Section Three
Myology

The human body contains three types of muscle： skeletal （骨骼肌）， smooth （平滑肌） and cardiac （心肌） ．Muscles of the locomotor system are skeletal muscles，which are also termed striated muscles because of the striped appearance of their cells under microscope．They are also often named voluntary muscles because individuals control many of them at will．Smooth muscle is distributed in the walls of internal organs and blood vessels．Cardiac muscle is found only in the heart．

Ⅰ．Muscle Structure

Each skeletal muscle consists of belly and tendon（Figure 2.36）．The muscle belly （肌腹） is composed of muscle fibers which produce contraction．The tendon （肌腱） is composed of connective tissue through which muscles are attached to bones．The flat tendon is called aponeurosis （腱膜） ．The structural unit of a muscle is a muscle fiber，which is covered with a connective tissue called the endomysium．Parallel muscle fibers are organized into bundles that are enwrapped by the perimysium．The epimysium surrounds the muscle itself．

Based on color，skeletal muscles are divided into red muscles and white muscles．Red muscles，known as slow muscles，are thin muscle fibers that are red in color，and contain a large amount of myoglobin and mitochondria．They have slow and sustained contraction because red muscles perform aerobic metabolism for energy．White muscles，known as fast muscles，are thick muscle fibers that are white in color，and contain a small amount of myoglobin and mitochondria．They have fast but soon fatigue contraction because white muscles perform anaerobic metabolism for energy．Each skeletal muscle contains red and white muscle fibers．Different muscles have different ratios of red and white muscle fibers．Exercise can change the ratios of red muscles to white muscles．

Ⅱ．Muscle Terminology

Most muscles are attached to bones at two points．One attachment is relatively fixed，named the origin （起点） ，the other is relatively movable，named the insertion （止点） （Figure 2.37）．Interconvert of the origin and the insertion might happen．The prime movers or agonists are the main muscles activated during a specific movement of the body．Muscles which work with an agonist are named synergists．Muscles that exert an opposite force are named antagonists．The same muscle may act as a prime mover，an antagonist，or a synergist under different conditions．

Ⅲ．Supplementary Structures of Muscles

Supplementary structures of muscles contain fascia and tendinous sheath．The fascia （筋膜） includes the superficial and deep fascia．The superficial fascia is composed of loose connective tissue and fat．Located between the dermis and underlying deep fascia，the superficial fascia contains nerves，blood and lymphatic vessels．It covers the entire body．The deep fascia is a dense connective tissue layer that encloses deep structures such as the muscles and neurovascular bundles．The tendinous sheath （腱鞘） is a double layered sheath that invests the tendons in the hand and foot（Figure 2.38）．Its outer layer is fibrous layer which limits the movement range of tendons．The inner layer is synovial layer which contains synovial fluid and reduces the friction between the tendons and bones．

Ⅳ．Nomenclature of Muscles

Skeletal muscles are named according to a number of criteria，which include action，shape，origin and/or insertion，multiple points of origin，location，size，and direction of their fibers．Such as extensor digitorum and flexor digitorum are named for their action．The trapezius and the deltoid are named by their shape．The origin and insertion is indicated by the sternocleidomastoid．Muscles having more than one origin include the biceps，triceps，and quadriceps．The tibialis anterior and the external intercostal muscle are named for their location．The pectoralis major and the gluteus maximus are named partly for their size．The rectus abdominis and the transverse abdominis are named partly for the direction of their muscle fibers．

Ⅴ．Muscles of Body

There are more than 600 individual skeletal muscles．Skeletal muscles are divided into muscles of head ，neck，trunk and limbs．

1．Muscles of Head

Muscles of head consist of the muscles of facial expression and mastication（Figure 2.39）．The muscles of facial expression （表情肌） lie beneath the skin of the scalp and face，and surround the orifices of the eyes，mouth and nose．The occipitofrontalis on the skull wrinkles the forehead．The orbicularis oculi closes eyes，and the orbicularis oris closes the mouth．The buccinator underlying the cheek is active during sucking，whistling and blowing．They move the skin and change facial expression to convey mood．The muscles of mastication （咀嚼肌） contain masseter （咬肌） ，temporalis，medial and lateral pterygoid muscles．They act at the temporomandibular joint and perform the action of chewing．The masseter is located on the lateral surface of the ramus of the mandible，and is a very strong muscle in closing the mouth by raising the mandible．The temporalis and medial pterygoid muscles assist in the action of the masseter．The lateral pterygoid muscle can open the mouth，and move the mandible from side to side．

