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SYSTEMATIC ANATOMY

Chapter 3  Myology

Section 1  Introduction

In this chapter the skeletal muscles will be discussed. The skeletal muscles are the muscles attached to the  bone, fascia or skin. The ability to move provides orgnisms with a means of reacting to the environment. The contractions of skeletal muscles are made mechanically effective by means of the tendons, aponeuroses, and fascia. Skeletal muscles also been called voluntary muscles because their contraction is under the voluntary control(Fig.-3-1,2----138,139). Muscle tissue account for approximately 40% of the body weights. Each skeletal muscle possesses a definite shape, structure, location and acceddory apparatus, and is supplied by abundant blood vessels, lymphatics and nerves, so that, it can be regarded as an organ.

 

. Morphology of Skeletal Muscle

The muscle fibers or cells  are the smallest structural unit of a muscle. The connective tissue wraps the muscle fibers, bundle of muscle fibers and the muscle, is called separately the endomysium , the perimysium and the epimysium (Fig. -3-3). The fleshy part of a muscle is often called the belly . At each end of the muscle, the connective tissue blends with the strong collagen bundles to form the tendon . The muscles vary extremely in their shape. It may be long, short, broad or circular-band-like(sphincter). Some muscles have more than one belly or one origin.

Some shape is related to the fibers and long axis of the muscles. The fibers are oblique to the long axis of the muscle. When muscle fibers have a linear or narrow origin resembling half a feather they are called the unipennate muscle , and when the fibers arise from a broad surface resembling a whole feather they are called the bipennate muscles , and when septa extend into the attachments of the muscles and dividing them into several featherlike portions they are called the multipennate muscles (Fig.-3-4).

. Origin, Insertion and Action of Skeletal Muscles

  Each skeletal muscle has at least two attachments to some part of the skeleton or mucous membrane or skin. For purposes of description, most muscle are commonly described as having an origin and an insertion. The origin is usually a fixed attachment in the end of the muscles, and the insertion may be movable one. However, some muscles may act in both directions in different circumstances. It is therefore best in most cases to use the terms proximal attachment and distal attachment, instead of origin and insertion.

By controlling of nervous system, the skeletal muscular cells(fibers) and tissues contract together which move parts of the body. During movements of the body, the main muscles are called prime movers or agonists . They contract actively to produce the desired movement. A muscle that opposes the action of a prime mover is called an antagonist . As a prime mover contracts, the antagonist progressively relaxes, so the movement is produced smoothly. The antagonists may protect the joint from injury. The muscles which cooperate in performing an action are called the synergists . Other muscles, are called the fixators , steady the proximal parts of a limb while movements are occurring in distal parts. A muscle may act as a prime mover, antagonist, synergist or fixator under different conditions.

.  Nomenclature of Muscles  

The muscles have been named by their shape, location, action, attachments, direction, number of parts and so on. The trapezius and rhomboid muscle are named for their shapes. The levator scapulae is named for its action in lifting the scapula. The gluteus maximus, gluteus medius and gluteus minimus are named because of their location in the gluteal region and for their contrast in size. The sternocleidomastoid muscle has attachments to the sternum, the clavicle, and the mastoid process of the temporal bone. The number of heads of the origin is indicated by the terms, for example the quadriceps femoris.  

. Supplementary Structures of Muscles 

They include the fascia , the synovial bursa , the synovial sheath of tendon  and sesamoid bones .

I..Fascia

It is formed by the connective tissue. There are superficial and deep two groups of fascia in the body(Fig.-3-5).

1) Superficial fascia    

It is found immediately under the skin and covering almost the entire body. It is a layer of variable thickness containing fat. In the scalp, the palm of hand and the sole of foot the superficial fascia is very dense, and it is so thick that is capable of being subdivied into several laminae in the groin. It provides for increased mobility of skin, and the adipose component both contributes to thermal insulation and constitutes a store of energy for metabolic use. Subcutaneous nerves, vessels, lymphatics and skin muscles are found in the superficial fascia.

2) Deep fascia   

They are also composed of collagenous fibers, but form the dense, inelastic and arranged to regularly fiber membrane. The deep fascia may form a strong separate sheath to muscles or the vessels and nerves. In the limbs they give off septa which separate the functional groups of muscle by attaching along the longitudinal axis of a bone. These fascia sheets are called intermuscular septa . In certain situations the deep fascia affords attachment for muscles. It is thickened at the wrist and ankle form the retinaeulum  that maintains its underlying tendon.

