Fascinating Animals

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Skeletal System – Skeletons, Joints & Bones – part one

human skeleton

Click on the “thumbnail” picture above to view it in larger size

Did you know there are 206 bones in the average human being?  Bones support and protect the various organs of the body, produce red and white blood cells and store minerals.  Bone tissue is a type of dense connective tissue. Bones have a complex internal and external structure making them lightweight, yet strong and hard.

The cranium or skull and some other groups of bones contain a number of duplicate bones.  You can find the numbers of each type of bone in the human body by scrolling down the following linked page. For example there are 26 individual vertebrae bones in the human being, and there are 24 ribs (or 12 pairs).   Ossicle bones are the bones of the ear.  The hyoid bone is the one and only bone of the throat.  It is a horseshoe-shaped bone that serves as an anchoring structure for the tongue.  The clavicle is called the collar-bone, and the scapula called the shoulder-blade.


The skeleton is an animal’s internal support frame. This bony structure helps to protect the organs and aids in movement. Some animals, including human beings, have an internal skeleton called an endoskeleton. Other animals have a skeleton on the outside of their body called an exoskeleton.

All vertebrates or animals with backbones (including humans) from the Phylum Chordata have endoskeletons or internal skeletons / internal skeletal structures, made up of bones and cartilage.  They have an axial skeleton or vertebral column made up of vertebrae (also called the back-bone) and the cranium or skull (which protects the brain), the facial bones, hyoid bone and ossicles (of the ear), and the bones of the thorax (sternum or breastplate and ribs); and an appendicular skeleton, or the rest of the skeleton. The skeleton protects vital organs, provides a support frame for the entire body form, and provides an attachment for muscles and ligaments, allowing movement of parts of the body, including the expansion or lifting up of  the ribs during breathing, to allow more air into the lungs.  The rib cage protects the heart and lungs.

During early embryonic development, the endoskeleton of vertebrates is composed of a notochord and cartilage. The notochord in most vertebrates is replaced by the vertebral column, and the cartilage is replaced by bone in most adults.

An endoskeleton may function purely for support (as in the case of sponges from the Phylum Porifera), but often serves as an attachment site for muscle and a mechanism for transmitting muscular forces, as well as a structure for giving shape, support and protection to the body.

Any internal support structure composed of mineralized tissue, contained within the skin, qualifies as an endoskeleton.  Some other animals besides those in the Phylum Chordata have got endoskeletons.  A spiny skeleton of calcium carbonate lies below the epidermis (outer protective layer of skin) of Echinoderms (star fish, sea urchins, sand dollars, brittle stars, sea cucumbers and sea lillies).  Because the definition of an endoskeleton is “a skeleton contained within the body”, animals from the Phylum Echinodermata are said to have endoskeletons.  The spines of the echinoderms are protuberances or projections from the endoskeleton, covered by skin (epidermis).  It is usually a great surprise to people learn that spines are actually internal structures.

Sponges from the Phylum Porifera also have endoskeletons.  The poriferan ‘skeleton’ consists of microscopic calcareous (calcium or calcium carbonate) or siliceous (glass like) spicules, and spongin, a protein that supports the spicules. Their endoskeleton gives shape, support and protection to the body and provides a mean of locomotion.

One of the types of tissue that makes up bone is mineralized osseous tissue. This tissue gives bone rigidity and a honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum, periosteum, nerves, blood vessels and cartilage.

There are 5 types of bone found within the human body. These are:

  • Long
  • Short
  • Flat
  • Irregular
  • Sesamoid

To see examples of these bone types, go to the page below.


A long bone consists of several sections:

  • Diaphysis.  This is the long central shaft
  • Epiphysis. Forms the larger rounded ends of long bones
  • Metaphysis.  The area where the diaphysis meets the epiphysis. It includes the epiphyseal line, a remnant of cartilage from growing bones.

Osseous tissue or Bone tissue comprises compact bone and spongy bone. Compact bone is a very dense tissue forming the outer layer of all bones and the thickened shafts or diaphyses of long bones.   Spongy bone forms the core of most flat and irregular bones. It is also found in the epiphyses (ends) of long bones.  The endosteum is a thin layer of connective tissue (made up of endosteal cells) and lines the marrow cavities and irregular surfaces of spongy bone.

The bone matrix consists of collagenous fibre bound together by a cementing substance. The inorganic (not containing hydrocarbons) components of the bone matrix make up 65% of the weight of bone and are chiefly calcium compounds.

