Mammalian Skeleton

Mammals are highly advanced vertebrate animals and have complex skeletons. The mammalian skeleton is important for protecting vital organs (e.g. heart, liver and brain) and for providing structural strength so mammals are able to grow into the largest and strongest animals on Earth.

The mammalian skeleton is split into the axial and appendicular skeletons. The axial skeleton consists of the skull, spinal column and rib cage and the appendicular includes the bones of the arms, legs, hands, feet, pelvis and shoulder.

Skull

The skull of humans includes a total of 29 bones: 8 cranial bones which make up the rounded top of the skull and protect the brain, 14 facial bones that help to protect the mouth and nose, 6 auditory bones which help mammals to hear sounds and 1 hyoid bone that supports the tongue.

Human skullThe mammalian skull has orbital complexes which are the large circular area that surrounds the eyes. Each orbital complex is formed from both cranial and facial bones. The maxillary bone, commonly known as the cheekbone, fuses with a bone on the side of the skull and forms an arch called the zygomatic arch. Chewing by a jaw is controlled by muscles attached between the jaw bone (or mandible) and the zygomatic arch.

There are a number of other features of the skull that have important functions. A number of holes in bones allow blood vessels and nerves to enter and exit the skull. The occipital condyles are spots on the underside of the skull where the skull connects with the spine.

The skull can be used to determine the sex of a human because there are a number of differences between the male and female skulls. The chin, cranium, brow ridges and mandible are all larger in male skulls.Elephant skull

Skull shape can differ significantly between different mammal species and eating habits play a large role in the structure of the skull for many mammals. Herbivorous mammals will often have a large gap between their fore and hind teeth to store food while they are eating. Carnivores tend to have large sites along the top of their cranium for large jaw muscles to attach to and give carnivores the strength to chew through tough materials such as bones.

Spinal Column

There are roughly 26 bones along the spinal column that are correctly known as vertebrae. The vertebrate can be split into five distinct regions of the spine: the cervical, thoracic, lumber, sacral and caudal regions. The main function of the spine is to protect the spinal cord which controls the communication between the body and the brain. It also functions as an attachment point for the appendicular skeleton and provides mammals with support against gravity.

The cervical vertebrae are the first seven bones of the spinal column and make up the neck of mammals. The top two cervical vertebrate are the atlas and axis which, together with the skull, allows mammals to twist their heads.

Spinal columnBelow the cervical vertebrae are the thoracic vertebrae. These bones each attach to one row of ribs from the rib cage. The thoracic region of the spinal column is often much larger in large four-legged mammals, such as horses and elephants because their stomachs are far too heavy to be supported by the muscles of their abdomen so their rib cage extends further down body.

The lumber vertebrae sit below the thoracic vertebrae and for two-legged mammals, such as humans, they are the main weight-bearing bones of the spinal column. Beneath the lumbar vertebrae is the sacrum, a fusion of five vertebrae that joins to the pelvic bones.

Finally the caudal vertebrae make up the bones of the tail, a section of the spinal column with great variation amongst mammalian species. In humans the main function of the caudal vertebrae is to support the contents of the our abdomens.

rib cage

The rib cage or thoracic cage is a collection of long bones that connect to the thoracic vertebrae and sternum. The main function of the rib cage in mammalian skeleton is to protect the heart and lungs. In larger mammals it also plays a big role in supporting the abdomen.

In humans we have a total of twelve ribs. The first seven attach directly to both the spinal column and the sternum; ribs 8, 9 and 10 attach indirectly to the sternum via cartilage; and ribs 11 and 12 are only attached to spinal column but not to the sternum.

Shoulder and collar-bone

Pectoral girdle - shoulder, shoulder blade and collarboneThe pectoral girdle is the collection of bones that make up the shoulder and include the collar-bone and shoulder blade (or clavicle and scapula). Humans have a relatively flexible pectoral girdle compared to many other mammalian skeletons which allows us to have a large range of movement in our arms. Four legged mammals have rigid, strong pectoral girdles which helps them to run faster.

Pelvis

The pelvis or pelvic girdle is made from three fused bones: the ilium, ischium and pubis bones. It is the connection point for the leg and sacrum of the spinal column. In humans the pelvis is much shorter and wider than other mammals because it holds organs such as our stomachs. In four legged mammals the pelvis is longer to give more area for muscles to attach to. The pelvis is also a useful bone in determining the sex of a human skeleton. The female pelvis in wider to accommodate childbirth and the shape of the pubis is much flatter.

Arms and legs

The arm of a mammalian skeleton is made up of three bones: the humerus, ulna and radius. The humerus fits into the pectoral girdle with a ball and socket joint. The ulna and radius connect to the bones of the wrist and humerus to make the elbow. The arrangement of the radius and ulna in humans allows the rotation of the hand from the elbow, a skill absent in four legged mammals.

Mammalian skeletonThe hand is made up from the carpel bones in the wrist, five metacarpals which make the palm phalanges in fingers and thumbs. Humans and other high order primates have a saddle joint between the metacarpal and carpel of the thumb which gives their thumbs a very large range of movements and creates the opposable thumb.

The leg also consists of three long bones very similar to that of the arm but in humans we also have a fourth bone, the kneecap or patella. The patella helps to lock the knee to save energy and prevent over-extension of the knee-joint.

The femur is the longest bone in the human body and is similar in length to the humerus in many four legged mammals. It connects to a socket in the pelvic girdle, similar to the humerus in the shoulder joint. The lower end of the femur connects to the tibia and fibula bones which extend from the knee to the foot.

The tibia is the major bone of the lower leg and provides the majority of strength; the fibula runs down the outside of the tibia and extends lower than the tibia to connect to the side of the foot. In horses the fibula is partially fused with the tibia.

The foot is very similar in structure to the hand. It has a number of tarsal bones that form the ankle, equivalent to the carpel bones of the wrist. The tarsals connect to five metatarsals which extend along the length of the foot to the toes. The bones of the toes, phalanges, are the same as the finger bones but are much shorter in humans and other primates.


Summary

  • The mammalian skeleton can be separated into the axial skeleton and the appendicular skeleton.
  • The axial skeleton includes the bones of the head, spinal column and rib cage.
  • The appendicular skeleton includes the bones of the arms, legs, pelvis and shoulders.
  • Skulls contain around 29 bones that make the cranium, face and ears of mammals.
  • The spinal column is separated into five regions – the cervical, thoracic, lumber, sacral and caudal regions.
  • The rib cage of the mammalian skeleton protects the heart and lungs and in larger mammals it supports the abdomen.
  • The pelvis connects the spine to the legs and can be used to identify the sex of a mammal.
  • Arms include the humerus, radius and ulna bones plus the bones of the wrist and hand.
  • The leg bones are the femur, tibia and fibula plus the tarsels, metatarsels and phalanges of the ankle and foot.

Last edited: 14 December 2015

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