HUMAN ANATOMY – VOLUME 1

Fig. 88. Synovial sheath of hand.

1 — tendinous sheath of flexor carpi radialis; 2 — common flexor sheath; 3 — tendinous sheath of m. flexor pollicis longus; 4 — synovial and fibrous sheaths of tendos of fingers.

bursa is covered with a fibrous membrane, while the cavity of the bursa is lined with a synovial membrane. The size of the bursae varies from several millimeters to several centimeters. The cavity of the bursa sometimes communicates with an articular cavity. Next to their point of attachment, tendons of some muscles pass over a bony prominence called a trochlea. Trochleae change the direction of the tendon, provide support and increase the angle of attachment of the tendon to the bone. By this it increases the force applied during contraction of the muscle.

WORK OF MUSCLES

The work of a muscle depends on its size, shape and structure. A single muscle fiber can develop a force of approximately 0.1–0.2 g. An

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Fig. 89. Anatomic (continuous line) and physiologic (dot line) diameters of muscles of various shapes.

I — flat muscle; II — fusiform muscle; III — unipennate muscle.

absolute force produced by a muscle is on the average 10 kg per 1 mm2, and varies for different muscles between 6.24 and 16.8 kg/mm2. The strength of a muscle is directly proportional to the number of muscle fibers. The total area of all muscle fibers on a transverse section, cut perpendicularly to the long axis of the muscle, is called the a n a t o m i c a l c r o s s s e c t i o n (Fig. 89).

The size of the p h y s i o l o g i c a l c r o s s - s e c t i o n depends on the structure of the muscle. The more fibers there are per unit on the transverse cut, the greater is the physiological cross section. Unipennate and bipennate muscles, which have a large number of short fibers obliquely attached to the tendon, have a greater physiological cross section than strap or fusiform muscles of the same size. The long muscle fibers of strap and fusiform muscles are parallel to their longitudinal axis, so the force of contraction is directly proportional to the length of the muscle. The force of contraction also depends on where the muscle is attached to the bone lever. It decreases as the point of attachment approaches the pivot joint; the speed of muscle movement, however, becomes increased. The force of contraction grows as the angle of attachment between bone and tendon approaches 90°. This also causes the useful component of muscle force to increase. The work of a muscle is also determined by the area of its origin. If this area is large, the work of the muscle is increased. If the point of origin is small, the muscle can do less work, but its movements can be quicker and more delicate.

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Fig. 90. Action of muscles upon various levers.

A — lever of balance; B — lever of strength; C — speed lever. 1 — pivot point; 2 — point of applying; 3 — point of resistance.

Based on the nature of their movement, muscles are divided into «strong» and «fast» (Fig. 90). «Strong» muscles have a large physiological cross section, large areas of insertion, distanced from the pivot joint, and well-developed intramuscular connective tissue. For example, the gluteus and soleus muscles. «Fast» muscles have small areas of origin and insertion, which are close to the pivot joint. Their physiological cross section is small, the intramuscular connective tissue is poorly expressed, and muscle fibers are relatively long. «Fast» muscles contract with greater speed and amplitude, but less force. Also, they become tired more quickly than «strong» muscles. Examples of «fast» muscles are the biceps and sartorius.

During contraction, the origin and insertion points of the muscle move closer to each other, bringing into movement the bones they are attached to, and thus carrying out work. Therefore, by contraction of appropriate muscles, the human body and its parts change their position, move, resist or succumb to gravity. Contraction of muscles also serves for holding the body in a static position. Accordingly, muscle work can be divided into overcoming, yielding and fixating.

Overcoming work is carried out when muscle contraction changes the positions of the body or its part, with or without an extra load, moving it against gravity.

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Work is called yielding when muscle force succumbs to the force of weight of the body part (extremity) or a load. The muscle is carrying out work, but its length increases instead of decreasing. An example of this is when the load on a muscle is heavier than it is capable of lifting or holding in place. In this case the load is lowered, even though the muscle is exerting force.

Fixating work is carried out when the force of contraction is used to hold a load in a certain position, without displacement. For example, when a person is standing or sitting still or holding an object. The force of muscle contraction is equal to the gravity force of the body or the object. In this case the length of the muscle is not changing (isometric contraction).

Overcoming and yielding types of work are also called dynamic work, since the force of muscle contraction is causing the body or its parts to move. Fixating work is also called static, because there is no movementtaking place.

Bones, which are connected by joints, are moved by muscles like levers. According to biomechanics, when the application of force and the resistance are takes place on different sides of the pivot point, or fulcrum, the system is called a first-class lever. In a second-class lever both forces are applied on the same side of the fulcrum, but at a different distance from it.

A first-class lever is a two-armed system, and is also called a «balance lever». The fulcrum is situated between the applied force (from muscle contraction) and the resistance point (weight of body part or load). An example is the connection between the vertebral column and the skull. The latter is in balance when the torque applied (product of force applied to the occipital bone and distance between the point of force application and the fulcrum) is equal to the torque created by weight of the head (product between the weight and the length of the lever arm, which is the distance between the weight and fulcrum).

