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In this article we will discuss about the Skeletal System of Humans.
Endoskeleton:
The main functions of endoskeleton are as follows:
1. Support:
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One of the main functions of the skeleton is to give support to the softer body parts.
2. Protection:
The endoskeleton protects the delicate body parts.
3. Muscle attachment:
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The endoskeleton parts provide attachment for large muscles.
4. Movement:
Due to the contractions of the muscles, the bones or parts of the bones are able to change their positions. Thus bones help in bringing about movements also.
5. Body form:
It forms a typical body shape and form of an individual animal.
6. Blood cells formation:
The red blood corpuscles and white blood corpuscles and platelets are produced by the bone marrow.
7. Mineral reservation:
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The bones maintain the calcium and phosphorus levels of the blood. In cases of great demand excessive amounts of calcium and phosphorus are removed from the bones.
8. Helps in breathing and hearing:
The cartilages of the larynx, trachea, sternum and ribs are helpful in breathing while ear bones of the middle ear transmit the sound vibrations from the tympanic membrane to the internal ear.
On the basis of the position of the skeletal structures in the body the endoskeleton is divisible into two parts.
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1. Axial skeleton:
It is present on the median longitudinal axis of the body. It consists of skull, vertebral column, sternum and ribs.
2. Appendicular skeleton:
It is situated at the lateral sides which actually extend outwards from the principal axis. It consists of pectoral and pelvic girdles and bones of arms and legs.
Skull:
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Skeleton of head is called skull.
The skull rests upon the upper end of the vertebral column and its bony structure consists of the following parts:
(i) Bones of the Cranium:
The cranium is formed by 8 bones. The bones which form the cranium are: 1 frontal bone, 2 parietal bones, 2 temporal bones, 1 occipital bone, 1 sphenoid bone and 1 ethmoid bone.
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Temporal bone has a projection called mastoid (L. Mastos = breast, Gr. eidos = resemblance, resembling a breast shaped) process. The cranial cavity, in which the brain is lodged, posteriorly opens by foramen magnum. The brain is connected to spinal cord at this foramen.
(ii) Ear ossicles:
There are present 6 ear ossicles (2 malleus, 2 incus and 2 stapes) in the skull.
(iii) Hyoid bone:
The hyoid bone is situated in the wall of the upper part of the throat, just above the larynx. Although it is not a bone of the skull proper, it is customarily considered with that portion of the skeleton. It serves as a point of attachment for some of the muscles of the tongue and floor of the mouth but does not articulate with any other bone.
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(iv) Bones of the Face:
There are 14 bones which form the skeleton of the face; 2 zygomatic bones, 2 maxilla, 2 nasal bones, 2 lacrimal bones, 1 vomer, 2 palatine bones, 2 inferior nasal conchae or turbinated bones and 1 mandible (dentary).
Thus the skull consists of 29 bones:
At the posterior end of the cranium there are two rounded protuberances, the occipital condyles, that articulate with the atlas (1st vertebra). Thus human skull is dicondylic.
Functions of the Skull:
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(i) The most important function of the skull is to protect the brain. This function is mainly carried out by the cranium,
(ii) The skull bears jaws which help the animal for cutting and masticating its food,
(iii) It also protects and supports the special sense organs. This function is carried out by the nasal-cum-auditory capsules and the orbits,
(iv) The skull also provides the rigid walls of a respiratory passage through which air is inhaled into the lungs,
(v) The hyoid apparatus protects and supports the throat.
Vertebral Column (Backbone):
The vertebral column is about 71 cm long. It is curved lying in the mid dorsal line of the neck and trunk. It is made up of 33 vertebrae. However it consists of 26 bones because five sacral vertebrae are fused to form one sacrum and four coccygeal vertebrae are fused to form one coccyx. The components of the vertebral column are called vertebrae (sing, vertebra).
Features common to most vertebrae (Fig. 20.13):
A typical (generalized) vertebra has a large, disc-like anterior, flattened portion, the centrum or body and a posterior portion, the neural arch. The latter encloses the spinal cord. The hole formed by the neural arch is the vertebral foramen.
The vertebral foramina of all twenty four vertebrae form the vertebral canal or neural canal. Each neural arch is formed by two processes of the centrum, the pedicles, and two laminae.
