The superficial back muscles are arranged in 3 layers and are primarily hypaxial in origin and appendicular in function.

CLASSIFICATION:

1. 1ST layer (superficial)

A. Trapezius

B. Latissimus dorsi

2. 2nd LAYER (intermediate)

A. Levator scapulae

B. Rhomboideus (major and minor parts)

3. 3rd LAYER (deep)

A. Serratus posterior superior

B. Serratus posterior inferior

• 5 trapezial truisms:

1. This muscle is NOT hypaxial in origin but is derived from the head somites which explains its cranial nerve (X1) innervation. It is derived from the gill arch levators of fishes. (You may forget this esoteric fact!).

2. Origin: occipito-vertebral (from the back of the occiput, the cervical ligamentum nuchae and the spinous processes of C7-T12).

3. Insertion: scapulo-clavicular (scapular spine, acromion and lat. 1/3of the clavicle).

4. Actions: (1) Scapula

A. elevates

B. depresses

C. adducts

D. rotates

(2) Head/Neck

A. extends

B. tilts (chin up on oppo. side)

5. Innervation:

1. Spinal accessory nerve (X1)

2. C3/4 (secondary and more recent innervations accrued during the caudal migration of the trapezius in land vertebrates).

• 5 things to know about the latissimus dorsi:

1. The "lats" are the widest (= "latissimus") muscles of the back (= "dorsum").

2. Origin: vertebro-ilial (from the spinous processes of T6 - L5 and the median sacral crest via the lumbar aponeurosis [ = "morbid fear of apes"], and the posterior iliac crest).

3. Insertion: humeral (floor of the intertubercular groove).

4. Action: gleno-humeral joint

1. extends

2. adducts

3. medially rotates

4. also depresses shoulder complex

* the "swimmer's" muscle

5. Innervation: brachial plexus via the nerve to the latissimus dorsi (aka the thoraco-dorsal or middle subscapular).

*The "lats" also have secondary costal and scapular (inferior angle) attachments.

• 5 things to know about the levator-rhomboid complex:

1. This is actually a single muscle sheet with different heads of origin.

2. Origin: (1) transverse head (= levator scapulae - from transverse processes of C1-C4)

(2) vertebral head (= rhomboideus - from spinous processes of C7-T5)

* The splenius capitis emerges between these heads forming "superior and inferior splenial triangles".

3. Insertion: scapular (to the entire vertebral border)

4. Actions: (1) scapular

A. adducts

B. elevates

C. rotates lateral angle downward

(2) neck (levator scapulae)

A. extends

B. laterally flexes

5. Innervation: (1) levator scapulae (C3-5)

(2) rhomboideus (C5 = dorsal scapular nerve from the brachial plexus)

• 0 things to know about the posterior serrati:

* They are highly variable and totally insignificant.

* If anything, they have something to do with stabilizing the ribs.

* In the dissecting room the superior is usually removed with the rhomboids and the inferior with the latissimus.

* They span the true, intrinsic, epaxial columns of deep back muscles.