Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Locomotor System
Muscular System
Muscles of the Trunk and Neck

Two groups of Muscles are distinguished on the trunk and neck: intrinsic muscles and extrinsic muscles. The former lie deep, directly against the BONES OF THE Axial Skeleton, and their contractions primarily move the Skeleton of the Trunk and HEAD. Extrinsic muscles, developing on the limbs or originating from the mesoderm of the visceral arches, appear on the trunk later and therefore lie superficially over its own musculature (Atlas, Figs. 26, 27). Extrinsic muscles differ from intrinsic ones in that they are primarily associated with the function of the upper limbs, although under certain conditions they can also move the trunk and head. Intrinsic muscles are found in all Regions of the trunk. Extrinsic muscles are located on the chest, back, and neck.

Muscles located along the midline of the trunk have a longitudinal fiber direction, whereas those situated laterally have an oblique direction. The former are derivatives of the ventral longitudinal trunk Muscle of fish-like ancestors, while the latter are homologues of intercostal muscles.

The Description of the Muscles of the Trunk and neck will be organized by region: chest, abdomen, back, and neck.

Muscles of the chest. The deep intrinsic musculature of the thoracic region retains a segmental Structure, much like the skeleton of this region. The muscles are arranged in three layers: 1) external intercostal muscles; 2) internal intercostal muscles; 3) transversus thoracis muscle. The Diaphragm is also functionally associated with these muscles.

The external intercostal muscles (mm. intercostales externi) occupy all intercostal spaces from THE Vertebral Column to the costal cartilages (Atlas, Figs. 29, 30, 36). Their fibers run downward and forward, so upon contraction they elevate the Ribs, increasing the volume of the thoracic cavity in the anteroposterior and transverse directions. These are among the primary Muscles of inspiration. Their most dorsal bundles, originating from the transverse processes of the thoracic vertebrae, are distinguished as the levatores costarum muscles.

The internal intercostal muscles (mm. intercostales interni) occupy the anterior 2/3 of the intercostal spaces (Atlas, Figs. 29–31). The muscle fibers are directed upward and forward; therefore, by contracting, they depress the ribs and, by reducing the size of the thoracic cavity, facilitate expiration.

The transversus thoracis muscle (m. transversus thoracis) is rudimentary. It is located on the inner side of the thoracic wall (Atlas, Fig. 32A). Contraction of this muscle assists in expiration.

The fibers of the intrinsic chest muscles lie in three intersecting directions. This arrangement reinforces the thoracic wall.

The diaphragm (diaphragma), or thoracoabdominal partition, separates the Abdominal cavity from the thoracic cavity (Atlas, Figs. 32A, 33). This muscle develops during the early Embryonic period from the cervical myotomes and, as The Heart and Lungs form, shifts posteriorly until it reaches its permanent position in the three-month-old fetus. Corresponding to its embryonic origin, the muscle is innervated by a branch arising from the Cervical plexus.

The diaphragm has a dome-shaped form. It consists of muscle fibers that attach by one end along the entire circumference of the inferior thoracic aperture, while the other end transitions into a central tendon that forms the apex of the dome. The heart rests upon the mid-left portion of the dome. The thoracoabdominal partition is perforated by openings through which pass the aorta, Esophagus, Veins, Thoracic duct, and nerve trunks. The diaphragm serves as the primary respiratory muscle. Upon contraction, its dome descends, increasing the vertical dimension of the thoracic cavity. This mechanically stretches the lungs and brings about inspiration.

The extrinsic muscles overlying the intrinsic chest muscles are powerfully developed in humans. They mobilize and secure the upper limbs to the trunk. These include the pectoralis major, pectoralis minor, and serratus anterior muscles (Atlas, Figs. 24, 26, 27, 29–31).

The pectoralis major muscle (m. pectoralis major) originates from the clavicular part of the clavicle, the edge of the Sternum, and the cartilages of ribs V–VI (Atlas, Figs. 24, 26, 29, 30). The muscle inserts into the crest of the greater tubercle of the humerus. A synovial bursa lies between the latter and the muscle tendon. Upon contraction, the muscle adducts and pronates the arm, drawing it forward.

The pectoralis minor muscle (m. pectoralis minor) lies beneath the pectoralis major (Atlas, Figs. 29–31). It originates from ribs II–V, inserts into the coracoid process, and upon contraction pulls the scapula downward and forward.

