THE SKELETAL SYSTEMLecture IIIWHAT IS THE SKELETAL SYSTEM?In biology, the skeletal system is the biological system that provides support in living organisms. TYPES OF SKELETAL SYSTEMS There are three diferent types of skeletal systems: Exoskeleton- A hard encasement deposited on the surface of an animal. Endoskeleton- Consists of hard supporting elements, such as bones, buried within the soft tissues of an animal.Hydrostatic Skeleton- Consists of fuid held under pressure in a closed body compartment.HYDROSTATIC SKELETONoHydrostatic skeletons are similar to a water-flled balloon. oLocated internally in cnidarians, (coral, jellyfsh etc.) and annelids (leeches, earthworms etc.), among others, these animals can move by contracting the muscles surrounding the fuid-flled pouch, creating pressure within the pouch that causes movement. oAnimals such as earthworms use their hydrostatic skeletons to change their body shape, as they move forward, from long and thin to shorter and wider.HYDROSTATIC SKELETONNot all skeletons are rigid; many invertebrate groups use their body fuids as an internal hydrostatic skeleton. Muscles in the body wall of the earthworm, for example, have no frm base for attachment but develop muscular force by contracting against the coelomic fuids, which are enclosed within a limited space and are incompressible, much like the hydraulic brake system of an automobile.Chaetae get lifted in regions of circular muscle contraction.

During longitudinal muscles contraction, chaetae anchor into the groundHYDROSTATIC SKELETONLocomotion in aquaticinvertebrates occurs byfuid ejections or jetting -Jellyfsh produce regularpulsations in bell-Squeezing some of water contained beneath it-Squids fll mantle cavitywith sea water-Muscular contractionsexpel water forcefully throughthe siphon, and the animalshoots backwardEXOSKELETONSome animals with exoskeletons:arthropods, such as insects, spiders and crustaceans other invertebrate animals, such as shelled mollusks* The exoskeletons of mollusks contain calcium carbonate.As the mollusk grows, the shell enlarges as well.* Jointed appendages are also present in organisms with an exoskeleton.MOLTINGENDOSKELETONoThe endoskeleton, also known as an internal skeletal system, consists of rigid (or semi rigid) structures, within the body. These structures are capable of being moved by the muscular system which surrounds it. oIf the skeletal structures are mineralized or ossifed, as they are in humans and other mammals, they are referred to as bones.o Cartilage is referred to as another common component of skeletal systems, supporting and supplementing the skeleton. Though less ridged, they are more fexible in comparison to the ossifed bone, allowing for manipulation in shape to enable bending. For example, the human ear, nose, and ribs all consist of cartilage. ENDOSKELETONBone is never formed in vacant space but is always laid down by replacement in areas occupied by some form of connective tissue. Most bone develops from cartilage and is called endochondral (within cartilage) or replacement bone.Cancellous (or spongy) bone consists of an open, interlacing framework of bony tissue, oriented to give maximum strength under the normal stresses and strains that the bone receives. All bone develops frst as cancellous bone, but some bones, through further deposition of bone salts, become compact.MICROSCOPIC STRUCTURE OF BONECompact bone is composed of a calcifed bone matrix arranged in concentric rings. The rings contain cavities (lacunae) flled with bone cells (osteocytes), which are interconnected by many minute passages (canaliculi). These passages serve to distribute nutrients throughout thebone. This entire organization of lacunaeand canaliculi is arranged into an elongated cylinder called an osteon (also called haversian system)MICROSCOPIC STRUCTURE OF BONEBone consists of bundles of osteons cemented together and interconnected with blood vessels and nerves. Because of blood vessels and nerves throughout bone, it is living tissue, although nonliving ground substance predominates. As a result of its living state, bone breaks can heal, and bone diseases can be as painful as any other tissue disease.PLAN OFF THE VERTEBRATE SKELETONThe vertebrate skeleton is composed of two main divisions: Axial skeleton, which includes skull, vertebral column, sternum, and ribs,Appendicular skeleton, which includes the limbs (or fns or wings) and pectoral and pelvic girdlesAXIAL SKELETONSKULLAmphibians and lizards have 50 to 95, and mammals, 35 or fewer. Humans have 29.VERTEBRAL COLUMNThe vertebral column is the main stifening axis of the postcranial skeleton. In fshes it serves much the same function as the notochord from which it is derived; that is, it provides points for muscle attachment and prevents telescoping of the body during muscle contraction.AXIAL SKELETONVERTEBRAL COLUMNIn amniote tetrapods (reptiles, birds, and mammals), the vertebrae are diferentiated into cervical (neck), thoracic (chest), lumbar (back), sacral (pelvic), and caudal (tail) vertebrae. In birds and also in humans the caudal vertebrae are reduced in number and size, and the sacral vertebrae are fused.The number of vertebrae varies among the diferent vertebrates. Pythons seems to lead the list with more than 400. In humans (Figure 31-9) there are 33 in a young child, but in adults 5 are fused to form the sacrum and 4 to form the coccyx.AXIAL SKELETONRIBSRibs are long or