A spinous process (neural spine) is found projecting dorsally. Six other processes originate on the neural arch; two superior articular processes, two inferior articular processes and two transverse processes. The centrum of human vertebrae is acoelous (= amphiplatyan).
The vertebrae are grouped into five groups:
(i) Cervical Vertebrae:
7 in number present in the neck, first cervical vertebra is called atlas, second cervical vertebra is known as axis. The atlas supports the head and is named for the mythological Atlas who supported the world on his shoulders. The atlas lacks body (centrum) and a spinous process. The axis has a peg like process called the odontoid process.
(ii) Thoracic Vertebrae:
12 in number present in the chest. They are larger and stronger than cervical vertebrae. They articulate with ribs.
(iii) Lumbar Vertebrae:
5 in number present in the abdomen. They are largest and strongest in the vertebral column. Their processes are short and thick. The spinous processes are thick and broad and project posteriorly.
(iv) Sacrum:
The five sacral vertebrae are fused in the adult, forming one structure called the sacrum, which lies between the innominate bones of the pelvic girdle. The female sacrum is shorter, wider than the male sacrum.
(v) Coccyx:
The four coccygeal vertebrae are fused to form a curved triangular bone, called the coccyx. In female, the coccyx points inferiorly but in male it points anteriorly. It is considered as a vestigial tail (Fig. 20.14).
Intervertebral Discs:
These are present between the bodies of adjacent vertebrae from second cervical vertebra to the sacrum. Each disc has an outer fibrous ring consisting of fibrocartilage and an inner soft, pulpy, highly elastic substance. The discs form strong joints, permit various movements of the vertebral column and absorb vertical shock.
Vertebral Formula:
The vertebral formula of human is C7T12T (5) C (4).
Curvatures (Bends):
Human vertebral column shows four curvatures:
(i) Cervical curvature— in the neck region, convex anteriorly,
(ii) Thoracic curvature— in the thorax, concave anteriorly,
(iii) Lumbar curvature— in the abdomen, convex anteriorly.
(iv) Sacral curvature— in the pelvic region, concave anteriorly.
Functions of Vertebral Column:
(i) It protects the spinal cord,
(ii) It carries the weight of the body during motion as well as in standing position,
(iii) Anterior cervical vertebrae allow free movement of the head,
(iv) The inter-vertebral discs present in between the vertebrae give axial flexibility to the animal,
(v) It serves as a strong beam from which viscera are suspended by mesentries in the body cavity.
Sternum or Breast Bone:
This is a flat bone which is present just under the skin in the middle of the front of the chest. It is about 15 cm long. Its shape is like a dagger and consists of three parts— the manubrium is the uppermost part, the body is the middle portion and the xiphoid process is the tip of the bone. The true ribs (7 pairs) are attached to the sternum.
Functions of Sternum:
It protects the internal organs in the thoracic region. It also provides the surface for muscle attachment. The sternum helps in the respiratory mechanism.
Ribs (Fig. 20.16 & 20.17):
There are 12 pairs of ribs which form the bony lateral walls of the thoracic cage.
1. The first seven pairs are called true ribs because their anterior ends are attached directly to the sternum by means of small pieces of hyaline cartilage, the costal cartilages.
2. The eighth, ninth and tenth pairs of the ribs are called false ribs. They articulate by cartilage with costal cartilage of the seventh rib and thus are attached indirectly to the sternum.
3. The last two pairs of ribs are called floating ribs because their anterior ends are not attached to either the sternum or the cartilage of another rib. The floating ribs protect the kidneys.
A typical (generalized) rib consists of two parts: vertebral part and sternal part. The vertebral part is long and bony and articulates with the thoracic vertebrae by facets. The sternal part is short and made up of hyaline cartilage which articulates with sternum or sternal part of its upper rib.
The head of a typical rib is wedge-shaped and consists of one or two facets that articulate with thoracic vertebrae. The neck is a constricted portion. A knob like structure, the tubercle articulates with the transverse process. The shaft (= body) is the main part of the rib. There is an abrupt change in the curvature of the shaft. This point is called the angle.
Functions of Ribs:
They protect the delicate organs like heart and lungs, etc. present in the thoracic cavity. Ribs provide the surface for attachment of the muscles concerned with respiratory mechanism. Floating ribs protect the kidneys. Ribs 6-10 show “bucket-handle” type of movement.