The serratus anterior muscle (m. serratus anterior) originates with nine digitations from ribs II–IX (Atlas, Figs. 26, 27, 29, 31). It inserts into the medial border of the scapula and its inferior angle, with which the majority of its bundles are connected. Upon contraction, the muscle pulls the scapula forward and its inferior angle outward, causing the scapula to rotate around the sagittal axis and elevating its lateral angle. If the arm is abducted, the serratus anterior muscle, by rotating the scapula, elevates the arm above the level of the shoulder joint. The arm now moves together with the shoulder girdle at the sternoclavicular joint.

The Fasciae of the chest are generally weakly developed.

Muscles of the abdomen. The abdominal wall is formed by a group of intrinsic muscles. These include: the rectus abdominis, pyramidalis, quadratus lumborum, and the broad muscles of the abdomen — the external and internal oblique, and transversus abdominis (Atlas, Figs. 24–26, 29–31). The broad muscles lie in the lateral walls of the abdomen. The tendinous fibers of their aponeuroses interlace anteriorly to form the Linea Alba of the abdomen along the midline of the abdominal wall, which is anchored superiorly to the xiphoid process of the sternum and inferiorly to the Pubic Symphysis. Flanking the linea alba is the rectus abdominis muscle with longitudinally directed fibers. The broad muscles have an obliquely directed fiber arrangement and, as in the chest, are disposed in three layers: the external oblique abdominal muscle is the continuation of the external intercostal muscles, the internal oblique corresponds to the internal intercostals, and the transversus abdominis corresponds to the muscle of the same name in the chest. The quadratus lumborum muscle forms the posterior abdominal wall.

The inferior wall of the abdominal cavity, or the floor of the lesser pelvis, is termed the Perineum.

The rectus abdominis muscle (m. rectus abdominis) originates from the cartilages of ribs V–VII and the xiphoid process of the sternum, and inserts lateral to the pubic symphysis (Atlas, Figs. 24, 26, 29, 30). It is intersected transversely by three or four tendinous inscriptions, which represent remnants of abdominal sclerotomes. The rectus muscle is enclosed within a fibrous sheath formed by the aponeuroses of the abdominal oblique muscles (Atlas, Figs. 26, 29–31, 32B).

The pyramidalis muscle (m. pyramidalis) is small and frequently absent (Atlas, Figs. 29, 30). It is a rudiment of the pouch musculature of mammals. Originating near the pubic symphysis and tapering superiorly, the muscle inserts into the linea alba, which it tautens upon contraction.

The external oblique abdominal muscle (m. obliquus abdominis externus) originates by eight digitations from the lower ribs (Atlas, Figs. 24, 26, 27, 29). Its fibers run downward and forward, inserting into the iliac crest. Anteriorly, the muscle transitions into an aponeurosis, the fibers of which contribute to The formation of the rectus sheath; along the midline, they interlace with the fibers of the contralateral oblique aponeuroses to form the linea alba of the abdomen.

The lower free margin of the aponeurosis is turned inward, with its thickening forming the inguinal ligament, the ends of which are anchored to the anterior superior iliac spine and the pubic tubercle (Atlas, Fig. 32B).

The internal oblique abdominal muscle (m. obliquus abdominis internus) originates from the thoracolumbar fascia, iliac crest, and inguinal ligament, runs upward and forward, and inserts into the lower three ribs (Atlas, Figs. 29, 30, 36). The lower bundles of the muscle transition into an aponeurosis that contributes to the rectus sheath and the linea alba.

The transversus abdominis muscle (m. transversus abdominis) originates from the lower ribs, lumbodorsal fascia, iliac crest, and inguinal ligament, and anteriorly transitions into an aponeurosis that participates in forming the rectus sheath and the linea alba (Atlas, Figs. 29, 31). The lowermost bundles of these latter two muscles descend within the Spermatic Cord into the Scrotum, where they envelop the Testis. These bundles are designated as the cremaster muscle (m. cremaster) (Atlas, Fig. 32B).

The Abdominal muscles perform diverse Functions. They form the wall of the ABDOMINAL CAVITY AND, through their tone, support the Internal Organs. By contracting, they constrict the abdominal cavity (primarily the transversus abdominis) and act upon the internal organs as an abdominal press, which facilitates urination, defecation, vomiting, coughing, and childbirth. The abdominal muscles pull the ribs downward, reducing the size of the thoracic cavity and thereby participating in expiration. Finally, these muscles flex the vertebral column forward (primarily the rectus abdominis), laterally, and rotate it around the longitudinal axis. The latter movement is executed by the simultaneous contraction of contralateral external and internal oblique muscles, with rotation occurring toward the side of the internal oblique, which originates from the iliac bones that are fixed while a person is standing.

