Fascinating Animals

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Skeletal System – Skeletons, Joints & Bones – part two



A joint is the point where two or more bones meet. There are three main types of joints – Fibrous (immoveable & held together by a ligament), cartilaginous (partly moveable, held together by cartilage) and the Synovial (freely moveable) joint. Synovial joints consist of a synovial capsule (collagenous structure) surrounding the entire joint, a synovial membrane (the inner layer of the capsule) which secretes synovial fluid (a lubricating liquid) and cartilage known as hyaline cartilage (a flexible connective tissue) which pads the ends of the articulating bones.  An example of a Fibrous joint is where the teeth are attached to their bony sockets. The vertebrae have cartilaginous joints between them.

Examples of Synovial Joints follow.

A hinge joint allows movement in a certain spot to take place. This joint is similar to the opening and closing of a door. Some examples of hinge joints are the elbow, knee, ankle and joints between the fingers. Hinge joints allow the body parts to bend and straighten.

Ball and socket joints allow twisting and turning movements. In a ball and socket joint, one of the bones has a rounded head which is the ball. The other bone has a cup-like area that is known as the socket. Some of these joints are the shoulder and the hip. The shoulder joint is the most flexible joint in the entire body. It allows movement in any direction.

There are other types of joints in the body. Gliding joints allow two flat bones to slide over each other like in the bones of the foot and wrist. A condyloid joint allows the head to nod and the fingers to bend. The thumbs have a saddle joint that allows enough flexibility for the thumb to touch any other finger.

Some invertebrate animals (animals without backbones), such as insects (from the Class Insecta) and crustaceans (from the Class Crustacea) have hard rigid protective structures on the outside of their skins, which are called exoskeletons (or shells). These groups both belong to the Phylum Arthropoda. The Phylum Arthropoda (arthro = joint; poda = foot) is the most numerous phylum of all living organisms, both in number of species and in number of individuals.  An arthropod has a segmented body covered by an exoskeleton made from chitin and other chemicals. This exoskeleton serves as protection and provides places for muscle attachment. Arthropods must molt because their exoskeletons don’t grow with them.

An important advantage of an endoskeleton over an exoskeleton is that the endoskeleton provides more structural support for a greater number of organs and muscles, and their growth, so the physical form (including size) and functioning of animals with endoskeletons is more complex than that of animals without an endoskeleton.

Below are some pictures showing the endoskeletons of other animals.

cat skeleton

Source: Feline Anatomy Pictures

Cats have an extra lumbar vertebra and two extra thoracic vertebrae, and caudal vertebrae in the tail, which give them enhanced spinal mobility and flexibility, compared to humans. Cats also have free-floating clavicle bones (collar bones) which allow them to pass their body through any space into which they can fit their heads. The vestigial (trace or small) clavicle is not attached to the shoulder joint (as in humans), so “floats” in muscle.  This allows the cat to squeeze through narrow openings and thread speedily through narrow passages, by placing one forelimb directly in front of the other.

The feline skeleton evolved for a lifestyle of speed and agility. A cat’s slender but robust legs support a narrow ribcage and a highly supple spine (backbone or vertebral column).  The scapula (shoulder blade) is connected to the spine (vertebral column) by muscles and ligaments, rather than,  as in the case of human beings, being connected to the clavicle and the sternum (breast-plate) by gliding joints.   This structure allows the cat more flexibility of movement of its fore-limbs.

The hard structure of the skeleton protects the internal organs, provides points of attachment for muscles, and acts as a system of levers and joints necessary for fluid movement. The thick cartilagenous discs between the spinal vertebrae are looser and more supple than similar joints in other species, providing a greater degree of flexibility in the torso.

You can see the general composition of the vetebral column, including the numbers of vertebrae in different regions of the human vertebral column, at Part 1 of this series, by clicking here,  if you like.

horse skeleton

Source: http://www.rainbowresource.com/prodlist.php?subject=11&category=9306

The horse skeleton is composed of approximately 210 individual bones, excluding those in the tail. The broad flat surface of the scapula or shoulder-blade and the formation of the lumbar vertebrae provide ample space for the attachment of powerful muscles required to move the horse’s fore and hind limbs. What would be the third metacarpal in our hand (the bone from the wrist to the knuckle of the middle finger) is called the cannon bone in a horse. The “middle finger” starts at the fetlock, and the fingertip (equivalent to the human distal phalange) is encased in a hoof.

Click here for a labelled picture of a horse skeleton. Click here to see the bones of a horse’s hoof, as compared to bones in the hand of a human being (below).

bones of the hand


For human foot bone anatomy, please click on this Health fitness site here.

You can download a PDF document fom the Oxford University Museum of Natural History showing different animal skeletons by clicking here.

Last, but not least, is a picture of the endoskeleton of a Humpback Whale.

Humpback Whale Skeleton

Source of picture: Museum of Osteology

Did you know that “Osteology” is the scientific study of bones?

Other sources of information for this 2 part series follow.











Some good book references on the internet about the anatomy of cats (feline anatomy), including the cat skeleton, are listed below:

Outwitting Cats

Animal Anatomy for Artists


Click  here  to go to Part One

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