Pectoral (Shoulder) Girdles:
Each pectoral girdle (Fig. 20.18) consists of two bones: 1 clavicle and 1 scapula. The scapula (shoulder blade) consists of a sharp ridge, the spine and a triangular body. The end of the spine projects as a flattened and expanded process called acromion. This process articulates with the clavicle.
At the lateral end of the superior of the scapula is a projection of the anterior surface called the coracoid process, to which the tendons of the muscles attach. At the point where the superior and lateral borders of the scapula meet there is the lateral angle which presents a shallow articular surface termed as glenoid cavity into which the head of the humerus is articulated.
Functions of Pectoral Girdle:
In addition to providing the glenoid cavity for articulating with the head of the humerus, the pectoral girdle is also meant for attachment of the arm muscles. The delicate organs of this area are well protected.
Each arm consists of the following 30 bones:
1 humerus, 1 radius, 1 ulna, 8 carpal bones, 5 metacarpal bones, 5 digits (14 phalanges). Phalangeal formula: 2, 3, 3, 3, 3.
Upper rounded end of the humerus is called head which articulates into the glenoid cavity of the pectoral girdle. A greater and a lesser tubercles occur near the head. The shaft of the humerus has a V-shaped deltoid ridge at about its middle.
A pully like trochlea is present between two ridges. Ulna is longer than radius. Its upper end has a larger olecranon process that forms the eminence of our elbow. The head of the radius articulates with the humerus.
Each wrist is composed of eight carpals which are arranged in two rows : scaphoid, lunate, triquetrum and pisiform in proximal row and trapezium, trapezoid, capitate and humate in distal row. Functions of Arm Bones The arms give support to the shoulders by articulating the head of the humerus with the glenoid cavity of the pectoral girdle.
Pelvic Girdle:
The pelvis or pelvic girdle is formed by two innominate bones (hip bones). The sacrum and coccyx also take part in the formation of the pelvis. Each innominate bone consists of three separate bones, the ilium, the ischium and the pubis. On its outer surface it has a deep depression called the acetabulum to which head of the femur is articulated forming the hip joint.
The acetabulum is formed by the ilium, ischium and pubis. The ilium has one large depression, the greater sciatic notch through which the sciatic nerve, the longest nerve in the body, passes.
The ischium also has one small depression the lesser sciatic notch. There is also present, the obturator foramen. The foramen is so named because even though blood vessels and nerves pass through it yet it is nearly closed by the fibrous obturator membrane (Fig. 20.20).
Pubic symphysis is made up of white fibrous cartilage.
Functions of Pelvic Girdle:
Apart from articulating with the legs, the pelvic girdle supports the posterior region of the trunk. The articulation of the iliac surfaces with the sacrum gives added strength to the wsacral region. It provides surface for the attachment of the muscles of the legs. Pelvic girdle protects the soft organs present in the pelvic cavity of this region.
Bones of the Legs:
Each leg consists of 30 bones: 1 femur, 1 tibia, 1 fibula, 1 patella (knee cap), 7 tarsal bones, 5 metatarsal bones, 5 digits (14 phalanges). Phalangeal formula: 2, 3, 3, 3, 3. The femur is the longest and strongest bone of the body.
Its upper end has a rounded head, a constricted neck and a greater trochanter. Head of femur articulates into acetabulum of pelvic girdle. There is a long shaft. The lower end is divided into two condyles with a inter-condylar fossa in between.
Patella (knee cap) is flat, sesamoid bone, formed by ossification in the tendon of quadriceps femoris muscle. Tibia is longer, thicker and lies more medially and front. Fibula is shorter, thinner and located more laterally and deeply. Each ankle is composed of seven tarsals which are calcaneum, talus, cuboid, navicular and first, second, third cuneiforms. Total bones of a leg are 30.
Functions of Leg Bones:
The leg bones are involved mainly in propulsion and support. The femur is articulated with the acetabulum of the pelvic girdle. Therefore, the leg bones work like an extensible movable axis, on which the main pelvic girdle is balanced and propelled at the hip joint. The weight of the body is also carried by the leg bones.
Types of Bones:
According to the shape and size, bones are classified into four categories.