The quadratus lumborum muscle (m. quadratus lumborum), originating from the iliac crest, inserts into the transverse processes of the lumbar vertebrae and the 12th rib (Atlas, Fig. 33). The muscle depresses the rib, participating in expiration, and flexes the vertebral column backward and laterally.

The perineal muscles are heavily modified tail muscles and certain other muscles derived from human evolutionary ancestors. Supporting the abdominal organs from below, the perineal muscles function simultaneously as sphincters of the anus and the Urethra.

Among the fascias covering the abdominal wall muscles, the most dense is the endoabdominal fascia, which lines the inner surface of the abdominal wall. This fascia takes part in forming the posterior wall of the rectus sheath and the Inguinal Canal. From the inside, the fascia is covered by the Peritoneum.

The mutual intersection of the broad muscle fibers, the fibrous sheath surrounding the rectus muscle, and its tendinous intersections all serve to reinforce the soft abdominal wall. However, certain Structural Features of the abdominal wall result in "weak spots" that can become sites for hernia formation. A hernia is defined as the protrusion of internal organs—such as the intestine, Stomach, greater omentum, Kidney, or Ovary—from the abdominal cavity, primarily beneath the Skin of the abdomen. The causes of hernias include muscular weakness or severe emaciation combined with a chronic increase in intra-abdominal pressure: prolonged constipation, crying in infants, heavy lifting, etc. Hernias appear at those "weak spots" of the abdominal wall that cannot withstand the intra-abdominal pressure.

Hernias may occur in the region of the linea alba due to the Separation of its fibrous fibers, or at the umbilicus, which represents the scar left after the newborn's umbilical cord is severed. Inguinal hernias form when organs protrude through the inguinal canal. The latter lies above the inguinal ligament (Atlas Fig. 32B) and is a muscular cleft through which the spermatic cord passes in males, and the round ligament of the Uterus in females. Femoral hernias occur when organs protrude beneath the skin inferior to the inguinal ligament; here, Blood and Lymphatic vessels pass into the thigh between the ligament and the pelvic bone, medial to which lie loose Connective Tissue and Lymph Nodes. Under certain conditions, this area becomes passable for internal organs.

Back muscles. On the back, as in the thoracic region, intrinsic muscles lie deep and are covered by extrinsic muscles, which mobilize the upper limbs and anchor them to the trunk. Among the intrinsic back muscles of ventral origin are two poorly developed muscles terminating on the ribs: the serratus posterior superior and serratus posterior inferior (Atlas Figs. 34, 35).

The serratus posterior superior muscle (m. serratus posterior superior) originates from the spinous processes of the two lower cervical and two upper thoracic vertebrae.

The serratus posterior inferior muscle (m. serratus posterior inferior) originates from the thoracolumbar fascia at the level of the two lower thoracic and two upper lumbar vertebrae. Both muscles participate in the act of Respiration: the upper elevates the ribs, while the lower depresses them. Acting simultaneously, they expand The thoracic cage.

Beneath both posterior serratus muscles, along the vertebral column, lie the Deep Muscles of the back. These are intrinsic trunk muscles of dorsal development. In humans, they retain a primitive, more or less metameric arrangement. The deep back muscles lie on both sides of the vertebral spinous processes, extending from the sacrum to the Skull. Four tracts can be distinguished within them, arranged sequentially in a deep-to-superficial direction.

Tract I (restricted to the neck) is represented by the splenius capitis and cervicis muscle (m. splenius capitis et cervicis), which originates from the spinous processes of the upper thoracic and lower cervical vertebrae and inserts into the transverse processes of the First and Second cervical vertebrae and the mastoid process of the Temporal bone (Atlas Figs. 34, 35). Upon bilateral contraction, the muscle extends the Head and Neck; upon unilateral contraction, it rotates them.

Tract II is formed by the erector spinae muscle (m. erector spinae), which originates from the posterior surface of the sacrum, the iliac crest, the spinous processes of the lumbar and lower thoracic vertebrae, and the thoracolumbar fascia (Atlas Fig. 35). The muscle extends the Vertebral Column and plays a major role in its statics.

