Zoology Notes: 010 Chapter 6

Chapter 6. The Skeletal System The skeleton is defined as any hardened part of an animal. It is the supporting framework of bones and serves as protection while providing levers for muscular action. The study of bones is known as osteology. The skeleton of a living animal is made up of bones that are themselves living structures. They have blood vessels, lymphatic vessels, and nerves; they are subject to disease, repair themselves, and adjust to changes in stress. General Functions of the Skeleton: 1. 2. 3. 4. 5.

It supports the surrounding tissues giving rigidity and form to the body. It protects vital organs and other soft tissues of the body. It assists in body movement, giving attachment to the muscles and providing leverage. It manufactures blood cells. This hematopoietic function occurs in the red bone marrow. It provides a storage area for mineral salts, especially phosphorus and calcium to supply body needs.

Three Types of Cells that Exists in Bones: 1. 2. 3.

Osteoblasts - bone-forming cells or the building blocks of bone matrix. Osteoclasts - associated with bone resorption. Osteocytes - formed when osteoblast on the bone surface are surrounded by mineralized matrix (lamellae). The central Haversian canal provides space for the network of blood vessels and nerves.

Two Types of Ossification

Fig. 6.1. Bone cell

Intramembranous Ossification. This process involves the direct mineralization of richly vascular dense connective tissue membrane, thus forming the bone. The membrane itself becomes the periosteum, while immediately within the periosteum can be found compact bone with an inner core of cancellous bone. Only flat bones of the cranium form completely by this process. Endochondral Ossification. Most bones form by this process where there is replacement of a “scale model” of hyaline cartilage by bone. The cartilage skeleton is completely formed at the end of three months of pregnancy. During the subsequent months of gestation, ossification and growth occurs. Types of Bone Tissue Compact bone tissues (substantia compacta and substantia corticalis) are dense and strong bone tissue. Cancellous or spongy bone tissue (substania spongiosa) has many open (marrow) spaces, giving the tissue a spongy appearance. It is elaborated in the extremities of long bones, forms the internal substance of short and irregular bones, and is interposed between the two compact layers of most flat bones. Types of Bones According to Shape and Function: Long Bones. Long bones (ossa longa) are greater in one direction in one dimension than any other. Each consists of a relatively cylindrical shaft (diaphysis) and two extremities called epiphysis with a metaphysic between each epiphysis and the diaphysis. They function chiefly as levers and aid in support, locomotion, and prehension. The cylindrical part, termed the shaft or body (corpus) is tubular, and encloses the medullary cavity, which contains Fig. 6.2. Long bone. the medulla or marrow. Ex. humerus (arm bone), radius and ulna (forearm bones), femur (thigh bones), tibia and fibula (leg bones). 33

