The spine divides the body into unequal anterior and posterior segments. The back or dorsum is located on the posterior of the thorax and abdomen or between the neck superiorly and the pelvis inferiorly.
The back consists of skin and fascia overlying the spine, scapulae, muscle groups, nerves, vessels, and the presacral vertebrae. The primary movements of the back are flexion/extension, lateral bending, and rotation of the trunk. Some of the back muscles attach to the lateral and posterior processes of the vertebrae and help the spine maintain an upright posture, while others are involved in upper extremity movement.
The back muscles are divided into three layers deep, intermediate and superficial. In some scientific records, these muscles can also be grouped as extrinsic and intrinsic back muscles. This article examines the back muscles in three layers as the deep, intermediate, and superficial groups.
Structure and Function
The back area has superficial and deep fascia. The superficial fascia act as a layer of thickness and strength. This fascia, which contains adipose tissue, is continuous with the superficial fascia of the cervical, gluteal, and upper extremities. The deep fascia, a dense fibrous structure, is attached to the occipital bone, spinous processes of the vertebrae, and the iliac crest. It covers superficial muscles. Moreover, there are a number of ligaments that lie between the thoracal vertebrae and the parts of the ribs and between successive vertebrae. One of these ligaments, the ligamentum nuchae, is a fibrous membrane. It starts from the external occipital protuberance attaches to the spinous process of the cervical vertebrae. While in lower animals, it helps to sustain the head position, it is exclusively the rudiment of an important ligament in humans.
The back serves many different functions in the human body. It primarily serves as the primary structural support for the human torso and allows flexibility for movement. Running centrally down the posterior midline is the spinal column.
The spinal column is composed of bony vertebrae, which house and protect the human spinal cord. The column is a continuation of the seven cervical vertebrae in the neck and is composed of twelve thoracic vertebrae, located more superiorly, and five inferior lumbar vertebrae. The column terminates in the sacrum. Ribs articulate with the twelve thoracic vertebrae. Two bony scapulae are situated on either side of the spinal column laterally. They have the function to provide a bony attachment for several muscles, including the rotator cuff muscles of the upper extremity.
In addition to the bones, three groups of muscles comprise the back. These muscles are grouped as the intrinsic muscle group, also known as the deep group, the superficial muscle group, and the intermediate muscle group. These groups serve to allow primary movements in the back, including flexion/extension, rotation, and side bending, a locomotor function of the limbs, and assistance in the respiratory effort. In addition to supporting respiratory function, these muscles are also involved in supporting trunk, posture, and locomotor functions of the whole body.
The development of the structures in the back stems from early divisions of several key embryologic structures. In the third week of human development, germ layers form during a process called gastrulation. The three germ layers in human development are the ectoderm, the mesoderm, and the endoderm. The paraxial mesoderm, which forms the dermis of the skin, also develops the skeletal muscles in the body and the majority of the axial skeleton. The epidermis of the skin in the back derives from the ectoderm. The spinal cord derives from an ectodermal structure called the neural plate. The neural plate develops neural folds bilaterally that rise, come together, and fuse to form the neural tube. The tube completely fuses and separates from communication with the amniotic cavity by day 27. Failure of this fusion to occur can result in anencephaly.
The superficial and intermediate muscles of the back are termed "extrinsic" because they originate embryologically from hypaxial myotomes. Intrinsic back muscles develop from epaxial myotomes.
Blood Supply and Lymphatics
Blood supply to the skin and muscles in the back is primarily from dorsal branches of the posterior intercostal arteries. These arteries arise from intercostal arteries or directly from the descending aorta in some variants. The intercostal arteries run in a groove along with the intercostal vein and nerve caudal to the ribs. The thoracic aorta runs anteriorly to the vertebral column and slightly lateral on the left side. The azygos and hemiazygous veins may also be present anterior to the spinal cord. The spinal cord itself has several different sources of blood supply, depending on location and embryological development. The anterior spinal artery, the posterior spinal arteries, and the artery of Adamkiewicz are responsible for vascular supply to the spinal cord. The anterior and posterior intercostal veins are responsible for the venous blood supply of the back.
Nervous supply to the back primarily arises from dorsal branches of spinal nerves, also known as posterior rami. The sensory innervation to the back organizes in a dermatomal pattern, which corresponds to a specific spinal nerve at different spinal nerves. In addition to providing sensation to the skin of the back, the dorsal branches also serve to innervate the intrinsic muscles of the back. This innervation is in contrast to the extrinsic muscles of the back, which are innervated by spinal nerves.
