SKELETAL SYSTEM1

Skeletal System

Composed of the body’s bones and associated ligaments, tendons, and cartilages.

Support

The bones of the legs, pelvic girdle, and vertebral column support the weight of

the erect body.

The mandible (jawbone) supports the teeth.Other bones support various organs and

tissues.

Protection

The bones of the skull protect the brain.

Ribs and sternum (breastbone) protect the lungs and heart.

Vertebrae protect the spinal cord

Movement

Skeletal muscles use the bones as levers to move the body.

Reservoir for minerals and adipose tissue

99% of the body’s calcium is stored in bone.

85% of the body’s phosphorous is stored in bone.

Adipose tissue is found in the marrow of certain bones.

Hematopoiesis

A.k.a. blood cell formation.

All blood cells are made in the marrow of certain bones.

Bones and Osseous Tissue

Bone, or osseous tissue, is a connective tissue in which the matrix is hardened by the deposition of calcium phosphate and other minerals.

The hardening process is called mineralization or calcification. (Bone is not the hardest substance in the body; that distinction goes to tooth enamel.)

Osseous tissue is only one of the tissues that make up a bone. Also present are blood, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue.

The word bone can denote an organ composed of all these tissues, or it can denote just the osseous tissue.

There are 206 named bones in the human body.

It is often stated that there are 206 bones in the skeleton, but this is only a typical adult count. At birth there are about 270, and even more bones form during childhood. With age, however, the number decreases as separate bones fuse.

Each belongs to one of 2 large groups:

AXIAL and APPENDICULAR

Axial skeleton

Forms long axis of the body.

The axial skeleton consists of the 80 bones in the head and trunk of the human body. It is composed of five parts; the human skull, the ossicles of the inner ear, the hyoid bone of the throat, the chest, and the vertebral column

These bones are involved in protection, support, and carrying other body parts.

Appendicular skeletonBones of upper & lower limbs and the girdles (shoulder bones and hip bones) that attach them to the axial skeleton.Involved in locomotion and manipulation of the environment.

The appendicular skeleton, consisting of 126 bones, makes body movement possible and protects the organs of digestion, excretion, and reproduction. The word appendicular means referring to an appendage or anything attached to a major part of the body, such as the upper and lower extremities.

Surface Features of

Bones

Bone surfaces exhibit a variety of ridges, spines, bumps, depressions, canals, pores, slits, and articular surfaces. It is important to know the names of these surface markings because later descriptions of joints, muscle attachments,and the routes travelled by nerves and blood vessels are based on this terminology.

The terms for the most common of these features are listed in table on the next slide and several of them are illustrated to.

How many bones to you know….

There are four basic bone shapes in the human skeleton.The terms used to describe each type are:

– Long

– Short

– Flat

– Irregular

The Shapes of BonesBones are classified into four groups according to their shapes and corresponding functions

1. Long bones are conspicuously longer than wide.Like crowbars, they serve as rigid levers that are acted upon by the skeletal muscles to produce body movements. Long bones include the humerus of the arm, the radius and ulna of the forearm, the metacarpals and phalanges of the hand, the femur of the thigh, the tibia and fibula of the leg, and themetatarsals and phalanges of the feet.

Long Bones Much longer than they are wide.

All bones of the limbs except for the patella (kneecap), and the bones of the wrist and ankle.

Consists of a shaft plus 2 expanded ends.

Your finger bones are long bones even though they’re short!

2. Short bones are more nearly equal in length and width. They include the carpal (wrist) and tarsal (ankle) bones. They have limited motion and merely glide across one another, enabling the ankles and wrists to bend in multiple directions.

Short BonesRoughly cube shaped.Bones of the wrist and the ankle

3. Flat bones enclose and protect soft organs andprovide broad surfaces for muscle attachment. They include most cranial bones and the ribs, sternum (breastbone), scapula (shoulder blade), and os coxae (hipbone).

4. Irregular bones have elaborate shapes that do not fit into any of the preceding categories. They include the vertebrae and some of the skull bones, such as the sphenoid and ethmoid.

