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HUMAN SKELETAL SYSTEM

SKELETAL SYSTEM IN MAN Human skeleton is made of 206 bones that are formed from some 300 bones through fusion. The bones are arranged into axial and appendicular skeleton. Number of bones in child = 350

Axial Skeleton It is part of endoskeleton which occurs along the middle longitudinal axis of the body. Axial skeleton has 80 bones forming four structures – skull, vertebral column, sternum and ribs. 1. Skull (29 bones) It constitutes skeleton of the head region. Skull consists of four parts – cranium, facial bones, hyoid and ear bones.

Skull of Human (lateral view)

(a) Cranium (Brain Box, 8 bones). It is the bony structure that encloses the brain and protects it. It is hollow and nearly rounded. Bones constituting the cranium are called cranial bones. They are eight in number. (i) Frontal-1 (ii) Parietal-2 (iii) Occipital-l (iv) Temporal-2 (v) Sphenoid-l (vi) Ethmoid-l. Cranial bones are flattened bones. They are jointed with each other by fixed interdigited joints called sutures. At the base of skull occipital has a large opening called foramen of magnum. Through foramen of magnum brain communicates with spinal cord. There are two lateral protuberances or occipital condyles, one on each side of foramen magnum. Because of the presence of two occipital condyles, human skull is called dicondylic. The two occipital condyles are articulated with atlas vertebra by hinge joints for performing nodding movements. Each temporal bones bear opening of external auditory meatus. Skull bones also posses fontanelles. Fontanelles are the soft areas present in foetal cranium. An infant may have many fontanelles at birth but generally six are recognized. Anterior fontanell or Frontal fontanell is largest and it closes by 18 to 24 months of age. Neurocranium is part of skull having brain and sensory capsules. Temporal bone has styloid process, zygomatic process and mastoid process. It also posses an external auditory meatus. (b) Ear Ossicles (6 bones). Each side or temporal region contains an auditory capsule or middle ear. It has three bones. These bones are small in size and named as ear ossicles or ear bones. They are outer malleus (hammer shaped), middle incus (anvil shaped) and innermost stapes (stirrup shaped). These bones are helpful in the amplification of sound 20-22 times. (c) Facial Bones (14 bones). Facial bones constitute front part of skull alongwith skeleton of lower jaw, hard palate and nose. They are fourteen in number. (i) Nasals-2 (ii) Maxillae-2 (iii) Zygomatic (cheek bones)-2 (iv) Lacrimals-2 (v) Palatines-2 (vi) Inferior nasal conchae-2 (vii) Vomer-1 (viii) Mandible-1. Frontal is common bone to both cranium and face. Maxillae form the upper jaw whereas mandible forms the lower jaw. Upper jaw is fused with cranium. Lower jaw or mandible is horse-shoe shaped bone which bears two processes on each side, a pointed one named coronoid and a rounded one called condyloid. Mandible is strongest and largest bone of face. Movable mandible helps in the mastication of food as well as speech. Jaws have sockets for the fixation of teeth.

(d) Hyoid Bone (Tongue Bone, 1 bone). It lies at the base of tongue and above the larynx. It is not joined to the rest of skeleton except to styloid process of temporal bone by muscles with the help of certain muscles. It is horse shoe-shaped slender bone. Upper portion of hyoid bone has a swelling known as the greater cornu. The lower portion has small projection known as lesser cornu. Hyoid bone and the tongue together form the hyoid apparatus. Hyoid bone is fractured during strangulation. FUNCTIONS OF SKULL (i) Cranium protects and houses the brain. (ii) It contains and supports the eyes. (iii) Skull contains nasal chambers where sense of small is perceived. (iv) It has respiratory passages. (v) Maxillae and mandible form upper and lower jaws having teeth. They help in cutting and mastication of food. (vi) Mandible being mobile plays a major role in mastication and speech. (vii) Hyoid apparatus supports the throat and tongue. (viii) Ear bones are mobile. They are helpful in amplification of sound. (ix) Facial bones support the upper portion of digestive tract and respiratory tract. (x) Muscles attached to facial bones are meant for the facial expression. 2. Vertebral Column (33/26, bones) It is also called back bone. Length of vertebral column in males is 71 cm, in females 61 cm. Vertebral column lies in the mid-dorsal region of neck and trunk. It is a string of 26 vertebrae which have two forward and two backward curves. Discs or pads of fibrocartilage occur in between adjacent vertebrae. They are called intervertebral discs. They provide flexibility to vertebral column. A ligament supports the vertebrae against dislocation. Vertebrae are of five types – cervical, thoracic, lumber, sacral and coccygeal with a vertebral formula of C7T12L5S5C4. Each vertebra has a large disc-like flattened body called centrum in front and neural arch behind. A neural canal or neural foramen occurs between the two. Spinal cord passes through it. Neural arch is formed of two rounded pedicles infront and two flattened laminae on the back side. Pedicles of adjacent vertebrae enclose intervertebral foramen for passage of spinal nerves. A neural spine occurs at the tip of neural arch. Transverse processes occur on the sides. Articular facets occur at the bases of tranverse processes. There are two superior articular factes or prezygapohyses. (sing. Prezygapophysis) and two inferior articular facets occur or post-zygapophyses. Superior and inferior articular facets of adjacent vertebrae form gliding type of joints.