The muscles of the back are divided into three categories.
The first category is the superficial back muscles; these are part of the extrinsic back muscle. These muscles are located superficially, and they assist in the movement of the upper limbs. The superficial muscles include:
Originates from the superior nuchal line, external occipital protuberance, nuchal ligament, and spinous processes of C7-T12
Attach to the lateral third of the clavicle, acromion, and spine of the scapula
Innervated by the accessory nerve (12th cranial nerve)
Originates from spinous processes of T7-L5, sacrum, thoracolumbar fascia, iliac crest, and 10th through 12th rib
Inserts to the intertubercular groove of the humerus
Innervated by the thoracodorsal nerve
Originates from the transverse processes of the first four cervical vertebrae
Inserts to the superior angle of the scapula
Innervated by the dorsal scapular nerve
Originated from the nuchal ligament and spinous processes of C7-T1
Inserts to the superior part of the medial border of the scapula (the line which is located between superior angle and trigonum scapula
Innervated by the dorsal scapular nerve
Originated from spinous processes of T2-T5
Inserts to the inferior part of the medial border of the scapula (the line which is located between inferior angle and trigonum scapula
Innervated by the dorsal scapular nerve
The second group of muscles is the intermediate group. It is part of the extrinsic back muscle. These muscles assist in the human respiratory function and intimately associate with the ribs. This muscle group consists of:
Serratus posterior inferior
Originated from the ligamentum nuchae and spinous processes of C7-T3 vertebrae
Inserts to the superior aspect of 2nd through 5th ribs
Innervated by the anterior rami of the T1-T4 spinal nerves
Serratus posterior superior
Originated from the spinous processes of T11-L3 vertebrae
Inserts to the inferior border of the 9th through 12th ribs
Innervated by the T9-T12 spinal nerves
The final group of muscles is known as intrinsic or deep back muscles. These muscles are responsible for the motion of the axial skeleton. A deep fascial layer covers the intrinsic muscles. The deep fascia forms a sheath known as the thoracolumbar fascia, which exists in the thoracic and lumbar regions. This fascia extends in the midline from the medial crest of the sacrum to the ligament nuchae and skull. It attaches laterally to the transverse processes and angles of the ribs. It has three layers as anterior, middle, and posterior. The posterior layer lies between the lumbar and sacral spinous processes laterally over the surface of the erector spinae muscles. The middle layer starts from the lumbar transverse processes attaches to the iliac crest and the last rib. The anterior layer covers the quadratus lumborum muscle and attaches to the transverse processes of the lumbar vertebrae and the iliac crest.
This group further subdivides into several categories in the back and neck.
The superficial layer consists of the splenius muscles located in the lateral and posterior neck region.
The main muscle groups in the intrinsic muscles are the erector spinae muscles and the transversospinalis group. The main movements of these muscles are flexion/extension, side bending, and rotation. The erector spinae group, which is the largest group of the intrinsic back muscles, consists of:
Iliocostalis, most laterally located
Longissimus, the largest part of the group
Spinalis, most medially located
The functions of this muscle group are to maintain posture, lengthen the spine, and bend the spine to the side. It is the primary extensor of the back when it runs bilaterally . It acts lateral bending and rotation of the spine, unilaterally 
The third component of the intrinsic muscles in the human back is the transversospinalis muscles. It is located between transverse processes and spinous processes of the vertebrae. These muscles lie deep to the erector spinae muscles. The transversospinalis group contains
Semispinalis is located in the cervical and thoracal region, has thoracic, cervicis, capitis muscles.
The multifidi lie deep to the semispinalis muscles, located along the entire spine, particularly in the lumbar region.
The rotatores are the deepest part of this group, located along the entire spine, particularly in the thoracic region.
Like the erector spinae group, the transversospinalis group is located bilaterally on the vertebral column. These muscles assist in bending the back posteriorly when contracted bilaterally. When unilateral contraction occurs, they assist with lateral bending and rotation.
It is known that a connection between the latissimus dorsi muscle and the teres major muscle may exist through muscle fibers (with a frequency of 10%) and that the latissimus dorsi muscle may be pierced by the radial nerve or a communicating branch between the radial nerve and the axillary nerve. 
The numerical variants of the vertebrae in the thoracolumbar region can be considered a potential risk factor for the clinical symptoms of the back.
The primary surgical considerations with the back have to deal with the placement of the needle for the administration of spinal anesthesia before surgical procedures. The conus medullaris is the most distal end of the spinal cord. It terminates at the level of L2/L3 in newborns and L1 level in adults. The surgeon inserts a needle into the dural sac between the L3/L4 levels in adults to avoid the conus medullaris during the administration of anesthesia prior to surgery.