Inferior to the XII rib, the erector spinae divides into three muscles: the iliocostalis, the longissimus, and the spinalis muscles of the back (Atlas Figs. 35, 36).

The iliocostalis muscle is the most lateral, inserting into the ribs and the transverse processes of the lower cervical vertebrae.

The longissimus thoracis muscle inserts into the transverse processes of all thoracic and cervical vertebrae and terminates on the mastoid process of the temporal bone.

The spinalis dorsi muscle inserts into the spinous processes of the thoracic and cervical vertebrae all the way up to the axis (epistropheus).

Tract III consists of the transversospinalis muscle (m. transversospinalis), which extends from the sacrum to the Occipital bone, with its fascicles running from the transverse processes to the spinous processes (Atlas Fig. 36). The muscles of this tract extend the vertebral column, bend it laterally, and also rotate it.

Tract IV is formed by the short back muscles—the intertransversarii and interspinales in the cervical and lumbar regions, and the short suboccipital-vertebral muscles (Atlas Figs. 33, 36).

The intertransverse muscles are located between the transverse processes of adjacent vertebrae: upon contraction, they assist in lateral flexion of the vertebral column.

The interspinous muscles are situated between the spinous processes of adjacent vertebrae; upon contraction, they participate in the extension of the vertebral column.

The short suboccipital-vertebral muscles, four in number, are located between the occipital bone, the atlas, and the axis. These muscles extend and rotate the head.

The Diversity of the deep back muscles is associated with the high differentiation of Movements of the spine and the entire body. The strength of this musculature ensures the upright posture of a human. Without the deep back muscles, the human torso would bend forward, since its center of gravity lies anterior to the vertebral column.

The group of extrinsic back muscles associated with the upper limbs is arranged in two layers. The superficial layer contains the trapezius muscle, which migrated from the head (of branchial origin), and the latissimus dorsi muscle, which migrated from the upper limb (Atlas Figs. 25, 27, 34).

The trapezius muscle (m. trapezius) originates from the superior nuchal line of the occipital bone, the nuchal ligament, and the spinous processes of all thoracic vertebrae (Atlas Figs. 25, 27, 34). The muscle fibers converge laterally and insert into the lateral third of the clavicle, the spine of the scapula, and the acromion. When the lower fascicles of the muscle contract, they depress the Pectoral Girdle; the middle fascicles pull it toward the vertebral column, and the upper ones elevate it. The upper fascicles act synergistically with the serratus posterior muscle when it abducts the arm above the level of the shoulder joint. When the pectoral girdle is fixed, the trapezius muscle pulls the head backward.

The latissimus dorsi muscle (m. latissimus dorsi) originates from the thoracolumbar fascia, the spinous processes of the lower IV–VI thoracic and all lumbar vertebrae, the lower four ribs, and the iliac crest (Atlas Figs. 25, 27, 29, 34). The muscle fibers converge superiorly and laterally, inserting via a flat tendon into the crest of the lesser tubercle of the humerus. A synovial bursa lies between the tendon and the tubercle. The muscle adducts, medially rotates (pronates), and extends the arm.

Deep to the trapezius muscle, In the second layer, lie the rhomboid muscles and the levator scapulae muscle.

The rhomboid muscle (m. rhomboideus) originates from the spinous processes of the lower cervical and upper thoracic vertebrae and inserts into the medial border of the scapula, pulling it medially and superiorly upon contraction (Atlas Figs. 25, 34).

The levator scapulae muscle (m. levator scapulae) originates from the transverse processes of the upper cervical vertebrae and inserts into the superior angle of the scapula. When contracted, it pulls the scapula upward while simultaneously depressing its lateral angle (Atlas, Figs. 34, 35).

The Muscles of the Upper Limb located on the trunk serve additional functions beyond those already described. For instance, the muscles that attach to the scapula do more than just mobilize it; when antagonistic muscle groups contract simultaneously, they fix the scapula in place. Furthermore, if the limb is immobilized by the tension of other muscles, their contraction acts not on the limb, but on the rib cage, expanding it and thereby functioning as accessory muscles of inspiration. The body recruits these muscles during forced or labored breathing, such as during running, heavy physical exertion, or certain respiratory disorders.