(i) Cervical Vertebrae (7). They are small sized vertebrae of the neck region. All humans have 7 cervical vertebrae. The first cervical vertebra is called atlas. It has reduced centrum, rudimentary neural spine and concave superior articular facets for fitting over two occipital condyles to provide for nodding movement of head. Inferior articular facets are circular. A median facet is also present for odontoid process of axis. Second cervical vertebra or vertebra or axis has a strong odontoid process or dens that fits into odontoid canal of atlas to provide sideways rotation of the head. Spines are bifid upto fifth cervical vertebrae. Third to seven cervical vertebrae are typical. At the lower portion of transverse processes of cervical vertebrae there is present an aperture called foramen transversarium. It forms a channel known as vertebrarterial canal for the passage of blood vessels from heart to the brain and back. (ii) Thoracic Vertebrae (12). Thoracic vertebrae are larger than cervical vertebrae. Transverse processes are stout. Spines are long and downwardly directed. Thoracic vertebrae have sites for attachment to ribs. They are twelve in number and form thoracic basket with the ribs for the protection of the lungs and heart. Tip of the transverse process has a facet for attachment with tubercle of rib. Head part of rib attaches to a facet formed jointly by two adjacent vertebrae near the pedicle. The first and the last thoracic vertebrae have complete facet for rib heads. The last three thoracic vertebrae do not have facets for rib tubercles.

(iii) Lumbar Vertebrae (5). They are five in number. Lumbar vertebrae occur in the abdomen. They are the largest, thickest and strongest of all vertebrae as they bear part of the weight of the trunk. Lumbar vertebrae possess long but narrow transverse processes. Neural spines are horizontal. (iv) Sacrum (5 sacral vertebrae). It is a triangular bony complex formed by the fusion of five vertebrae. Sacrum is concave in front and convex behind. It is attached to pelvic girdle by sacroiliac joints. The attachment transfers weight of the body to legs through pelvic girdle. Sacrum bears transverse lines indicating position of different sacral vertebrae. It also possesses foramina for exit of spinal nerves. (v) Coccyx (4-5 coccygeal vertebrae). It is a small vestigial tail bone which is formed by the union of 4 (or 5) nodule-like coccygeal bones. Coccyx is attached to base of sacrum. CURVATURES OF VERTEBRAL COLUMN Vertebral column has four curvatures : cervical, thoracic, lumber and sacral. Cervical and lumber curvatures are directed forwards while thoracic and sacral curvatures are directed backwards. The curves help in keeping the body straight. They also keep the centre of gravity in between in the heels so that erect posture is maintained during walking. Functions : (i) Vertebral column supports head region. (ii) With the help of atlas and axis vertebrae, vertebral column allows movement of head in different directions. (iii) Vertebral column encloses and protects the spinal cord. (iv) Through its curvatures, vertebral column helps in maintaining an erect posture. (v) It support ribs and forms the rib cage along with ribs and sternum. (vi) Vertebral column provides sites for attachment to girdles. (vii) It provides axial flexibility. (viii) Vertebral column shifts weight of the body to lower limbs through ilio-sacral joints. (ix) It supports suspension of viscera in the body cavity. 3. Ribs There are twelve pairs of ribs that join the thoracic vertebrae of the vertebral column on the dorsal side and the sternum on the ventral side to form the thoracic basket. Each rib is composed of two parts sternal and vertebral part. Sternal part of rib is attached to the sternum in front. The vertebral part of rib is attached to the vertebral column at the back. Each rib. Has a head, a tubercle (both for the attachment to thoracic vertebrae), neck, a shaft (body) and costal cartilage (for the attachment with sternum), Ribs are of three sub types – true, false and floating. (a) The Ribs. First seven pairs of ribs are known as true ribs they directly attached to sternum as well as vertebral column. Their size increases gradually from first to seven. The seventh rib is longest. (b) False Ribs. 8th, 9th and 10th pair of ribs are called false ribs as they are attached to one another as well as to the costal side of 7th pair of ribs. False ribs are also called vertebrochondrial ribs.

(c) Floating Ribs. 11th and 12th pairs of ribs are known as floating ribs because their one end is joined to vertebral column whereas the second end is free. Floating ribs play a very important role to protect the kidneys. Functions : (i) Ribs make the human thorax wider from side to side than from the front to back. It is an adaptation for the erect posture and helps to maintain equilibrium. (ii) They protect two delicate organs, heart and lungs. (iii) Ribs provide attachment to intercostal muscles that take part in breathing movements. (iv) Floating ribs protect the kidneys. 4. Sternum (Breast Bone) It is a flat and narrow dagger-like bone of about 15 cm length which is present in the exact centre of the body to which ribs are attached. Sternum is composed of 3 parts – manubrium, body and xiphoid. (a) Manubrium. It is wide flat plate which forms the upper part of sternum. It has facets for attachment to clavicles (collar bones) and first pair of ribs. (b) Body (Mesosternum). It is elongated middle part of sternum. Body of sternum is often equated to the blade of a dagger while manubrium is equated to the handle of dagger. Body has facets for attachment of 2–7 rib pairs. (c) Xiphoid (Xiphisterum). It is lower part of sternum which is cartilaginous in the young but becomes ossified in adults. Normally, it is small and pointed but can be bifid, curved or deflected. Xiphoid is often equated to the point of the dagger.

Functions : (i) Sternum is a component of rib cage. (ii) It has facets for attachment of clavicles and ribs. (iii) It protects diaphragm. (iv) Sternum has red bone marrow and is important area of haemopoiesis.