the human skeletal system(1)

7/31/2019 The Human Skeletal System(1)

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The human Skeleton System

Introduction

Two things about the human skeleton set it off from most or all otherskeletons: it is built erect, as opposed to walking on four legs, and thehand has an opposable thumb. The skeleton, made up of 206 bones inadulthood, is what gives us the shape we have, and the power to move.The skeleton is divided into two groups: the axial skeleton and theappendicular skeleton, each with its own purpose. Figure 1 illustratesthe structure of human skeleton.

Figure 1: The skeleton skull

Axial skeleton

The axial skeleton, making up 80 of your 206 bones, encompasses allyour upper body bones. It is subdivided into three groups: the skull, thevertebral-sound column, and the bony thorax-sound. The axialskeleton main purposes are to protect your vital organs, such as thebrain, heart, and lungs, and to provide an efficient structure to perform avariety of work.

The Skull


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The skull, or cranium-sound (see Figure 2), could be thought of as themost important structure in your skeleton, especially considering that ithouses your brain. Your skull provides the framework for most of yoursensory organs, such as eyes, ears, tongue, nose, and some skin. Yourskull is made up of 22 cranial or facial bones, plus the three in each ear.

Most are fixed joints (all but the mandible) separated by cartilage as ababy, but fuse together a later as you grow. Once fused, they are lockedtogether, forming immovable joints, called a suture-sound.

Figure 2: The human skull

The Backbone

The backbone, or vertebral or spinal column, though called a "bone", isreally a flexible structure made of 26 bones as represented in Figure 3. As

a baby, you have 33 vertebrae, or back bones, but the lower four fuse toform the coccyx-sound, and the next lower five fuse to become thesacrum. The backbone serves several important functions itself. Itprovides structure from which all other upper body structures branch, andit protects the spinal nerve, which is the "highway" that all theinformation your brain sends to your body travels. If the spinal columnwere to be damaging, also damaging the spinal nerve, it would be like thehighway collapsed, and all landmarks past that point could no longer bevisited, causing paralyzation from that point in your back and down.Figure 1below represents a backbone.


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Figure3: The Backboneor vertebral or spinal column

The backbone is approximately 28 inches, or 70 cm, long, and isseparated into five regions. The last two, the coccyx and sacrum-sound,are separated by the fused vertebrae. The remaining three aredistinguished mostly by concavity with respect to your front. Theremaining regions are: the cervical-sound curvature made of 7

vertebrae and concave; the thoracic-sound region made of 12 vertebraeand convex, and the lumbar-sound curvature made of 5 vertebrae andconcave.

The Bony Thorax

The bony thorax is basically your chest, comprising your breast boneand ribs. Your breast bone, or sternum-sound, is around 6 inches (15cm) tall, spanning about half the length of your ribs. You have twelveribs, forming the structure for your chest. One primary purpose of your

ribs is to protect your lungs are heart. Except for your floating ribs, eachrib connects to the sternum by cartilage on the tips. Your top seven ribsare called true ribs because they connect directly to the sternum. Thenext four ribs are called false ribs because they attach to the sternum soindirectly if at all. If they do not connect to the sternum, they do connectto upper cartilage for support. The last two ribs are called floating ribsbecause they do not connect to the sternum or any other support on theend.

The Appendicular Skeleton


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The appendicular skeleton refers to your arms and legs. They are calledappendicular (from "append") because they are attached by girdles, whichbridge each with the main body, as if they had been appended after themain body was formed. These girdles give these appendages aremarkable range of movement unique from anywhere else in the body.

Obviously the arms are the same allowing symmetry, and the legs aretoo. But ignoring size and shape, and instead focusing on joints andrelative placement, your arms and legs are the same, too. The pectoral-sound, or shoulder, girdle connects the arm to the axial body. Thescapula-sound, or shoulder blade, and the clavicle, or collarbone makesup the girdle.

The Upper Appendages

The main purpose of the arms is to do work. They are lighter and are

made to focus on detail. In your upper arm is your humerus whichconnects to the girdle as a ball-and-socket joint. It connects to yourforearm with a hinge joint. The forearm is really two bones, the radius-sound and the ulna-sound. The ulna is the bone that joins with thehumerus-sound in the elbow joint. Having two bones instead of oneallows for a wide range of twisting of the wrists.

