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.
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.
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
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.
or growth plate
Figure 2.2. A typical long bone in cross section. Image US Fed Govt.
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.
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
Cranium
Maxilla
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
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.
Figure 2.3. The hard palate as seen
from below, divides the mouth from
nasal cavity. Image GaylaLin
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.
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
Ruthvertebral
Lawson, Otago
Polytechnic
The
column
or
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.
Vertebral canal
Processes
Figure 2.5. A lumbar vertebra, note
the central hole the vertebral canal
where the spinal cord is located.
Image: Gray’s Anatomy
Vertebral body
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.
Cervical
Thoracic
Lumbar
Sacrum(between
pelvic bones)
Coccygeal
Figure 2.6. Different parts of the vertebral column. Image adapted from C
Meyer
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.
Scapula
Pelvis
Figure 2.7. The scapula makes up the thoracic girdle and bones of
the pelvis the pelvic girdle. Image from C Meyer
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.
Figure 2.8. Lateral view of the scapula of the dog
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 2.9 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.
Figure 2.10. Bones of the forelimb
in a dog. Image Ruth Lawson, Otago
Polytechnic
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.
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.
Figure 2.12. Lateral (side-on) view of the foot of the hind limb on a dog. Image: M Lawley
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.
Phalanges: See phalanges section in forelimb.
Pelvis
Femur
Tibia
Fibula
Tarsus/hock
Metatarsal bones
Phalanges
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.