Flat BonesThin, flattened, and usually a bit curved.Scapulae, sternum, (shoulder blades), ribs

and most bones of the skull

Irregular BonesHave weird shapes that fit none of the 3

previous classes.Vertebrae, hip bones, 2 skull bones

(sphenoid and the ethmoid bones

General Features of BonesKnowing the terms used to describe a long bone will help you to understand the anatomy of the other types. The diagram shows a longitudinal section through a long bone. You will note immediately that much of it is composed of a cylinder of dense white osseous tissue; this is called compact (dense) bone.

The cylinder encloses a space called the medullary cavity, which contains bone marrow. At the ends of the bone, the central space is occupied by a more loosely organized form of osseous tissue called spongy (cancellous) bone

The skeleton is about three-quarters compact bone and one-quarter spongy bone by weight. Spongy bone is found at the ends of the long bones and in the middle of nearly all others. It is always enclosed by more durable compact bone.

In flat bones of the skull, two layers of compact bone enclose a middle layer of spongy bone like a sandwich. The spongy layer is called the diploe. A moderate blow to the skull can fracture the outer layer of compact bone, but the diploe may absorb the impact and leave the inner layer of compact bone unharmed.

The principal features of a long bone are its shaft, called the diaphysis, and an expanded head at each end called the epiphysis.

The diaphysis provides leverage, while the epiphysis is enlarged to strengthen the joint and provide added surface area for the attachment of tendons and ligaments.

The joint surface where one bone meets another is covered with a layer of hyaline cartilage called the articular cartilage. Together with a lubricating fluid secreted between the bones, this cartilage enables a joint to movefar more easily than it would if one bone rubbed directly against the other. Blood vessels penetrate into the bone through minute holes called nutrient foramina

Externally, a bone is covered with a sheath called the periosteum. This has a tough, outer fibrous layer of collagen and an inner osteogenic layer of bone-forming cells described later in the chapter. Some collagen fibers of the outer layer are continuous with the tendons that bind muscle to bone, and some penetrate into the bone matrix as perforating (Sharpey fibres).

The periosteum thus provides strong attachment and continuity from muscle to tendon to bone. The osteogenic layer is important to the growth of bone and healing of fractures. There is no periosteum over the articular cartilage. The internal surface of a bone is lined with endosteum, a thin layer of reticular connective tissue and osteogenic cells that give rise to other types of bone cells.

The Spinal Column

The vertebral column physically supports the skull and trunk, allows for their movement, protects the spinal cord, and absorbs stresses produced by walking, running, and lifting. It also provides attachment for thelimbs, thoracic cage, and postural muscles. Although commonly called the backbone, it does not consist of a single bone but a chain of 33 vertebrae with intervertebral discs of fibrocartilage between most of them.

The adult vertebral column averages about 71 cm (28 in.) long, with the 23intervertebral discs accounting for about one-quarter of the length.

As shown in figure, the vertebrae are divided into five groups, usually numbering 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the chest, 5 lumbar vertebrae in the lower back, 5 sacral vertebrae at the base of the spine, and 4 tiny coccygeal vertebrae.

To help remember the numbers of cervical, thoracic, and lumbar vertebrae—7, 12, and 5— you might think of a typical work day: go to work at 7, have lunch at 12, and go home at 5.

Variations in this arrangement occur in about 1 person in 20. For example, the last lumbar vertebra is sometimes incorporated into the sacrum, producing four lumbar and six sacral vertebrae. In other cases, the first sacral vertebra fails to fuse with the second, producing six lumbar and four sacral vertebrae.

The coccyx usually has four but sometimes five vertebrae. The cervical and thoracic vertebrae are more constant in number.

Beyond the age of 3 years, the vertebral column is slightly S-shaped, with four bends called the cervical, thoracic,lumbar, and pelvic curvatures

Abnormal Spinal Curvatures

Abnormal spinal curvatures (fig. 8.21) can result from disease, weakness,or paralysis of the trunk muscles, poor posture, or congenital defects in vertebral anatomy. The most common deformity is an abnormal lateral curvature called scoliosis.

It occurs most often in the thoracic region, particularly among adolescent girls. It sometimes results from a developmental abnormality in which the body and arch fail to develop on one side of a vertebra. If the person’s skeletal growth is not yet complete, scoliosis can be corrected with a back brace.

An exaggerated thoracic curvature is called kyphosis (hunchback, in lay language). It is usually a result of osteoporosis, but it also occurs in people with osteomalacia or spinal tuberculosis and in adolescent boys who engage heavily in such spine-loading sports as and weightlifting.