2.  Synovial Bursa 

It is often found between a tendon and a bone or a ligament, or between two tendons. This is a specialized structure that allows an efficient freedom of motion between contiguous connective tissue surfaces. It is a relatively small, circumscribed pocked of complete separation, lubricated surface of a joint and help to the motion.

3.  Synovial Sheath of Tendon 

It is thought of as a synovial bursa that extends around the tendon to envelop it. So it has a double layers synovial sheath where they can be seen in the hand and foot(Fig.-3-6). The internal layer is attached by loose to the tendon, and the external one is attached to connective tissue. There are some synovial fluids between two layers. The internal and external layers are in continuity at each end of the sheath, whereby nerves and blood vessels may arrive to the tendon. This structure is called the mesotendon .

4. Sesamoid Bones 

They have large change of the number, the size and shape, and are often developed in the tendon where afford for a great amount of pressure, for example in the palm and the sole. They can change the direction of pulling force and increase the strengthen of the muscle. The patella is the largest sesamoid bone.

 

Section 2  Muscles of Trunk

The muscles of trunk may by divided into four parts: the muscles of back , the muscles of thorax , the diaphragm , and the muscles of abdomen .

 

. Muscles of Back 

They are located in the posterior aspect of the trunk there are two groups of muscles, superficial and deep in the back(Fig.-3-7----143). The superficial group is extrinsic back muscles that are concerned with limb movement and respiration. They are composed of the trapezius latissimus dorsi , levator scapulae , rhomboid muscles . The deep group constitutes the intrinsic back muscles that are concerned with movement of the vertebral column. They consist of a complex group of muscles. The main muscle of them is the erector spinae (sacrospinalis) .

 

 

 

 

 

Fig.-3-7.  Muscles of back

1. Trapezius

The trapezius is a large, triangular muscle in the upper back that extends horizontally from the base of the skull and the cervical and thoracic vertebrae to the shoulder. Its fibers are arranged in three groups as upper, middle and lower. Together these fibers rotate the scapula. The upper fibers acting alone raise the scapula and shoulder, as when the shoulders are shrugged to express a feeling of indifference. The middle fibers pull the scapula toward the vertebral column, and the lower fibers draw the scapula and shoulder downward. When the shoulder is fixed in position by other muscles, the trapezius can pull the head backward or one side.

2. Latissimus Dorsi

The latissimus dorsi is a very broad, flat, triangular muscle in the lower part of the back, which is overlapped by the lower part of the trapezius(Fig.-3-7). It takes origin from spinous processes of the lower six thoracic vertebrae, from spinous processes of all the lumbar and upper sacral vertebrae, and from the medial part of the iliac crest. There are additional slips of origin from the lower four ribs and a small slip of attachment to the inferior angle of the scapula. The insertion of this muscle is usually not seen in this particular dissection. It does attach to the humerus in such a way that contraction of this muscle pulls the arm posteriorly, or extends it, and rotates it medially.

3. Levator Scapulae 

This narrow elongated muscle is situated at the dorsal and lateral part of the neck(Fig.-3-7). It arises by tendinous slips from the transverse processes of the atlas and axis and from the posterior tubercles of the transverse processes of the third and fourth cervical vertebrae. It descends deep to the sternocleidomastoid muscle, along the floor of the posterior triangle of the neck, and then under cover of the trapezius to be inserted into the vertebral border of the scapula, between the superior angle and the triangular smooth surface at the root of the spine. The muscle can elevate the superior angle of the scapula, tends to draw the scapula medially, and rotates it so as to lower the lateral angle. With the scapula fixed, the levator scapulae bends the neck laterally and rotates it slightly toward the same side.

4. Rhomboid Muscles  

This muscle may be divided into rhomboid major  and rhomboid minor ( Fig.-3-7). The rhomboid major arises tendinous fibers from the spinous processes of the second to fifth thoracic vertebrae. The fibers course inferolaterally to be inserted by a narrow tendinous arch to medial border of the scapula to the inferior angle of the scapula below. The rhomboid minor arises from the inferior part of the ligamentum nuchae and from the spinous processes of the seventh cervical and first thoracic vertebrae. Its fibers course inferolaterally just superior to the rhomboid major to be inserted into the base of the root of the spine-edge of the scapula. They adduct the scapula by pulling it medially toward the vertebral column and also support the scapula by elevating it.