The Hand

The carpus, or wrist of the hand, is made of eight small bones in two

irregular rows connect with gliding joints. These eight bones give yourwrist the flexibility it has. Five metacarples sound extend from thecarpus, covered with skin, form the palm. Looking at just the skeleton,they look like the base of really long fingers. The tips of these bones arethe knuckles you see when you make a fist. At the beginning of this, theskeletal, topic, one of the characteristics that make the human skeletonunique is the opposable thumb. The first metacarpal, the thumb base, isjointed differently that the rest of the metacarples. They lay in a singleplane, while the thumb metacarpal is connected with a saddle joint, givingit a range of movement. This is what makes grasping things as easy as it

is. The rest of the hand comprises the phalanges, what we see as thefingers. Each finger has three phalanges, except for the thumb, which hastwo.

The Lower Appendages

As said earlier, the upper and lower appendages are structurally similar.One difference is that the lower appendages are thicker and stronger tosupport the incredible stress put on them when running and jumping.They are designed mostly for movement. The pelvic-sound, or hip,girdle does the same thing as the upper body girdles. One majordifference is that the two girdles in the upper body are replaced by one


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for the lower body. The femur is the lower appendage equivalent of thehumerus. It's the longest, largest and strongest bone of the body, mustbe so because of the incredible forces placed upon it. Jumping hard canput as much as two tons of pressure per square inch on it! The tibia andfibula are the equivalents of the ulna and radius. In the leg, however, the

range of movement has been restricted to almost none. The tarsals arelike the carpals, the metatarsus-sound like the metacarples, and thephalanges-sound have the same name.

In the leg, the foot meets the leg a right angles, unlike the hand. This isto support our weight and move us in a direction. Though a single bonecould replace the foot, have many bones make it adaptable to irregularterrain. The foot is also arched to support the weight it has on it. When itneeds to, the curve gives away a little, and then bends back when theweight is taken away. Like the thumb, the big toe has a fairly unique

function. Much larger and stronger that the other toes, it supports theweight of our body, especially when we walk. Notice that it is the last partof your foot to leave the ground when we step.

Composition of the Bone

The bone has five main functions:1. SupportThat's about everything just covered above

2. ProtectionThey keep organs away from damage, such as the rib cage and theskull.3. MovementThat's covered above, also.4. StorageThe bone stores fat and some minerals.5. Blood cell formationThe bone is the place that most of your blood components aremade.

The inside of the bone is made of three primal tissue layers:periosteum-sound, compact bone, and spongy bone.

The periosteum is a glistening double layerd tissue which covers the hardbone called the compaact bone. The periosteum cannot be seen by thenaked eye. The periosteum is exceedingly important since its lower layerhouses bone forming cells called osteoblast-sound. Also, the doublelayer serves as a place where tendons and ligaments can insert andanchor into the bone.


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The Compact bone seems very hard when seen by the normal eye, butat a microscopic level, the compact bone is very hollow. Canals calledcanaliculi-sound, which are filled with blood vessels, channel throughthe bone and all join to many major canals called Haversian canals-sound. These canals make the bone hollow. The compact bone is also

rich with nerves. The hardness of the bone is made by spider likeostecytes-sound, mature bone cells, which surround the caniculli andHeversian canals.

Figure 4: Structure of compact and spongy bones

Spongy Bone

Spongy bone is not exactly spongy. Towards the center of the bone, thebone gets more hollow. This is why it is called spongy. Located within thespongy bone is red bone marrow-sound and yellow bone marrow-sound. Red bone marrow is what makes red blood cells. The majority ofthe red bone marrow for an adult is located in the head of the femur andhemerus. Yellow bone marrow is stored fat. This marrow can sometimesturn in to red bone marrow when a person is very anemic (meaning lack

of blood, or is a decrease in number of red blood cells).

Links:http://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.html

http://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect9.html
http://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.htmlhttp://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.htmlhttp://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.htmlhttp://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect9.htmlhttp://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect9.htmlhttp://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect9.htmlhttp://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.htmlhttp://library.thinkquest.org/2935/Natures_Best/Nat_Best_High_Level/Page_Shells/Skeletals_Shell.html

the human skeletal system(1)

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