An exaggerated lumbar curvature is called lordosis (swayback, in lay language). It may have the same causes as kyphosis, or it may result from added abdominal weight in pregnancy or obesity.

General Structure of a Vertebra

A representative vertebra and intervertebral disc are shown in figure 8.22. The most obvious feature of a vertebra is the body, or centrum—a mass of spongy bone and red bone marrow covered with a thin layer of compact bone. This is the weight-bearing portion of the vertebra. Its rough superior and inferior surfaces provide firm attachment to the intervertebral discs.

Dorsal to the body of each vertebra is a triangular canal called the vertebral foramen. The vertebral foramina collectively form the vertebral canal, a passage for the spinal cord. The foramen is bordered by a bony vertebral archcomposed of two parts on each side: a pillarlike pedicle and plate like lamina.

Extending from the apex of the arch, a projection called the spinous process is directed toward the rear and downward. You can see and feel the spinousprocesses as a row of bumps along the spine. A transverse process extends laterally from the point where the pedicle and lamina meet. The spinous and transverse processes provide points of attachment for the spinal muscles.

When two vertebrae are joined, they exhibit an opening between their pedicles called the intervertebral foramen. This allows passage for spinal nerves that connect with the spinal cord at regular intervals. Each foramen is formed by an inferior vertebral notch in the pedicle of the superior vertebra and a superior vertebral notch in the pedicle of the one just below it

Intervertebral DiscsAn intervertebral disc is a pad consisting of an inner gelatinous nucleus pulposus surrounded by a ring of fibrocartilage, the annulus fibrosus. The discs help to bind adjacent vertebrae together, support the weight of the body, and absorb shock. Under stress—for example, when you lift a heavy weight—the discs bulge laterally.

Excessive stress can crack the annulus and cause the nucleus to ooze out. This is called a herniated disc (“ruptured” or “slipped” disc in lay terms) and may put painful pressure on the spinal cord or a spinal nerve.

Regional Characteristics of Vertebrae

We are now prepared to consider how vertebrae differ from one region of the vertebral column to another and from the generalized anatomy just described. Knowing these variations will enable you to identify the region ofthe spine from which an isolated vertebra was taken. More importantly, these modifications in form reflect functional differences among the vertebrae.

The cervical vertebrae (C1–C7) are the smallest and lightest ones other than the coccygeals. The first two (C1 and C2) have unique structures that allow for head movements (fig. 8.24). Vertebra C1 is called the atlas because it supports the head in a manner reminiscent of the Titan of Greek mythology who was condemned by Zeus to carry the world on his shoulders

There are 12 thoracic vertebrae (T1–T12), corresponding to the 12 pairs of ribs attached to them. They lack the transverse foramina and bifid processes that distinguish the cervical, but possess the following distinctive features of their own.

No other vertebrae have ribs articulating with them. Thoracic vertebrae vary among themselves mainly because of variations in the way the ribs articulate. In most cases, a rib inserts between two vertebrae, so each vertebracontributes one-half of the articular surface.

There are five lumbar vertebrae (L1–L5). Their most distinctive features are a thick, stout body and a blunt, squarish spinous process . In addition, their articular processes are oriented differently than on other vertebrae.

The sacrum is a bony plate that forms the dorsal wall of the pelvic cavity . It is named for the fact that it was once considered the seat of the soul. In children,there are five separate sacral vertebrae (S1–S5).

They begin to fuse around age 16 and are fully fused by age 26. The anterior surface of the sacrum is relativelysmooth and concave and has four transverse lines that indicate where the five vertebrae have fused. This surface exhibits four pairs of large anterior sacral (pelvic) foramina, which allow for passage of nerves and arteries to the pelvic organs.

The coccyx usually consists of four (sometimes five) small vertebrae, Co1 to Co4, which fuse by the age of 20 to 30 into a single triangular bone. Vertebra

Co1 has a pair of hornlike projections, the cornua, which serve as attachment points for ligaments that bind the coccyx to the sacrum. The coccyx can be fractured by a difficult childbirth or a hard fall to the buttocks. Although it is the vestige of a tail, it is not entirely useless; it provides attachment for muscles of the pelvic floor.