5. Erector Spinae (sacrospinalis) prolongations in the thoracic and cervical regions lay the groove on the side of the vertebral column. They are covered in the lumbar by the thoracolumbar fascia. This large muscular and tendinous mass arises from the anterior surface of a broad and thick tendon, which is attached to median sacral crest, to the spinous processes of the lumbar and lower tow thoracic vertebrae and their supraspinal ligaments, and to the lateral crests of the sacrum. The muscular fibers form a large fleshy mass which splits, in the upper lumbar region, into three columns: a lateral, the iliocostalis ; an intermediate, the longissimus ; and a medial, the spinalis . They are inserted separately into ribs and vertebrae. From these bones it runs continuously upward to insert into the mastoid process of the temporal bone. When acting on one side it bends and rotates the spinal column toward the opposite side. When acting on both sides it extends the spinal column(Fig.-3-7).

. Muscles of Thorax 

They are divided into the thoracicoextrinsic muscles or thoracico- upper-limb muscles , including the pectoralis major , the pectoralis minor  and the serratus anterior , and the thoracicointrinsic muscles , including the external intercostal muscles and the internal intercostal muscles .

1.  Pectoralis Major 

The clavicular portion of the pectoralis major arise from medial half of the clavicle, the costosternal portion from the anterior surface of the sternum and costal cartilages of the upper six ribs, and the abdominal portion from the aponeurotic tendon of the external abdominis oblique muscle. The insertion is on the crest of the greater tubercle of the humerus, the clavicular portion overriding the costosternal part. Its main action is to flex, adduct, and medially rotate at the shoulder joint, an act that brings the arm across the chest(Fig.-3-8).

 

2.  Pectoralis Minor 

This muscle lies deep to the pectoralis major and arises from the external surface of three ribs—usually the third, fourth and fifth—and converges superiorly and laterally to insert on the coracoid process of the scapula. The pectoralis minor draws the scapula forward and downward and also rotates the scapula so as to lower the lateral angle during adduction of the arm (Fig.-3-8).

3.  Serratus Anterior 

The muscle is a thin muscular sheet that is situated between the ribs and the scapula, spreading over the lateral part of the chest. It arises by fleshy digitations from the outer surfaces and superior borders of the eight or nine ribs, and from the aponeuroses covering the intervening intercostal muscles(Fig.-3-9). It is inserted into the entire anterior surface of medial border of the scapula. This muscle draws the medial border of the scapula anteriorly close to the thoracic wall, thereby preventing the bone from protruding backward in reaching and pushing movements. Its lower fiber helps to rotate glenoid cavity of scapula upward when the arm is raised above the head. By fixing the scapula it helps the inspiration.

4.  External Intercostal Muscles

The external intercostal muscle is the most superficial muscle in the intercostal spaces(Fig.-3-9). Its many fasciculi arise from the rib above and insert on the rib below. It reaches as far as the tubercles of the ribs posteriorly and to the costochondral joint anteriorly, from this point on to the sternum, the muscle has disappeared and a fibrous sheet—the external intercostal membrane—is all that remains. The muscle main action is to elevate the ribs in inspiration.

5.  The Internal Intercostal Muscles

The Internal Intercostal Muscles occupies the same area as just mentioned for the external intercostal muscle but at a deeper level(Fig.-3-9). It arises from the rib above and course inferiorly to insert on the rib below, and ends posteriorly to form the internal intercostal membrane. Its main action is problematical, but most feel that the interchondral portion aids in inspiration and the interosseous portion involved with expiration.

 

. Diaphragm 

It is a dome-shaped musclofibrous septum that separates the thoracic from the abdominal cavity. Its convex upper surface forms the floor of the thorax, and its concave inferior surface forms the roof of the abdomen. Its peripheral part consists of muscular fibers that take origin from the circumference of the thoracic outlet and converge to be inserted into a central tendon of diaphragm ( Fig.-3-10).

