular border superomedially. When only one scapula is fixed, the head and neck flexes and rotates ipsilaterally while it extends the neck contralaterally.

The order and shape of the muscular slips is interesting, as the slip from the transverse process of the Atlas (C1) twists posteriorly and descends to insert as the most posterior and inferior fibers in the medial border of the scapula. The other slips follow a similar pattern, which is what allows this muscle to rotate the neck. This indicates that the fibers of the levator scapulae muscle are spiral and the fibers follow the contour of the neck. This makes (to my knowledge) the levator scapulae the only spiral muscle of the body. This is shown as "A" in the second side image; "B" represents the misconception on the direction of the fibers in this muscle.

Since it is a common sign of stress and bad posture to raise the shoulders, this muscle can spasm, causing neck pain and in some cases be a trigger for headaches.

Note: The first side image shown in this article is from “Gray’s Anatomy” (1918) which is in the public domain. The second side image is from Arnold’s “Reconstructive Anatomy” (1968).

Levator scapulæ muscle.
Click on the image for a larger depiction 

Levator scapulæ muscle fibers.
Click on the image for a larger depiction

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
2. "Tratado de Anatomía Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015
5. “Reconstructive Anatomy, A Method for the Study of Human Structure” Arnold, M. W.B. Saunders. 1968“Gray’s Anatomy” Henry Gray, 1918

The supraspinatus muscle is found in the supraspinatus fossa of the scapula, and one of the four muscles that forms the rotator cuff. The muscle attaches to the medial two thirds of the floor of the fossa directly on the bone and on the deep aspect of the supraspinatus fascia which covers the muscle. The supraspinatus fascia and the supraspinatus fossa form an osteofascial case for the origin of this muscle.

The fibers of the muscle converge and pass deep to the acromion, forming an osseous tunnel that could entrap the muscle and tendon causing a supraspinous impingement syndrome. The side image in this article has the acromion cut off to show the muscle better. The animated image at the bottom of the article shows the supraspinatus muscle and its relation to the acromion process.

The supraspinatus tendon attaches to the capsule of the glenohumeral joint at the level of the highest of the three impressions that form the greater tubercle of the humerus.

The main function of the supraspinatus muscle is to abduct the arm. As part of the shoulder’s rotator cuff it helps prevent subluxation of the glenohumeral joint by keeping the head of the humerus in situ.

Note: The side image modified from the original by Henry VanDyke Carter, MD. Public domain. Animated image below by Wikimedia Commons - Anatomography [CC BY-SA 2.1 jp (https://creativecommons.org/licenses/by-sa/2.1/jp/deed.en)]

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995

The subscapular muscle or subscapularis is a large triangular muscle which is found on the anterior aspect of the scapula, in close relation to the posterolateral aspect of the thorax. It is covered by a well-defined fascia layer, the subscapularis fascia. It is one of the muscles that forms the rotator cuff.

It originates from the internal aspect of the medial border of the scapula, in close proximity to the insertion of the serratus anterior (magnus), and the internal aspect of the inferolateral border of the scapula, where it is separated from the teres major muscle by a thick aponeurosis. It also takes origin directly from the subscapular fossa, where some of the muscular fibers attach directly to the bone.

The muscle inserts by way of a tendon in the lesser tubercle of the humerus and the anterior aspect of the glenohumeral joint capsule. The tendon of the muscle is separated from the neck of the scapula by a large bursa (the infratendinous bursa of the subscapularis) which communicates with the cavity of the glenohumeral joint through an aperture in the capsule.

It receives innervation by two subscapular nerves, both branches of the brachial plexus.

This muscle rotates the head of the humerus medially. When the upper extremity is raised, it draws the humerus anteroinferiorly.  As part of the shoulder’s rotator cuff it helps prevent subluxation of the glenohumeral joint by keeping the head of the humerus in situ.

Note: The image shown in this article is from “Gray’s Anatomy” (1918) which is in the public domain

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015

Image modified from the original by Henry VanDyke Carter, MD. Public domain

The serratus anterior or serratus magnus is a is a wide, thin muscle sheet situated on the posterolateral aspect of the thorax and extends between the ribs and the scapula. It is formed by well-defined separate muscular digitations that originate in the external surface and superior aspect of the first superior eight (or nine) ribs. These originating fibers also arise from the fasciae covering the intercostal muscles. This is especially true for the first or most superior digitation which arises from the first and second rib and the intervening external intercostal fascia.

These digitations cover the lateral aspect of the thorax, pass deep to the scapula and converge to insert on the deep aspect of the medial border of the scapula. Some of its fibers may even hug the medial border of the scapula and insert on its anterior aspect. The first digitation is inserted into a triangular area on the ventral surface of the medial scapular angle. The next two digitations spread out to form a triangular sheet, the base of which is directed posteriorly and is inserted into nearly the whole length of the ventral surface of the vertebral border. The lower five or six digitations converge to form a fan-shaped mass, the apex of which inserts into a triangular impression on the ventral surface of the inferior scapular angle. The lower four slips of the serratus anterior interdigitate with the superior five muscular slips of the external oblique muscle.

The word “serratus” is derivates from the Latin word [serro] meaning “saw”. Serratus means “serrated” referring to the multiple tooth-like anterior digitations of the muscle. The plural form for "serratus" is " serrati". The Latin term “magnus” means “great”, “large”, or “mighty”. It points to the fact that this is the largest of three muscles that carry the same name “serratus”. The other two are the serratus posterior superior and the serratus posterior inferior.

Note: The image shown in this article is from “Gray’s Anatomy” by Henry Gray (1918) which is in the public domain. It depicts the serratus anterior in situ and shows the scapula retracted posteriorly.  The scapula is covered on its internal aspect by the subscapularis muscle (number 3 in the image). A better image can be found in “An Illustrated Atlas of the Skeletal Muscles” by Bowden (2015) which we cannot publish for copyright reasons.

Sources:
1. “Gray’s Anatomy” Henry Gray, 1918
2. "Tratado de Anatomia Humana" Testut et Latarjet 8th Ed. 1931 Salvat Editores, Spain
3. "Gray's Anatomy" 38th British Ed. Churchill Livingstone 1995
4. “An Illustrated Atlas of the Skeletal Muscles” Bowden, B. 4th Ed. Morton Publishing. 2015

Image modified from the original by Henry VanDyke Carter, MD. Public domain

The deltoid or deltoideus (Latin) is a large, thick, triangular muscle, which covers the glenohumeral joint anteriorly, superiorly, and posteriorly.  It can be described as having three components or segments, anterior, middle, and posterior. The anterior portion originates from the lateral third of the clavicle, on the clavicle’s superior border. The middle portion originates from the lateral border of the scapula’s acromion, and the posterior portion originates from the spine of the scapula. All three portions insert laterally by means of a thick tendon on the deltoid tuberosity of the humerus. At its insertion the muscle gives off an thick connective tissue expansion to the deep fascia of the arm.

The three portions of the deltoid muscle are usually well defined. As an anatomical variation, the clavicular or the acromial portion of the muscle may be absent.

This muscle is innervated by the axillary nerve (C5, C6), a branch of the brachial plexus, and supplied by the posterior circumflex humeral artery and the deltoid branch of the thoracoacromial artery.

The word deltoid is derivates from the Greek word [δελτοειδής] which is itself formed by the terms [δέλτα] (délta), referring to the triangular shape of the letter delta (uppercase Δ, lowercase δ or 𝛿) and [-οειδής] (-oeidís), a the Greek suffix meaning “similar to”. Delt-oid then would mean “similar to a Δ (delta)".

Image modified from the original by Henry VanDyke Carter, MD. Public domain