A second-class lever is a one-armed lever. There are two types of sec- ond-class levers, depending on where the force application and resisting weight are situated relative to each other. The first type is considered to be a power lever. In such a system the lever arm of force application is longer than that of the weight resistance. An example of this is the foot, in which the heads of metatarsal bones serve as the pivot point, the force is applied at the calcaneus (by the triceps of the leg), and the weight of the body acts upon the talocrural joint. This lever system has an advantage in power, but a disadvantage in speed. The other type of one-armed system (otherwise called a third-class lever) is considered a speed lever. The lever arm force in this system is shorter than the lever arm of the counteracting weight. In case of the elbow joint, much greater force is required from the flexor muscles for acting upon the weight, which is situated at a considerable distance from

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the fulcrum. This lever has an advantage in speed and amplitude of movement of a longer lever, but there is a disadvantage in power.

DEVELOPMENT OF MUSCLES

The origin of skeletal muscle in embryogenesis is the middle embryonic layer called mesoderm, which contains somites. The dorsomedial section of a somite, called the myotome. Initially contains a cavity — the myocele. As the myotome grows and turns into a syncytium mass, its cavity gradually disappears. Then, the cell mass differentiates into striated muscle fibers with a metameric arrangement. The myotome divides into cylindrical sections of muscle fibers. The dorsal sections of myotomes form the deep (proper) muscles of the back. The ventral sections form the deep muscles of the chest and anterior and lateral abdominal walls.

Muscles of the head and some muscles of the neck are formed from the ventral non-metameric part of the mesoderm of the cranial end of the embryo. This group of muscles (visceral muscles) includes the masticatory muscles, some muscles of the neck, which form through transformation of the first visceral arch; the muscles of facial expression (including the platysma) and some other muscles that form from the second visceral arch. The sternocleidomastoid and trapezius muscles develop from the brachial arch musculature. This group also includes some muscles of the perineum, for example the levator ani muscle. Some muscles develop from myotomes of cranial somites. These include the muscles that move the eyeball.

Some muscles develop from mesenchymal rudiments of the limbs, and their proximal ends later move onto the body. This group of muscles includes the pectoralis major, pectoralis minor, latissimus dorsi and psoas major muscles. There is also a group of muscles, which move from the body onto a limb. These include the trapezius, rhomboid, sternocleidomastoid, serratus anterior, omohyoid and levator scapuli muscles. The muscles of this group develop from the ventral sections of myotomes and from the brachial musculature. Their distal ends move from the body onto the bones of the skull or extremities. There are also muscles, which develop from mesenchyme of the limbs and remain on the extremities.

Questions for revision and examination

1.Name the parts of a muscle.

2.Which anatomic formations make up the auxiliary apparatus of muscles?

3.How are muscles divided according to their shape and structure?

4.Name the different types of levers and give a functional characteristic of each.

5.What does the force exerted by a muscle depend on? What are the anatomic and physiological cross sections and what is their practical significance?

6.What types of work can be performed by muscles? Characterize each type.

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MUSCLES AND FASCIAE OF THE BACK

The upper border of the back considered is to be the external occipital protuberance and the upper nuchal line of the occipital bone. Its lower border is the iliac crest and the sacrum. The lateral borders are drawn along the posterior axillary lines (table 8).

The back is divided into vertebral, lumbar, sacral, scapular and infrascapular regions. For convenience, muscles of the posterior region of the neck are described together with the muscles of the back.

The muscles of the back are paired; they are situated in layers and can be divided into superficial and deep.

SUPERFICIAL MUSCLES OF THE BACK

The superficial muscles of the back are attached to the bones of the shoulder girdle and the humerus. They are situated in two layers (Fig. 91). The first layer includes the trapezius and the latissimus dorsi muscles. The second layer consists of the major and minor rhomboid muscles, the levator scapulae, the serratus posterior superior and inferior muscles.

The trapezius muscle (m. trapézius) is flat and triangular. It begins with a short tendon on the external occipital protuberance, the medial part of the upper nuchal line, the nuchal ligament and the spinous processes of the C7 vertebra and all thoracic vertebrae. Its upper fascicles pass downwards lateraly, the middle fascicles pass almost horizontally and the lower bunches pass upwards and to the lateral. The trapezius muscle is inserted into the lateral third of the clavicle, the acromion and the scapular spine.

F u n c t i o n: by simultaneous contraction of all its parts, the trapezius moves the scapula to the vertebral column. The upper fascicles of the muscle bring the scapula up. During simultaneous contraction of its upper and lower fascicles, the trapezius moves the lateral scapular angle upwards medially, while its lower angle moves forwards laterally. If the scapulae are fixed, contraction of both sides of the trapezius causes the cervical part of the spine to straighten. Contraction of only one side turns the face toward the opposite side.