The muscular portion is divided into three parts according to the origin of its fibers. The sternal part arises from the posterior aspect of the xiphoid process. The costal part arises from the internal surface of the lower six ribs and costal cartilages. The lumbar(vertebral) part arises by two crura from the upper 2-3 lumbar vertebrae and the arcuat ligaments on the surface of the psoas major and the quadratus lumborum. Between the three original parts of the diaphragm, i.e. between the origins of the sternal part and the costal part and between the costal part and the lumbar part, there are usually triangular spaces without muscular tissue, called the lumbocostal triangle  and the sternocostal triangle . These triangles are the common site for a diaphramatic hernia and an eventration of the diaphragm.

 The central tendon of the diaphragm is a thin but strong aponeurosis situated near the center of the muscle. There are three openings in the diaphragm. The aortic hiatus  lies in front of the 12th thoracic vertebra and is formed by the two crura of diaphragm and the vertebral body of the 12th thoracic vertebra. It transmits the abdominal aorta, the thoracic duct and often the azygos vein. The esophageal hiatus  is located in the right crus of the diaphragm at the level of the 10th thoracic vertebra, 2-3cm to the left and upward of the aortic hiatus. It transmits the esophagus, the branches of the left gastric artery and vein, and the anterior and posterior vagal trunks. The vena caval foramen is located in the central tendon of the diaphragm at the level of the disc between the 8th and 9th thoracic vertebrae, 2-3cm to the right the median plane. It is the highest of the opening in the diaphragm. The inferior vena cava and some branches of the right phrenic nerve and lymph vessels pass through the foramen.

The diaphragm is the principal muscle of inspiration. When it contracts, the muscle descends and draws the central tendon downward with it, so increases the vertical diameter of the thoracic cavity, resulting in air being taken into the lungs. Diaphragmatic movements are also important in the blood circulation because the increased abdominal pressure and decreased thoracic pressure accompanying contraction of the diaphragm help to return blood to the heart.

 

. Muscles of Abdomen and fasciae

Muscles of Abdomen include the anterior group and posterior group of the abdominal wall.There are four important paired muscles in the anterior abdominal wall: three flat muscles, external oblique muscle of abdomen, internal oblique muscle of abdomen , and transversus abdominis muscle and one rectus abdominis muscle. The flat muscles cross each other that strengthen the abdominal wall and affords considerable protection to the abdominal viscera.

1Anteriolateral group of abdominal wall.

1  External Oblique Muscle of Abdomen

This is the largest and the most superficial muscle of the muscles of abdomen. (Fig.-3-11) It is located in the anterolateral part of the abdominal wall. The muscle arises from the external surface of 5th to 12th ribs, and its fibers run inferoanteriorly and medially in the same direction as do the extended digits. As the fibers pass medially, they become aponeurotic that ends medially in the linea alba. The muscle insert into linea alba, pubic tubercle and anterior half of iliac crest. 

 

 

 

 

 

 

 

 

 

 

2  Internal Oblique Muscle of Abdomen

The internal oblique muscle of abdomen is the intermediate of the three flat abdominal muscles(Fig.-3-11). It arises from the thoracolumbar fascia, anterior two-thirds of iliac crest, and lateral half of inguinal ligament. The fibers run superoanteriorly, and also become aponeurotic and the aponeurosis splits to form sheath for the rectus abdominis muscle. The muscle inserts into inferior borders of 10th to 12th ribs, linea alba, and the pubis via the conjoint tendon.

3Transversus Abdominis  

This is the innermost of the three flat abdominial muscles(Fig.-3-11,12). It arises from the internal surfaces of 7th to 12th costal cartilages, thoracolumbar fascia, iliac crest, and lateral third of inguinal ligament and inserts into linea alba with aponeurosis of internal oblique, pubic crest, and pecten pubis via conjoint tendon. Its fibers run more or less horizontally and end in an aponeurosis which contributes to the formation of the rectus sheath.

The three flat abdominal muscles and their extensive aponeurosis form a strong expandable support to the anterolateral abdominal wall, which protects for the abdominal viscrea. They can increase the intraabdominal pressure for defecation, micturition and parturition.

4Rectus Abdominis

This is a long, broad, strap muscle and the principal vertical one of the anterior abdominal wall(Fig.-3-11). It arises from pubic symphysis and pubic crest, and inserts into xiphoid process and 5th to 7th costal cartilages. The two muscles are separated by the linea alba and enclosed in the rectus sheath, formed by the aponeurosis of three flat abdominal muscles. The anterior layer of the rectus sheath is firmly attached to the rectus muscle at three or more tendinous intersections. The muscle forms the anterior abdominal wall and prevents tilting of the pelvis by the weight of the lower limbs.

