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    
 `	`	 A tumour is a mass of new tissue growth that does not
react to normal controls or the organizing influence of other
tissues, and it has no useful function in the body.  This applies
to both types of tumours, malignant and benign.  Malignant, also
known as, cancerous tumours, are additionally defined by their
invasion of local tissue and their ability to spread to other parts
of the body.
 
Benign Tumours
 `	`	 A benign tumour which is not cancerous, is less serious
than malignant tumours because they do not spread to other parts of
the body, but they may cause damage by local growth and pressure on
other structures, producing serious complications such as bleeding. 
Benign tumours generally grow slowly and kill the host only if it
occupies or attaches to an organ so as to interfere with a critical
function.  The cells of benign tumours closely resemble the cells
of the tissue of origin.  Surface benign tumours include warts and
moles.

Malignant Tumours
 `	`	 A malignant tumour always kills (unless treated) because
of its invasive and metastatic characteristics.  The tumour grows
locally by spreading into surrounding tissues.  Solid tumours,
which develop in the breast, colon, lung, and other organs, contain
an inner core with high pressure zones that compress and collapse
blood vessels, often preventing the penetration of bloodborne    h)        0*0*0*  ԌH'   2  
cancer treatments.  It spreads to distant sites by the breaking off
of malignant cells, which move through the blood and lymphatic
systems, attach themselves, and begin to grow as new colonies. 
Malignant tumours are diagnosed by examination of their
vascularity, shapes, forms of cells division, and differentiation. 
More than a hundred different types have been identified in humans. 
In general, those derived from epithelial tissue are carcinomas,
and those from connective tissue are sarcomas.
 `	`	 The most common form of malignant tumour of the
respiratory tract is lung cancer, which began increasing in
frequency at an alarming rate about 1940.  In 1980 it was the
leading cause of cancer deaths in men and is also rapidly
increasing in woman.  It is attributed to cigarette smoking and
environmental pollution:  cancer of the lung is rare in nonsmokers,
and exposure to materials such as asbestos, chromium, and
radioactive substances increases the probability of developing lung
cancer.  
 `	`	 Malignant tumours, also known as lymphomas, one of the
main types of lung cancer, arise in the lymph nodes related to the
lungs and other body tissue.  the other main type of lung cancer is
Sarcomas, it may originate in the lungs or in some other structure
such as a bone.  Sarcomas have a poor prognosis, but recent
advances in the treatment of lymphomas have increased the long term
survival.
 `	`	 The factors controlling tumour growth are poorly
understood, although genetics seems to play a role  Tumours in    (        0*0*0*  ԌH'   3  
laboratory animals may be transplanted to a second host using only
a single tumour cell.  This suggests that only one normal cell
needs to become cancerous for tumour growth to begin.  Tumours have
been experimentally induced in animals by chemical, physical, and
viral agents, and by radiation.

 `	`	 Cancer researchers no longer believe that a single drug
will be able to cure cancer.  Experts now believe that a
combination of drugs will be the best method to kill tumours.  One
route being researched utilizes antibodies that bind to specific
receptors on the tumour cell, thereby inhibiting tumour growth by
blocking certain tumourgrowth factors from entering the cell. 
Tumour necrosis factor (TFN), an immunesystem protein, has been
found effective in cutting off the blood supply to tumours,
although too much TFN causes severe side effects.
 `	`	 The work of physician Francis Peyton Rous, gave rise to
the virus theory of the causes of cancer.  In 1960, Rous found that
he could transmit a cancerous tumour (sarcoma) from one hen to
another by using an injection of tumour filtrate.  The sarcoma
virus was the first tumour virus identified, and it opened up a
whole new area of cancer research.  Rous shared the 1966 Nobel
Prize for physiology or medicine for his work.
 `	`	 