2．Muscles of Neck

Muscles of neck are divided into the superficial，intermediate and deep groups．The superficial group has the sternocleidomastoid muscle （胸锁乳突肌） ，which lies the anterolateral aspect of the neck（Figure 2.40）．When the muscles contracts，head is bent to the same side and the face is turned toward opposite side．The intermediate group includes the suprahyoid and infrahyoid muscles that elevate and depress the larynx．The deep group has scalene muscles，which flex neck laterally．In addition，the platysma is a broad thin layer of muscle beneath the skin of the neck．It belongs to the muscle of facial expression．This muscle can make the neck a smooth appearance and assist in drawing the angle of the mouth downward．

3．Muscles of Trunk

Muscles of trunk consist of muscles of the back，thorax，abdomen，perineum，and the diaphragm．

Musclesofback lie on the back of the trunk and are divided into superficial and deep groups．The superficial group mainly contains the trapezius and latissimus dorsi（Figure 2.41）．The trapeziusmuscles （斜方肌） are a paired triangular muscles，and located on the nucha and the upper part of the back of the trunk．They can raise，depress and retract the shoulder．The latissimusdorsi （背阔肌） is the largest flat muscle on the lower part of the back of the trunk．It can extend，adduct and medially rotate humerus．The deep group mainly contains the erectorspinae（sacrospinalis ，竖脊肌） ，which is located in the groove on the each side of spinal column．It can extend and bend the spinal column．The thoracolumbar fascia is diamond-shaped fascia and aponeuroses enclosing the erector spinae and quadratus lumborum in the middle and lower back．It is one of the common sources of back pain．

Muscles of thorax mainly contain the superficial pectoralis major，pectoralis minor，serratus anterior and the deep external and internal intercostal muscles（Figure 2.42）．The pectoralis major （胸大肌） is a large，thick muscle on the upper front of the chest wall．It adducts，medially rotates and flexes the shoulder joint，also acts as an accessory muscle of inspiration．The pectoralis minor （胸小肌） lies deep to the pectoralis major，and draws the scapula inferiorly and anteriorly．It also helps raise the ribs during inspiration．The serratus anterior （前锯肌） lies on the side of the chest．It draws the scapula against the chest．The external and internal intercostal muscles （肋间外肌、肋间内肌） are located between the intercostal space（Figure 2.43）．The external intercostal muscles elevate ribs and aid in inspiration，while the internal intercostal muscles depress ribs and aid in forced expiration．

Muscles of abdomen extend from the thorax to the pelvis and form the abdominal walls．The midline long muscle is the rectus abdominis （腹直肌） ，and the lateral broad muscles are external oblique （腹外 斜肌）， internal oblique （腹内斜肌） and transverse abdominis （腹横肌） from the outer to the inner（Figure 2.42）．The aponeuroses of three lateral broad muscles enclose the rectus abdominis and are intermingled with each other in the anterior median line to form the linea alba．The inguinal ligament （腹股 沟韧带） is the thickened inferior border of the external oblique aponeurosis．Above the ligament is abdomen（torso），below it is the lower limb．The inguinal canal is situated above the medial half of the inguinal ligament．The inguinal canal （腹股沟管） is a passage in the anterior abdominal wall which in men transmits the spermatic cord and in women transmits the round ligament（Figure 2.44）．The canal is larger and more prominent in men．When abdominal contents（such as intestine） exit through the inguinal canal，the condition is known as an indirect inguinal hernia．This condition is far more common in men than in women．A direct inguinal hernia protrudes through a weakened area in the fascia of the abdominal wall．

Muscles of perineum form the pelvic floor（Figure 2.45）．They support the internal organs in the pelvic cavity and occlude the urethra，vagina and anal canal．The outlet of the pelvis corresponds to the perineum．It is lozenge-shaped and is divided into anterior the urogenital triangle and posterior the anal triangle．The major muscles of the urogenital triangle are the perineal muscles．The triangle transmits the urethra in the male，and the urethra and vagina in the female．The anal triangle contains the levator ani and transmits the rectus．If these muscles are weak，protrusion of pelvic organs through opening may occur．Uterine prolapse often occurs in women，and unrepaired injuries during childbirth will increase such probabilities．