Actions: The anterolateral group of abdominal muscles support and protect the viscera. The abdominal muscles compress the abdomen to maintain and to increase the intraabominal pressure. The muscles are therefore important in respiration, defecation, micturition, parturition, cough and vomiting. Acting separately, they can move the vertebral column in flexion,rotation and maintain posture.

5 Fasciae of abdomen

1) Superficial fasciae Over the greater part of the anterolateral ahdominal wall. The superficial fascia consists of layers containing a variable amount of fat. At lower part of abdominal wall, it may be divided into two layers, a fatty superficial layer (Camper,s fascia) and a membranous deep layer (Scarpa's fascia) containing elastic fibers.

2) Deep fascia is several layers covering separately the surface and deep surface of the muscles.

3) Inner investing fascia  It covers all the inner surface of the abdominal cavity. The transverse fascia (Fig.3-9) covers the deep surface of the transverses   abdominis and its aponeurosis and is continuous from side to side deep to the sheath of rectus ahdominis and the rectus abdominis directly below the arcuate line. Above the middle of the inguinal ligament it forms the deep Iing of inguinal canal. The other parts of the inner investing fascia of the abdominal wall are named according to the structures on which they lined, i.e., the diaphragmatic fascia on the diaphragm, the iliac fascia on the iliacus and psoas fascia on the psoas, and the pelvic fascia in the pelvis.

Clinical Notes 

 Aponeurosis of anteriolateral 3 flat muscles of abdominal wall to form same structures which are important significance for the clinic surgery.

1) Sheath of rectus abdominis(Fig.-3-12)  It is a strong, incomplete fibrous compartment of  the rectus abdominis. At the lateral margin of the rectus abdominis, the aponeurosis of obliquus internus abdominis splits into two layers, one passing anterior to the muscle, and another posterior to it. Its anterior layer joins with the aponeurosis of obliquus externus abdominis to from the anterior layer of the sheath of rectus abdominis. The posterior layer of the sheath of rectus abdominis is deficient. At 1ower1/4 of the posterior layer of the sheath of rectus abdominis has deficient. Here the aponeurosis of obliquus internus ahdominis does not split and all the aponeuroses of the three flat muscles pass anterior to the rectus abdominis to form the anterior layer of the sheath of rectus ahdominis. The lower limit of the posterior layer of the sheath is marked by a crescentic border called the arcuate line.The position of this line is usually midway between umbilicus and pubic crest. From this line downward rectus abdominis is directly in contact postertorly with the transverse fascia.

 

 

 

2Linea alba is placed on the anterior median line of the abdomen. It extend from the xiphoid process to the pubic symphysis which is formed by fibers of the sheath of rectus abdominis of two sides

3Inguinal canal is an oblique passage, 4~5cm long, through the abdominal wall (Fig. -3-13). It passes downwards and medially from the deep to superficial inguinal ring. It lines paralleled and immediately above the inguinal ligament. The inguinal canal is occupied in male by the spermatic cord and in female by the round ligament of uterus.

 4Inguinal hernias  They are divided into direct and indirect hernia(). In the indirect hernia the abdominal contents enter the deep ring and tranverse inguinal canal to emerge from the superfial ring. In same persons the hernia is congenital or acquired. However in the direct inguinal hernia the viscus protrudes immediately forward superficial ring which sometimes form the hernial sac. Typical direct hernia with the sac lies meadial to the inferior epigastric vessels. Direct hernia involves the wall in the region of the Hesselbach’s triangle.

  2posterior group of abdominall wall     

It consists of the psoas major and the quadratus lumborum(Fig. -3-12). The psoas major will be described in the  " The Muscles of Lower Limb. The quadratus lumborum is a roughly quadrilateral thick muscular sheet attached below to the iliac crest, above to the last rib, and medially to the tip of transverse processes of the 1st to 4th Lumbar vertebrae. The muscle is directly in contact posterior with the erector spinae.  

Actions: Fixes and lowers the 12th rib. Acting alone, it bends the trunk toward the same side. Acting together, they help to extend the lumbar region of the vertebral column and give it lateral stability

 

 

LiuWen:  Nankai University School of Medicine