Birds have a totally different internal anatomy. Though they have some of the parts common to us their functioning and arrangement are completely different from us. When talking about the internal anatomy the first thing that comes into your body is about the muscles and the skeleton system. Both these parts are specially designed to help the bird to adapt to its flight.
Now lets examine the internal parts of the birds one by one.
Brain is the part of the central nervous system that encloses the cranium. The brain is divided into three main sections. They are hindbrain, midbrain and the forebrain. Medulla oblongata, spinal cord, and cerebellum is located in the hindbrain, and the mid brain contains the pendunles and cortex. The fore brain is divided into thalamencephalon and cerebral hemispheres. The weight of the brain depends upon the size and the species of the birds. It is with the help of this small brain birds surprise us with their learning capacity.
Like all the other living things in the world the spinal column of the bird runs throughout its body length and it consists of the spinal cord. Spinal cord is also the part of the nervous system and it also act as the messenger sending messages to the brain from all other parts of the body.
Trachea is the long tube that is found from the throat of the bird to the its lungs and it helps in the transport of fresh air helping the birds to breathe.
This narrow tube is used to transport the food swallowed by the bird to the crop where it is stored till the food gets digested.
The lungs serves the same purpose as in other human beings. Lungs carry the oxygen through out the body through the blood streams. In addition the birds have a air sacs which allow the air to flow through the lungs in only one direction.
Gizzard is used to grind the food and to pass it to the intestine. So it is a tough muscle that contains roughage that is useful for grinding the food.
The liquids that the birds intake are passed to the kidneys which then filter then the waste that has to be expelled later.
Birds have four chambered heart that pumps oxygen rich in blood content through out the body. Since birds are small creatures they have higher fast beats then any other animals in the earth. Some bird species have their heart beat to 500 beats per minute.
The main function of the liver is to eliminate any toxins from the body. So it acts like a large filter.
Ureter is a tube that is extended from the kidney. Its main function is to expel the liquid waste from the body. And rectum allows the solid waste to be expelled from the body of the bird.
Also see External Anatomy