Among the back fasciae, the thoracolumbar fascia is particularly well-developed, enclosing the deep back muscles both anteriorly and posteriorly (Atlas, Figs. 34, 35). By attaching its deep lamina to the transverse processes of the lumbar vertebrae and its superficial lamina to the spinous processes of nearly all vertebrae, it forms an osteofibrous canal for these muscles. The superficial lamina, which is especially robust, gives rise to the latissimus dorsi, the serratus posterior inferior, as well as the transversus abdominis and internal oblique abdominal muscles.

Muscles of the neck. The neck musculature comprises the muscles located on the anterior and lateral aspects of the spine (those situated posteriorly belong to the back muscles). These include the longus colli and longus capitis, as well as the rectus capitis muscles. By flexing the HEAD AND TORSO, they act as antagonists to the back muscles that extend onto the neck.

The intrinsic muscles of the neck, which are also deeply situated but have obliquely oriented fibers, include the anterior, middle, and posterior scalene muscles (Atlas, Figs. 37, 40). They are homologous to the oblique and intercostal muscles of the abdomen. Originating from the transverse processes of the cervical vertebrae, the scalene muscles insert into the 1st (anterior and middle) and 2nd (posterior) ribs. They elevate the ribs to aid in inspiration and, when the rib cage is fixed, flex the cervical spine.

The intrinsic Neck Muscles with longitudinally oriented fibers comprise the superficially located muscles, mostly lying inferior to the Hyoid bone (Atlas, Fig. 37). They represent the cervical continuation of the ventral longitudinal muscles of the trunk. Their origins and insertions are clear from their names: sternohyoid, omohyoid, sternothyroid (attaching to the thyroid Cartilage of the Larynx), and thyrohyoid.

These muscles pull the hyoid bone, and consequently the larynx, downward.

Among the muscles situated superior to the hyoid bone, the intrinsic group includes the geniohyoid muscle (m. geniohyoideus). It extends from the mental spine of the Mandible to the hyoid bone, which it pulls upward upon contraction.

The extrinsic muscles of the neck (or "migrant" muscles) include those located above the hyoid bone that develop from the mesoderm of the visceral arches: the digastric, mylohyoid, stylohyoid, and platysma (Atlas, Fig. 37). Another extrinsic muscle, derived from the mesoderm of the branchial arches and extending along the entire length of the neck, is the sternocleidomastoid.

The posterior belly of the digastric muscle (m. digastricus) originates from the mastoid process, while its anterior belly attaches lateral to the mental spine of the mandible. An intermediate tendon between the two bellies is anchored to the hyoid bone by a fibrous loop.

The mylohyoid muscle (m. mylohyoideus) forms the floor of the Oral Cavity and lies inferior to the geniohyoid muscle. It originates from the mylohyoid line on the internal surface of the mandibular body; the medial borders of these paired muscles form a midline raphe that attaches posteriorly to the body of the hyoid bone.

The stylohyoid muscle (m. stylohyoideus) extends from the Base of the styloid process of the temporal bone to the hyoid bone.

All these muscles pull the hyoid bone and the larynx upward and forward; when the hyoid bone is fixed, they assist in depressing the mandible.

The suprahyoid and Infrahyoid muscles, when contracting simultaneously, fix the hyoid bone, thereby facilitating movements of the larynx. Consequently, they participate in speech articulation and singing—activities aimed at producing sounds generated within the larynx.

The platysma (m. platysma) is well-developed in certain animals (such as hedgehogs and horses), spreading beneath the skin across their entire body. In humans, this muscle is vestigial and appears as an extremely thin muscular sheet. It originates from the Fascia of the pectoralis major and deltoid muscles, extends upward along the lateral surface of the neck, inserts into the fascia overlying the masseter muscle (see below) and the inferior border of the mandible, and partially blends with the facial expression muscles. When tense, the platysma tightens the skin of the neck and protects the subcutaneous veins from compression. It develops alongside the facial expression muscles from the mesoderm of the hyoid arch.

The sternocleidomastoid muscle (m. sternocleidomastoideus) is superficially located on the neck and significantly shapes its contour (Atlas, Figs. 24, 26, 27, 40). Originating from the superior border of the manubrium sterni and the sternal end of the clavicle, the muscle extends obliquely upward to insert into the mastoid process of the temporal bone. Bilateral contraction of the muscle pulls the head backward, whereas unilateral contraction rotates the head to the opposite side while turning the face upward.