(2) Classification by body tissue:
Cancers are identified according to the type of body tissue in which they originate. This leads us to divide them into two main groups: carcinomas and sarcomas.
• Carcinomas are cancer that starts in epithelial tissue which is the tissue that forms the skin and the linings of inner organs including (Skin, breast, digestive, reproductive
respiratory and urinary system).
• Sarcomas are the cancer that begin in connective tissues, which forms the body's supporting structures, e.g. bones and cartilage.

CAUSES OF CANCER
Before we know the causes of cancer we have to identify carcinogens.
Carcinogens are agents that cause or enhance cancer such as chemicals, certain forms of radiation and viruses. To develop cancer the people have prolonged (long time) contact with one or more carcinogens. Carcinogens attack normal cells and may eventually cause one of them to become cancerous which is the start of cancer with changes in a single cell. Then it passed on to all the cell's descendants.
The pathway to introduce carcinogens into the body is through the nose, mouth or some other opening and some carcinogens attack through the skin. The combination of two or more agents increases the risk of cancer.
How can scientists detect carcinogens ?
If scientists observe a particular population group which shows an unusually high cancer rate, experts look for substances in the group's environment that may cause the cancer then they test it in laboratory on animals.
If high percentage of the animals develops cancer then they suspect that the agent might also cause cancer in people.

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