What is a vertebrate?

Vertebrates are those animals with a bony column of vertebrae (the spine) supporting the

body and head and includes the classes of fish, amphibians, reptiles, birds and mammals

e.g. fish, amphibians, reptiles, birds, mammals.

Our discussion will primarily be related to mammals but there is some extra information

related to bird anatomy in the appendices.

The skeleton is the supporting framework of the body comprised of hard, rigid connective

tissue structures called bones.

The functions of bones are:

Provide support for the body

Provide attachments for muscles to allow for movement or locomotion

Provide a store of minerals such as calcium

Production of blood cells in marrow of some bones

Figure 2.1. A typical vertebrate, a dog, showing skull attached to spinal

column, thoracic girdle and forelimb, and pelvic girdle and hind limb.

Image C Meyer.

Types of bones

Bones are sometimes classified into groups depending on their structure or shape. Categories

based on shape include:

Long bones

Short bones

Irregular bones

Flat bones

Long bones are those longer than they are wide and many of the bones of the limb, e.g.

femur, humerus, metacarpal bones.

Short bones are usually of approximately the same length, width and thickness, examples

include bones of the carpus (‘wrist’ in humans) and of the tarsus (‘ankle’ of people).

Irregular bones include bones such as the vertebrae which possess irregular projections for

muscle attachments.

Flat bones include those of the skull and the ribs and often serve a protective surface.

Other bones may be classified according to their location, e.g.

Sesamoid bones: these are bones that are situated within tendons

Visceral bones: these bones sit within organs, an example is the os penis of the dog.

This cylindrical shaped bone sits within the penis to provide extra rigidity to assist in

mating. (Not all animals have an os penis)

Structure of bones

The typical long bone has a shaft called the diaphysis and the ends of the bones are called the

epiphysis. The epiphyses normally flare out to help form the joints. The bones are partly

hollow and contain the marrow. The central aspect of the diaphysis contains yellow marrow,

a reserve of fat cells. The central part of the epiphyses contains sponge-like bone which

contains red marrow, the site of blood cell production. The hard outer bony layer is called

compact or cortical bone ( or the cortex).

The bone receives its blood supply from blood vessels that penetrate through a small hole in

the cortical bone called the nutrient foramen.

Bone growth in immature animals occurs near the ends of the bone from a cartilage structure

called the growth plate. When the animal reaches maturity this growth plate becomes bony

and growth no longer occurs; this layer of hard bone in the epiphysis may be seen on

radiographs as a line.

The epiphyses are normally covered by a thin layer of cartilage called hyaline or articular

cartilage. This forms part of the joint. The remainder of the bone is covered by a very thin

layer of nerve reach connective tissue called the periosteum.

Most bones have bony projections to assist with the attachments tendons and ligaments.

Tendons are the attachments of muscle to bone, whereas ligaments attach bone to bone. Both

tendons and ligaments are composed of very dense connective tissue called collagen.

or growth plate

Figure 2.2. A typical long bone in cross section. Image US Fed Govt.

We shall examine the different parts of the skeleton in turn commencing at the skull.

The skull

The skull is a bony case enclosing the brain, and other important structures assisting the

animal with some of the essential senses e.g. hearing, sense of smell, and vision.

The skull is actually comprised of many bones which become fused together when the animal

reaches adulthood. The bones are not fused in juveniles so as to accommodate for growth of

the skull.

The skull may be divided into three main parts:

Cranium: This is the bony case enclosing brain, the orbits are located at the front of

the cranium and house the eyes.

Mandible: This is the lower jaw

Maxilla: the upper jaw

Figure 2.2. Dog skull showing

lower jaw mandible, upper jaw

maxilla, and the cranium.

Holes in the skull are places

where nerves and other

structures run. Image Ainal

Maxilla

Cranium

Mandible

The maxilla houses the upper teeth and also the nasal cavity. The nasal cavity is divided from

the mouth by a horizontal flat bony structure called the hard palate. The nasal cavity

contains scrolls or rolls of very delicate bone called the nasal turbinates. These bones

increase the surface area over which air flows when entering the body and will be discussed

in more detail in later chapters.

Within the skull there are other chambers that communicate with the nasal cavity. These are

called sinuses and assist in warming and humidifying (making it moist) air as it is inhaled.

The vertebral column or

Figure 2.3. The hard palate as seen

from below, divides the mouth from

nasal cavity. Image GaylaLin

Figure 2.4. Section of a dog skull

demonstrating the proximity of the

sinuses to the nasal cavity. The hard

palate can be seen dividing the

mouth from the nasal cavity. Image

Ruth Lawson, Otago Polytechnic

The Spine or Vertebral Column

Irregularly shaped bones called vertebrae joined together to form a flexible column along the

length of the animal. The mass of nerves called the spinal cord is protected by sitting within

the vertebra in the spinal canal.

Each vertebra is an irregularly shaped bone with projections or processes which aid in

attachments of muscles and ligaments.