short skeletal structures that articulate medially with vertebrae and extend into the body wall. Fishes have a pair of ribs for every vertebra they serve as stifening elements in the connective tissue septa that separate the muscle segments and thus improve the efectiveness of muscle contractions.AXIAL SKELETONRIBSMammals such as sloths have 24 pairs of ribs, whereas horses posses 18 pairs. Primates other than humans have 13 pairs of ribs; humans have 12 pairs, although approximately 1 person in 20 has a thirteenth pair. APPENDICULAR SKELETONAPPENDAGESMost vertebrates, fshes included, have paired appendages. All fshes except agnathans have thin pectoral and pelvic fns that are supported by the pectoral and pelvic girdles, respectively APPENDICULAR SKELETONAPPENDAGESIn nearly all tetrapods the pelvic girdle is frmly attached to the axial skeleton, since the greatest locomotory forces transmitted to the body come from the hindlimbs. The pectoral girdle, however, is much more loosely attached to the axial skeleton, providing the forelimbs with greater freedom for manipulative movements.The skeleton system of mammals is broadly divided functionally into Axial and Appendicular portionsThe axial skeleton consists of braincase(cranium),backbone and ribsThe limbs and their girdles constitute the appendicular skeletonIn addition to these two, visceral elements such as jaw, auditory ossicles of the middle ear constitute visceral skeletal systemSKELETAL SYSTEM OF MANIn man ,the bones are named based on the positionIn human body,there are 206 bonesAxial skeleton-80 bonesAppendicular skeleton-126 bonesSKULLIt protects the brain and provided support for organs of vision,hearing,smell and tasteThe lower jaw or mandible remains specially attached to the skullSKULLThe bones in our skull are joined my sutures to form a complex box like structure.The sutures are immovable.The major bones in frontal view includes frontal bone, zygomatic bone, mandibleThe openings in the skull include orbits(for theeyes)and the nasal cavityA large opening present in the base of the skull is the foramen magnum. Through this opening,medulla oblangata descends as the spinal cordVERTEBRAL COLUMNThe vertebraes makes up a slightly S shaped vertebral column.The vertebral column contains 26 bones,which includes cervical(7), thoracic(12), lumbar(5) ,sacral(1), coccygeal(1)RIB CAGEThere are 12 pairs of ribs.First 7 pairs are directly articulated with the sternum.hence they are TRUE RIBS8,9,10 th pairs are fused and are attached to sternum.hence they are known as FALSE RIBS11th,12th pairs are not attached to sternum.hence they are known as FLOATING RIBSAPPENDICULAR SKELETONPectoral girdle:both of our hands are attached to it loosely by muscles to the body, this arrangement facilates wide range of movement of our hand.APPENDICULAR SKELETONPelvic girdle:Also known as the pelvis, is formed by the sacrum and paired bones known as coxae or hip bones, the pelvic girdle contains the acetabulum which is meant for the articulation of our lower limb.UPPER LIMB:the part of the hand from shoulder to elbow is the arm.it contains one long bones known as the HUMERUSFOREARM:this is the part inbetween the arm and the wrist, the forearm has two important bones, they are RADIUS,ULNA.RADIUS AND ULNARadius:the radius is present on the lateral or thumb side of the forearmUlna:ulna is present on the side of the little fngerWRIST AND HANDWrist:this region is composed of 8 carpel bones.on the anterior surface carpal tunnel is present through which tendons,blood vessels passes to the hand.Hand:there are fve metacarpels.each digit other than the thumb contains three phalanges while the thumb contains only two phalanges.LOWER LIMBSFemur:the upper region of leg which is the thigh contains a single largest bone known as the FemurKnee:contains a large,fat bone called as patella which is articulated with the femurLEG,ANKLE,FOOTLeg:lower limb between knee and ankle.it consists of two bones namely Tibia and Fibula. Tibia is larger and supports most of the weight of legADVANTAGES OF THE HUMANSKELETAL SYSTEMThe design of the skeleton accounts for the demand for support based on the size of the bodyThe diferent types of joints allow for a wide range of motionThe human skeleton provide the body with a support system as well as fexibility and locomotionSOME INTERESTING FACTSSmallest bone-stapes(1.14mm)Largest bone-thigh bone(26.74% of length)Vermes-soft bodied.organisms that lack skeleton.Teeth-teeth are frst included under skeletal system but since they are functional part in digestive system they are included under digestive systemBONE MARROWIt is an fexible tissue present interior to the bones.In humans RBC are produced by the heads of long bones by a process known as HAEMATOPOIESISOn average bone marrow constitutes abt 4% of the total body weight in humans.in an adult weighing 65 kg the weight of the marrow is nearly 2.6kgBone marrow is also a key to lymphatic system,producing lymphocytes-immunityFUNCTIONS OF BONESMINERAL STORAGE:bones act as a reserve for minerals,specifcally calcium,phospateProtection:protect internal organsShape:provides a frame to support bodyAcid base balance:bone bufers the blood against excessive ph changes by absorbing or releasing alkaline saltsDetoxyfcation:bone tissues can remove heavy metals and other foreign substances from the blood and store them reducing their efects on other tissues.these substances are later excreted