I n n e r v a t i o n: the accessory nerve, cervical plexus.

B l o o d s u p p l y: transverse artery of the neck, suprascapular artery, occipital artery, posterior intercostal arteries.

The latissimus dorsi muscle (m. latíssimus dórsi) is flat and triangular. It begins from the spinous processes of the lower six thoracic and all lumbar vertebrae, the iliac crest, the median sacral crest and the 3–4 lower ribs. Fascicles of the muscle are directed upwards and laterally. Near the posterior margin of the axillary fossa the muscle continues into a flat thick tendon, which attaches to the crest of the lesser tubercle of humerus.

Fig. 91. Superficial muscles of back.

1 — trapezius; 2 — splenius capitis; 3 — rhomboid muscles major and minor; 4 — serratus posterior; 5 — thoracolumbar fascia; 6 — lumbar triangle; 7 — latissimus dorsi.

F u n c t i o n: The latissimus dorsi muscle causes adduction of the arm to the body and lowering of a raised arm, and pronation and extension in the shoulder. When the arms are fixed, this muscle pulls the body towards them (for example, when doing pull ups).

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I n n e r v a t i o n: thoracodorsal nerve.

B l o o d s u p p l y: thoracodorsal artery, posterior circumflex humeral artery, posterior intercostal arteries.

The levator scapulae muscle (m. levátor scápulae) begins with tendon fascicles from the transverse processes of 3–4 upper cervical vertebrae, passes downwards and attaches the upper part of the medial margin of the scapula. In its upper third it is covered by the sternocleidomastoid muscle, and in its lower third — by the trapezius.

F u n c t i o n: This muscle lifts the scapula, bringing it closer to the spine. If the scapula is fixed, the muscle bends the cervical spine toward itself.

I n n e r v a t i o n: dorsal scapular nerve.

B l o o d s u p p l y: cervicalis ascendens artery, transverse cervical artery.

The rhomboid major and minor muscles (mm. rhomboidei minor et major) are situated beneath the trapezius muscle.

The rhomboid minor muscle begins from the spinous processes of the C7 and T1 vertebrae. The muscle stretches downwards and laterally, and attaches to the medial margin of scapula, above the scapular spine.

The rhomboid major muscle begins from the spinal processes of T2T5 vertebrae and ends at the medial margin of the scapula, below the scapular spine. The rhomboid major and minor muscles are often accreted with each other.

F u n c t i o n: The rhomboid muscles move the scapula towards the vertebral column, raising it up.

I n n e r v a t i o n: dorsal scapular nerve.

B l o o d s u p p l y: transverse cervical artery, suprascapular artery.

The serratus posterior superior muscle (m. serrátus postérior superior) is thin and flat. It is situated beneath the rhomboid muscles. It begins with a flat tendon from the spinous processes of the C6, C7, T1 and T2 vertebrae. The muscle stretches downwards laterally, and attaches to the back surfaces of ribs 1–5 lateral of their angles.

F u n c t i o n: This muscle lifts the ribs. I n n e r v a t i o n: intercostal nerves.

B l o o d s u p p l y: posterior intercostal arteries, deep cervical artery.

The serratus posterior inferior muscle (m. serrátus postérior inferior) is flat and thin; it is situated to the front of the latissimus dorsi muscle. It begins on the spinous processes of the T11, T12, L1 and L2 vertebrae and attaches to the lower 4 ribs.

F u n c t i o n: It lowers the ribs.

I n n e r v a t i o n: intercostal nerves.

B l o o d s u p p l y: posterior intercostal arteries.

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DEEP MUSCLES OF THE BACK

The deep muscles of the back are situated in three layers. The superficial layer consists of the splenius capitis muscle, the splenius cervicis muscle and the erector spinae muscle (Fig. 92, 93). The middle layer of the deep muscles includes the tranversospinales muscles, and the profound layer is formed by the interspinales and suboccipital muscles.

Fig. 92. Deep muscles of back. To the left back from the m. erector spinae superior and inferior serrate muscles are kept; and from the right they’re removed.

1 — splenius capitis; 2 — semispinalis capitis; 3 — splenius cervicis; 4 — iliocostalis thoracis; 5 — longissimus; 6 — spinalis; 7 — external intercostal; 8 — thoracolumbar fascia (removed); 9 — obliqus externus abdominis; 10 — serratus posterior inferior; 11 — erector spinae; 12 — serratus posterior superior.

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Fig. 93. Deep muscles of back; from the right erector spinae is revealed; from the left — spinotransversalis.

1 — iliocostalis; 2 — longissimus; 3 — spinalis; 4 — semispinalis cervicis; 5 — longissimus capitis and cervicis; 6 — semispinalis capitis; 7 — rectus capitis posterior major; 8 — obliqus capitis inferior; 9 — semispinalis capitis; 10 — semispinalis cervicis; 11 — levatores costarum.

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