The spine may be divided into separate sections based on the anatomical location and shape

of the vertebrae. These sections, starting closest to the skull are:

Cervical spine (neck region)

Thoracic spine(chest region, have associated ribs)

Lumbar spine (lower back area)

Sacrum (fused vertebrae next to pelvis)

Coccygeal vertebrae (tail vertebrae)

The number of bones in each of these segments may vary from species to species. Dogs for

example have 13 thoracic vertebrae whilst horses normally have 18 thoracic vertebrae. The

vertebrae vary in their appearance in these regions depending on the amount of muscle

attached to the various bony projections or processes.

Processes

Vertebral body

Vertebral canal

Figure 2.5. A lumbar vertebra, note

the central hole the vertebral canal

where the spinal cord is located.

Image: Gray’s Anatomy

The individual vertebrae are assigned numbers according to their positions. The cervical

vertebrae are assigned a prefix C then the order in which it is found. For instance the first

cervical vertebra is termed C1, the next C2 and so on until the first thoracic vertebra which is

termed T1, lumbar vertebrae are given the prefix L.

The first two cervical vertebrae are sometimes given their own specific terms, i.e. C1 is called

the Atlas, and C2 the Axis.

The Ribs

The ribs form a bony cage protecting the lungs and heart and move to assist breathing. The

ribs are attached dorsally by a joint to the thoracic vertebrae, ventrally they are attached to a

bony plate called the sternum.

The Thoracic and Pelvic Girdles

These structures support the limbs; the scapula or ‘shoulder blade’ comprises the thoracic

girdle, and the bones of the pelvis comprise the pelvic girdle. The limbs attach to these

structures and they act as shock absorbers when the animal runs or jumps.

Coccygeal

Lumbar

Thoracic Cervical

Sacral (between wings of the pelvis)

Figure 2.6. Different parts of the vertebral column. Image adapted from C

Meyer

Lumbar

Sacrum

Sacrum(between

pelvic bones)

The scapula or shoulder blade is a wide flattened bone sitting laterally to the ribs. There is

one scapula on either side. The bone is wide and flattened to facilitate muscular attachment.

In some animal such as the cat the scapula is very mobile.

The pelvis is bony box attached to the spine via the sacrum. It is actually comprised of a

number of bones that fuse together when the animal stops growing.

Figure 2.7. The scapula makes up the thoracic girdle and bones of

the pelvis the pelvic girdle. Image from C Meyer

Scapula Pelvis

Figure 2.8. Lateral view of the scapula of the dog

Figure 2.9 View of the pelvis of a

dog.

Bones of the forelimb

The bones comprising the forelimb include:

Humerus

The radius and ulna

The carpal bones

Metacarpal bones and

Phalanges or toes

All mammals possess these bones in their forelimb, but the number and shape of some may

vary.

Humerus: this is the bone of the upper aspect of the forelimb and proximally adjoins with the

scapula, distally it adjoins the radius and ulna bones to form the elbow.

Radius and ulna: these two bones are positioned next to each other, in humans this

represents the forearm. Distally the radius and ulna adjoin the bones of the carpus (carpal

bones- wrist in people, called the knee in horses)

Carpal bones: these are short bones positioned to form the carpal joint, distally they adjoin

the metacarpal.

Figure 2.10. Bones of the forelimb

in a dog. Image Ruth Lawson, Otago

Polytechnic

Metacarpal bones: These are long bones, in the dog there are four main bones with

sometimes a fifth making up the dewclaw. In horses most of the metacarpal bones are no

longer present and the main remaining bone is very elongated.

Phalanges: These are the bones of the toes or digits. There are 3 bones for each digit. In

some animals there may only be one toe, e.g. the horse, others such as cattle and sheep have

two toes, while dogs and cats have 4 main toes and sometimes a smaller toe on the medial

side of the foot called a dew claw.

Bones of the hind limb

The bones comprising the hind limb include:

Femur

The tibia and fibula

The tarsal bones

Metatarsal bones and

Phalanges or toes

All mammals possess these bones in their hind limb, but the number and shape of some may

vary.

Femur: this is the bone of the upper aspect of the hind limb and proximally adjoins with the

pelvis to form the hip, distally it adjoins the tibia and fibula bones to form the knee or stifle.

Tibia and fibula: these two bones are positioned next to each other, in humans this

represents the shin. The fibula is located laterally and is much smaller than the tibia. Distally

the tibia and fibula adjoin the bones of the tarsus (tarsal bones- ankle in people).

Tarsal bones: these are short bones positioned to form the tarsus joint or the hock, distally

they adjoin the metatarsus.

Metatarsal bones: metatarsal bones are the hind limb equivalent of the metacarpal bones.

Figure 2.12. Lateral (side-on) view of the foot of the hind limb on a dog. Image: M Lawley

Phalanges: See phalanges section in forelimb.

Joints of the body

Junctions between bones are called joints. Some of these are highly movable whereas others

may have minimal movement.

Joints of the body may be classified as follows:

Fibrous joint (immovable) e.g. teeth in sockets, joints between bones of the cranium

Cartilaginous joint are partly movable joints, examples include joints between

vertebrae

Synovial joints highly movable joints, examples include elbow, wrist, and knee

joints.

Femur

Pelvis

Tarsus/hock

Fibula

Metatarsal bones

Phalanges

Tibia