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Respiratory system
Lungs are the respiratory organs. The passage way for air is formed by external nostrils, pharynx, larynx, trachea and bronchi. Lungs contain large number of minute alveoli in which external respiration takes place. Surrounding the lungs and the heart is a thoracic cage containing long, curved bones called ribs. Below the lungs is a diaphragm which divides the trunk into an upper thorax and a lower abdomen. The ribs and the diaphragm play a major role in the breathing movements inspiration and expiration.
Excretory system
The excretory system is represented by the kidneys, the ureters and the urinary bladder. Kidneys are composed of a large number of functional units called nephrons. Kidneys take up the function of excretion and osmoregulation. The excretory system is closely associated with the reproductive system to form the urinogenital system.Nervous system
The nervous system consists of a central nervous system, a peripheral nervous system and an autonomic nervous system.The central nervous system consists of a brain and a spinal cord. The peripheral nervous system consists of 12 pairs of cranial nerves and the 31 pairs of spinal nerves. The autonomic nervous system consists sympathetic and parasympathetic nerves.
In the brain cerebral hemispheres are highly enlarged and contains many convolutions.Reproduction and development
In rats, as in all vertebrates, sexes are separate. Male and female exhibit several differences in their external features.The Male reproductive system consists of a pair of testis that are enclosed in scrotal sacs, a pair of vas deferens, a seminal vesicle, ejaculatory duct and penis. The female reproductive system consists of a pair of ovaries, a pair of fallopian tubes, a uterus, and a vagina. (or birth canal). Fertilisation and development are internal.
Fertilisation takes place in the fallopian tube. The zygote undergoes an intitial development and then reaches the limb (uterus). The attachment of the embryo to the uterine wall is called implantation. The entire duration of development inside the uterus, is called gestation period. After completing development, the young one is born. Hence rat is described as a viviparous animal.