(i) Long bones:
E.g., humerus of upper arm, radius and ulna of forearm, femur of thigh and tibia and fibula of shank.
(ii) Short bones:
E.g., metacarpals of palm, metatarsals of sole, phalanges of fingers and toes.
(iii) Flat bones:
E.g., scapula of pectoral girdle, sternum and cranial bones.
(iv) Irregular bones:
E.g., vertebrae, carpals of wrist and tarsals of ankle.
Joints (Fig. 20.22 — 20.25):
The structural arrangements of tissues by which bones are joined together are called joints. According to the mobility they are classified as fibrous or fixed or immovable joints, cartilaginous or slightly movable joints and synovial or freely movable joints.
1. Fibrous or Immovable Joints:
In this type of joints there is no movement between the bones concerned. As the name suggests, there is white fibrous tissue between the ends of the bones. Examples of this type include the joints between the bones of skull called sutures and the joints between the teeth and the maxilla and teeth and mandible.
2. Cartilaginous or Slightly Movable Joints:
In this type there is a pad of white fibrocartilage between the ends of the bones taking part in the joints which allows for very slight movement. Movement is only possible because of compression of pad of cartilages. Examples of cartilaginous joints include the pubic symphysis of pubis and the joints between the vertebrae (intervertebral discs).
3. Synovial or Freely Movable Joints:
A considerable movement is possible at all synovial joints.
Structure of a Generalized Synovial Joint:
There is present a membrane called synovial membrane. This membrane is composed of secretory epithelial cells which secrete a thick sticky fluid, of the consistency of the white of an egg called synovial fluid. It acts as a lubricant to the joint, provides nutrient materials for the structures within the joint cavity and helps to maintain the stability of the joint.
It prevents the ends of the bones from being separated as does a little water between two glass surfaces. Bone ends are covered by articular cartilages (composed of hyaline cartilage).
Ligaments join the bones and tendons connect the bones with muscles. Little sacs of synovial fluid or bursae are found in some joints. Their position is such that they act as cushions to prevent friction between a bone and a ligament or tendon, or the skin where a bone inside a joint capsule is near the surface.
Types of Synovial Joints:
According to the shape of the bones and types of the movements they allow, the synovial joints are of six types: gliding, hinge, pivot, ellipsoid, saddle and ball and socket.
(i) A gliding joint is the simplest of the synovial joints. The articular surfaces of two bones are usually flat, permitting only back-and-forth and side-to-side movements. No rotation or twisting is possible because the bones are packed closely together or held in place by ligaments. Gliding joints are found between the carpal bones and between the tarsal bones. It is monaxial joint.
(ii) A hinge joint allows movement primarily in one plane. In a hinge joint spool (reel) like surface of one bone fits into the concave surface of another bone. The elbow, the knee, ankle and inter-phalangeal joints are examples of hinge joints. Articulation between the occipital condyles of skull and atlas vertebra also forms a hinge joint. It is monaxial joint.
(iii) A pivot joint also allows movement in only one plane. In a pivot joint rounded or pointed bone fits into a shallow depression in another bone. The joints between the atlas and axis and between the radius and ulna (radioulnal joint) just below the elbow are examples of pivot joints. The primary movement at a pivot joint is rotation. It is monaxial joint.
(iv) Condyloid or ellipsoid joint allows movement in two planes, back and forth and side-to-side. In an ellipsoid joint an oval shaped condyle of one bone fits into an elliptical cavity in another bone. The joints between the metacarpals and phalanges (metatarsophalangeal joint) of the fingers are examples of ellipsoid joints. It is biaxial joint.
(v) A saddle joint allows the same movements as an ellipsoid joint, but the movements are free. The projection of one bone fits in saddle-shaped depression in another bone. The joint between the carpal and metacarpal of thumb of the hand (carpometacarpal joint of thumb) is an example of saddle joint. It is biaxial joint.
(vi) A ball-and-socket joint is the most freely movable of all joints. In a ball-and-socket joint as the name suggests, a ball-like structure on the one bone fits into a socket like structure in another bone. Shoulder and hip joints are the examples of ball and socket joint (between glenoid cavity of the pectoral girdle and head of the humerus and between acetabulum of pelvic girdle and head of the femur). It is multi-axial joint.