Generalized back pain is a common presenting symptom for patients. The underlying etiology of the pain most commonly traces to a skeletal muscle strain. The differential for back pain is vast, and lumbar disc issues, including herniation or rupture, and vertebral fractures, all merit consideration. Additionally, osteoarthritis and spondylolisthesis, a condition where a vertebra slips anteriorly compared to the others in the column, can both be causes of back pain.
Spina bifida is a congenital condition seen on the back when there is incomplete closure of the vertebral column. There are three different subcategories of the condition.
Spina bifida occulta results from the incomplete fusion of the vertebral arch. The patients are usually asymptomatic, and the only clinical index of suspicion may be a small tuft of hair overlying the defect.
Spina bifida cystica with meningocele occurs when the bony vertebral arch fails to form, and there is a herniation of the spinal meninges.
Spina bifida cystica with meningomyelocele is the most severe and involves herniation of the meninges and the spinal cord.
The thoracolumbar fascia (TLF), located in the distal aspect of the dorsal region, is the main connection that provides force transmission between the upper and lower extremities.
The triangle of auscultation is located on the upper half of the back region. It is bordered by the trapezius, latissimus dorsi, and the medial margin of the scapula. The floor of the triangle of auscultation contains endothoracic fascia and pleura.  For this reason, pulmonary auscultation can be performed on the triangle of auscultation. Intervention to the back through the auscultation triangle can be preferable due to decreased morbidity compared to conventional thoracotomy.
(Click Image to Enlarge)
Major and Minor Oblique muscles of the Head, Occipital Bone, Oblique Superior and Inferior, Rectus posterior Major and Minor, Semispinalis Capitis, Longissimus Capitis, Semispinalis Cervicis, Longissimus Cervicis, Semispinalis Dorsi, Spinalis Dorsi, Longissimus Dorsi, Iliocostalis Dorsi, Iliocostalis Lumborum,Multifidus, Quadratus Lumborum, Sacrospinalis, Crest of the Ilium
Contributed by Gray's Anatomy
(Click Image to Enlarge)
Surface anatomy of the back, Trapezius, Spine of Scapula, Rhomboideus major, Teres Major, Deltoideus, Inferior Angle of Scapula, Sacrospinalis, Iliac crest, Latissimus Dorsi, Glutaeus Medius and Maximus
Contributed by Gray's Anatomy Plates
Kocbek L,Rakuša M, Common trunk of the posterior intercostal arteries from the thoracic aorta: anatomical variation, frequency, and importance in individuals. Surgical and radiologic anatomy : SRA. 2018 Apr; [PubMed PMID: 29532168]
Dewhurst C,O'Neill S,O'Regan K,Maher M, Demonstration of the course of the posterior intercostal artery on CT angiography: relevance to interventional radiology procedures in the chest. Diagnostic and interventional radiology (Ankara, Turkey). 2012 Mar-Apr; [PubMed PMID: 22125216]
Choi S,Trieu J,Ridley L, Radiological review of intercostal artery: Anatomical considerations when performing procedures via intercostal space. Journal of medical imaging and radiation oncology. 2010 Aug; [PubMed PMID: 20718909]
Ranade AV,Rai R,Rai AR,Dass PM,Pai MM,Vadgaonkar R, Variants of latissimus dorsi with a perspective on tendon transfer surgery: an anatomic study. Journal of shoulder and elbow surgery. 2018 Jan [PubMed PMID: 28939333]
Sugiura K,Morimoto M,Higashino K,Takeuchi M,Manabe A,Takao S,Maeda T,Sairyo K, Transitional vertebrae and numerical variants of the spine : prevalence and relationship to low back pain or degenerative spondylolisthesis. The bone & joint journal. 2021 Jul [PubMed PMID: 34192932]
Uçar İ,Karartı C,Cüce İ,Veziroğlu E,Özüdoğru A,Koçak FA,Dadalı Y, The relationship between muscle size, obesity, body fat ratio, pain and disability in individuals with and without nonspecific low back pain. Clinical anatomy (New York, N.Y.). 2021 Nov; [PubMed PMID: 34378252]
Prakash S,Kalra P,Khan Y,Dhal A, Ventral scapular osteochondroma excision through 'triangle of auscultation': A case series. Journal of orthopaedic surgery (Hong Kong). 2020 Jan-Apr [PubMed PMID: 31916491]
Use the mouse wheel to zoom in and out, click and drag to pan the image