Short Bones. Short bones (ossa brevia) are somewhat cuboid or approximately equal in all dimensions and are not merely shorter versions of long bones. Short bones function in absorbing or diffusing concussions, and they are found in complex complex joints. Ex. carpals (wrist bones) and tarsals (ankle bones) Sesamoid Bones. Sesamoid bones (ossa sesamoidea) are small and rounded, enclosed in tendon and fascial tissue and are found adjacent to joints. They reduce friction or change the course of tendons. They may also change the angle of pull of muscles and thus give greater mechanical advantage. Ex. patella (kneecap) Flat Bones. Flat bones (ossa plana) expanded in two directions and are found wherever there is a need for protection of soft body parts or for a provision of extensive muscle attachment. They consist of two plates of compact substance: the lamina externa and the lamina interna, separated by spongy bone called diploe. Ex. ribs, scapula, parts of pelvic girdle and the bones of the skull. Irregular Bones. Irregular bones (ossa irregulata) have the same basic structure as short and flat bones; comprising however of peculiar and differing shape. They are located medially and include the vertebrae and some of the unpaired bones of the skull. Ex. the ossicles of the ear, bones of the vertebral column. Kinds of Bone Marrow Red Bone Marrow (medulla ossium rubra). The many spaces within the cancellous bone of the ribs, vertebrae, sternum, and pelvis are, in normal adults, filled with red bone marrow. This marrow, which is richly supplied with blood, consists mainly of blood cells and their precursors. Its primary function, referred to as hematopoiesis, is the formation of red and white blood cells and megakaryocytes, which fragment to form platelets (necessary for blood clotting). Yellow Bone Marrow (medulla ossium flava). Yellow bone marrow is a connective tissue that consists chiefly of fat cells and is found primarily in the shafts of long bones, within the marrow cavity. Divisions of the Skeletal System: Axial Skeleton. The axial skeleton is composed of bones found along the longitudinal axis of the body. The axial skeleton includes the: 1. Skull or Cranium – houses the brain and the sense organs. The skull has two set of bones: a. Cranial bones – encloses and protects the fragile brain tissue. It is composed of eight large bones including the paired parietal and temporal bones b. Facial bones – composed of 14 bones 2. Hyoid apparatus – a flat structure supporting the floor of the mouth 3. Bony thorax – protective cone shape cage of slender bones around organs of the thoracic cavity. It includes: a. Sternum – breastbone of vertebrates which result from the fusion of three bones: the manubrium, the body, and the xiphoid process b. Ribs – twelve pairs of the ribs form the walls of the thoracic cage 4. Vertebral column – backbone or spine a. Cervical vertebrae (7) – forms the neck region of the spine b. Thoracic vertebrae (12) – the spinous shape is long and hooks sharply downward c. Lumbar vertebrae (5) – massive, block-like bodies; sturdiest vertebrae d. Sacrum (5) – fusion of 5 vertebrae e. Coccyx – human tail bone

34

Appendicular Skeleton. The appendicular skeleton includes the bones of the limbs (appendages) and the pectoral and pelvic girdles w/c attach the limbs to the axial skeleton. The appendicular skeleton includes the:

1. Bones of the Thoracic Limb a. shoulder girdle – consists of the scapula (shoulder blade), and clavicle (collar bone) b. arm bone – humerus c. bones of the forearm – radius and ulna d. bones of the hand (manus) – consists of the wrist bones (carpals), bones of the palm Fig. 6.3. Bones of the hand.

(metacarpals), and bones of the fingers (phalanges) 2. Bones of the Pelvic Limb a. pelvic girdle – consists of the ilium, ischium, and pubis; V-shaped bone which articulates with the sacral bones and support the bones of the hind limb. b. thigh bone – femur; heaviest and strongest bone c. bones of the legs – tibia, fibula and patella (kneecap) d. bones of the foot (pes) – consists of the ankle bones (tarsals), bones of the foot (metatarsals), and bones of the toes (phalanges)

Fig. 6.4. Some of the 206 bones that make up the

The Vertebral Column. The vertebral column or human skeletal system. spine extends from the skull which it supports to the pelvis where it transmits the weight of the body. It is formed from 26 irregular bones. The cervical vertebrae form the neck region of the spine. It is composed of 7 vertebrae in mammals. The atlas and axis (the first and second cervical vertebrae, respectively) are different because they perform functions, (atlas has no body and provides a nodding motion; axis acts as pivot for the rotation of the atlas). The 12 thoracic vertebrae provide attachment for the ribs protecting the chest area. The 5 lumbar vertebrae are located at the small of the back. The sacrum is formed by the fusion of 5 vertebrae. It serves as an attachment to the bones of the posterior limb. The coccyx, or tail bone, is also made up of fused bones and forms the last bone of the vertebral column. Intervertebral discs made of cartilage cushion the vertebrae and absorb shocks.

35

The number of vertebrae in a given species is fairly constant in each region except the last, so that the vertebral formula may be expressed (for humans for example) as follows: C7T12L5S5Cy1-3.

Fig. 6.5. The vertebral column.

36