The diaphragm （膈） is a dome-shaped muscle separating the thoracic and abdominal cavities（Figure 2.46）．It has three apertures，which permit the esophagus，aorta and inferior vena cava to pass by between the thorax and abdomen．The diaphragm is the chief muscle for respiration．It descends during inspiration，and arises during expiration．When the diaphragm contracts together with the abdominal muscles，it increases the intraabdominal pressure，and aids in defecation，parturition，cough and vomiting，etc．

4．Muscles of Limbs

Muscles of limbs are divided into muscles of the upper limb and lower limb．

Muscles of the upper limb include muscles of the shoulder，arm，forearm and hand．The major muscle of the shoulder is the deltoid muscle．The deltoid （三角肌） ，a triangular muscle which forms the fleshy part of the shoulder and abducts the arm．The muscles of the arm are divided into anterior and posterior groups．The anterior group contains the biceps brachii （肱二头肌）， coracobrachialis （喙肱肌） and brachialis （肱 肌） that flex the arm （Figure 2.47）．The posterior group is the triceps brachii （肱三头肌） that extends the arm （Figure 2.48）．The muscles of the forearm are also divided into the anterior and posterior group（Figure 2.42）．The anterior group contains nine muscles which are anterior to the radius and ulna．They flex wrist and fingers，and pronate the forearm （Figure 2.47）．The posterior group has ten muscles which are behind the radius and ulna．They extend wrist and fingers，and supinate the forearm （Figure 2.48）．The muscles of the hand are divided into the lateral，intermediate and medial groups（Figure 2.48）．The lateral group is also named the thenar muscles （鱼际肌） that abduct，adduct，flex，extend and opponens the thumb．The intermediate group includes interosseous muscles that abduct and adduct the fingers，and lumbricales that flex the metacarpophalangeal joints and extend the interphalangeal joints．The medial group is also termed the hypothenar muscles （小鱼际肌） that abduct，flex and opponens the little finger．

Muscles of the lower limb include muscles of the hip，thigh，leg and foot．Muscles of the hip include the anterior and posterior group．The anterior group contains the iliopsoas that consists of the major psoas and iliacus（Figure 2.49）．It is anterior to the hip joint and flexes it．The posterior group mainly include the gluteus maximus （臀大肌） ，gluteus medius，gluteus minimus，and piriformis（Figure 2.50）．The gluteus maximus is a thick quadrilateral muscle that forms the prominence of the gluteal region．It extends and laterally rotates the hip joint．The other three muscles are deep to the gluteus maximus．They laterally rotate and abduct the hip joint．Muscles of the thigh are divided into the anterior，medial and posterior groups．The anterior group mainly consists of the quadriceps femoris and sartorius．The quadriceps femoris （股四头肌） has four heads：the vastus medialis，vastus lateralis，vastus intermedius，and rectus femoris（Figure 2.49）．The vastus intermedius is behind the rectus femoris，between the vastus medialis and vastus lateralis．The tendons of the four heads unite to form a strong tendon，which encloses the patella，and is inserted into the tibia as the patellar ligament （髌韧带） ．The quadriceps femoris is the strong extensor of the knee joint．The sartorius is a narrow straplike muscle which flexes both the thigh and the leg．The medial group lies on the medial part of the thigh and adducts the thigh．The posterior group is located in the posterior part of thigh and contains three muscles（hamstring muscles）：the lateral side is the biceps femoris；the medial side is the semitendinosus and the semimembranosus（Figure 2.50）．They flex the knee joint and extend the hip joint．Muscles of the leg are divided into anterior，lateral and posterior groups．The anterior group is in front of the tibia and fibula（Figure 2.51），and consists of the tibialis anterior，the extensor hallucis longus，and the extensor digitorum longus from medial to lateral．The muscles of this group can extend the ankle joint，toes and invert foot．The lateral group includes the peroneus longus and the peroneus brevis，which evert the foot．The major muscles of the posterior group are the triceps surae （小腿三头肌） （Figure 2.52），the flexor digitorum longus， the flexor hallucis longus and the tibialis posterior． The triceps surae consists of gastrocnemius （腓肠肌） and soleus （比目鱼肌） and forms a strong tendo calcaneus （ Achilles tendon ，跟腱） ．The other three muscles are deep to the triceps surae．They are flexor hallucis longus，tibialis posterior，flexor digitorum longus from lateral to medial．The posterior group muscles of the leg can plantarflex the foot and flex the knee joint．Muscles of the foot include the dorsal muscles offoot and muscles of sole．The distribution of the muscles of sole is similar to those of hand．The muscles of sole play a role of maintaining the arches of foot．