Osseous tissue contains three types of cells.

These are associated with bone formation and are located in the periosteum, which is the dense membrane lining the surface of bones (except at the joints of long bones), that contains blood vessels and nerves which nourish the bone cells.  Periosteum is a connective tissue. Osteoblasts contain the enzyme alkaline phosphatase used to calcify the osseous matrix.

These are osteoblasts that have been enclosed within the bony matrix in a space called the lacuna.  Osteocytes are networked to each other via long cytoplasmic extensions that occupy tiny canals called canaliculi.  Although osteocytes have reduced synthetic activity and, like osteoblasts are not capable of mitotic division, they are actively involved in the routine turnover of bony matrix, i.e. they are involved with bone maintenance.

These are larger multi-nuclear cells found on the surfaces of osseous tissue, usually in shallow depressions called Howship’s Lacunae.  They are responsible for the dissolution and absorpton of bone.  Bone is a dynamic tissue that is continuously being broken down and restructured in response to such influences as structural stress and the body’s requirement for calcium. The osteoclasts are the mediators of the continuous destruction of bone.  The lacunae are thought to be caused by erosion of the bone by the osteoclasts’ enzymes. Osteoclasts are formed by the fusion of many cells derived from circulating monocytes in the blood . These monocytes (a type of white blood cell), in turn, are derived from the bone marrow.

Compact bone consists of cylindrical units called osteons. Each osteon contains concentric lamellae (layers) of hard, calcified bone matrix with osteocytes lodged in lacunae (spaces) between the lamellae.   Spongy bone or Cancellous bone consists of a complex pattern of “lattice-work” struts enclosing spaces.

compact and spongy_bone

Bone marrow is the tissue (myeloid tissue) found in the hollow interior of bones.  In adults, marrow in large bones produces new blood cells.  There are two types of bone marrow: red marrow and yellow marrow (consisting mainly of fat cells).  Both types of bone marrow contain numerous blood vessels and capillaries.

 Red marrow is found mainly in the flat bones such as hip bone, breast bone (sternum), skull, ribs, vertebrae and shoulder blades, and in the spongy bone at the ends of the femur and humerus.  Red blood cells, platelets and most white blood cells arise from red marrow.

inside a long bone

Yellow marrow is found in the hollow interior of the long bones (called the medullary cavity), as shown above.  The color of yellow marrow is due to the higher number of fat cells. Basically, these fat cells are the last resort for the body’s energy requirements and can be consumed in an event of extreme hunger. But, the important function that it performs in relation to formation of cellular elements is its ability to convert itself into red marrow in case of large volume blood losses which would deprive the body of oxygen carrying capacity in certain instances. The yellow marrow would be able to convert itself within 1 – 2 hours to take over the role of red marrow and this is one of the natural reserves to sustain life in extreme events.

Information about marrow is from an article by Dr Pandula Siribaddana, September, 2010

human vertebrae, vertebral column

Picture Link

Vertebrae – the Vertebral Column

There are 33 vertebrae found in an adult human’s backbone. Seven of them are in the cervical region, twelve in the thoracic, five in the lumbar, five in the sacral and four in the coccygeal region.  The vertebral column surrounds and protect the spinal cord, which is the human body’s nerve centre.

Cervical means neck.

Thoracic relates to the thorax or chest (i.e. the part of the trunk in humans and higher vertebrates between the neck and the abdomen, containing the cavity, enclosed by the ribs, sternum, and thoracic vertebrae, in which the heart and lungs are situated).

Lumbar refers to the abdomen or abdominal segment of the torso or trunk (the body apart from the limbs), between the diaphragm (at the bottom of the ribs) and the sacrum (pelvis).

The sacrum is a large, triangular bone, situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones;  its upper part or base articulates with the last lumbar vertebra, its apex with the coccyx, and consists of five segments that are fused together.

The coccyx is the tail-bone which consists of 4 small bones fused together at the bottom of the spine.

Note, that the sacral bones or sacrum are part of the vertebral column, while the pelvic bones (hip bones), shown below, are made up of the ilium, the pubis and the ischium fused together, and are not part of the vertebral column.   To find out the different number of vertebrae in different animals, you will need to “google” the answer (sorry, but I can’t find a single source listing the number of vertebrae in different animals, so you will need to research the information relevant to particular animals, besides human beings, yourself).  See also part 2 of this series for the cat skeleton including vertebrae.

sacrum bones

The hip bones or pelvic bones –  Picture Link


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