The Dog

Domestic dog, carnivorous mammal, generally considered the first domesticated
animal. The domesticated dog has coexisted with human beings as a working
partner and household pet in all eras and cultures since the days of the cave
dwellers. It is generally believed that the direct ancestor of the domestic dog
is the wolf, originally found throughout Europe, Asia, and North America.
Remains of a dog, estimated to be 10,500 years old, have been found in Idaho.

TAXONOMY Kingdom: Animalia Phylum: Chordata Class: Mammalia Order: Carnivora
Family: Canidae Genus: Canis Species: Canis familiaris


Little is known about wild dogs of the past but that they are carnivores:
hunters and scavengers. This means that they are secondary consumers in web
chains. Eventhough they are carnivores they sometimes accept eating green plants.
The ecology of dogs right know is that it helps the human in many fields of life.
Since the cave dweller times, dogs have been domesticated by humans and it has
helped him to hunt, in herding, protection, etc. It has been very important as a
work animal and as a psychological support for humans. The habitat of the dog is
where itís owner lives. Different dogs have different adaptations to their
ancestral habitat but nowadays, this is not applicable.



The skeleton of the dog is the articulated structure, moved by the muscles, that
holds the dogís body and protects some organs and the nervous system. It also
functions as mineral and blood deposit of the body. The skeleton of a dog is
made up of approximately 321 bones: 134 form the axial skeleton (skull,
vertebrae, ribs, etc.), and 186 form the appendicular skeleton (appendages). An
extra bone has to be added for male dogs which is the penile bone. The dog is a
digigraded animal (it walks with itís toes). It lies on itís third phalanges
which are protected by palm cushions. The dogís toes are arranged in an angle
which gives more facility of rest after running or other activities. The teeth
of the dog is composed of 42 teeth which include canines, molars, incisors, etc.


Joints permit the movement of the bones. There are three types of joints in a
dog: fixed joints, movable joints, and semi-movable joints. Fixed joints, such
as the ones in the skull donít permit any movement but keep the bones together.
The semi-movable joints are those that permit a little movement. They are
represented in the spinal column. The movable joints are those present in the
rest of the bones. Inside of this group of joints there are various types: the
hinge, the ball-and-socket, the pivot, and the gliding joints. The most movable
joints are present in the appendages. Joints are held together by a fibrous
wrapping, the joint capsule, which reinforced by ligaments. Muscles and tendons
also help keeping the bones together.


There are three types of muscles in a dog: the skeletal muscle, cardiac muscle,
and smooth muscle. The skeletal muscle works in pairs, a flexor and an extensor.
It permits the movement of the skeleton and it also moves the skin of the dog
(cutaneal muscle is very developed in dogs). The cardiac muscle is the muscle
which is exclusively in the heart. The smooth muscle is the one present in the
walls of the digestive organs, arteries, veins, in the diaphragm (which
separates the two cavities of the body: the thorax and abdomen), and in some
other internal organs.


The digestive system of a dog is very similar to a human one. It ensures the
ingestion of food and itís transformation (by mechanical and chemical acts) to
simple substances which the dogís body can absorb and assimilate. It all starts
in the mouth, where food is broken down mechanically and also a little
chemically (saliva and teeth). Then they pass through the esophagus to the
stomach to the small intestine (only 3 meters long but it has a very strong
digestion) and to the large intestine where the feces are made. Then excretion
is made through the rectum and then through the anus. A series of added glands
produce substances which are used in digestion and they perform various
important jobs. The most important one is the liver (which is an organ).


In the respiratory system, air goes through the nostrils in the snout. In the
nasal cavity air is purified, moistened and warmed. Then it passes through
pharynx, larynx (where vocal cords for barking are), and then to the trachea.
Air is then canalized through the two bronchi and then inside the lung by the
bronchioles which are subdivisions of the bronchi. The last