“Medical education… calls for the possession of certain portions of many sciences arranged and organised with a distinct practical purpose in view.” - Abraham Flexner -
Tumours
What is a tumour? The first important observation in answering this question came from another great English clinician, the surgeon Sir Percivall Pott. In 1775 Pott made the following famous observation about chimney sweeps: This ‘noisome, painful and fatal disease’ was scrotal cancer – and this statement is a landmark in epidemiology and occupational medicine for it contains the first realisation that tumours are not solely random events. Researchers listened, and began to focus on what tumours were, and what it was about the chimney sweeps that predisposed them to scrotal cancer. Of particular confusion, chimney sweeps in continental Europe did not seem to suffer such a high prevalence of the disease.
This dilemma was not solved until 1915, when Japanese researcher Katsusaburo Yamagiwa and his assistant, Koichi Ichikawa, provided a fundamental scientific insight into the nature of cancer.  Hypothesising that it was the chemical irritation from the coal tars that caused the cancers observed by Pott, Yamagiwa and Ichikawa painted tar on the ears of rabbits.
“The fate of these people seems singularly hard; in their early infancy they are most frequently treated with great brutality, and almost starved with cold and hunger; they are thrust up narrow, and sometimes hot chimnies, where they are bruised, burned, and almost suffocated; and when they get to puberty, become particularly liable to a most noisome, painful, and fatal disease.”
To their amazement, after some time of tar exposure, the rabbits did indeed develop cancer. This was an impressive finding, but Yamagiwa and Ichikawa’s next experiment was the crucial one. They again painted on the coal tar, but this time they removed it after a few repeated exposures and left the rabbits alone. Unfortunately for the rabbits, they still developed cancer. For the first time, Yamagiwa and Ichikawa had discovered a ‘cause’ for a type of cancer. They also proved that the 18th century British chimney sweeps were dirtier than their European counterparts. Celebrating his achievement, Yamagiwa composed the following Haiku: "Cancer was produced. Proudly I walk A few steps."
So with this work understood, let us now attempt to define a tumour:
A tumour is an abnormal mass of tissue, the growth of which exceeds and is uncoordinated with that of the normal tissues, and persists in the same excessive manner after cessation of the stimuli which evoked the change.
This definition fulfils the ‘laws of a definition’ (described later) In the sieve tumours are further classified as benign or malignant according to their behaviour. Benign tumours do not invade other tissues, but grow locally and generally slowly.
Malignant tumours, however, often grow rapidly and infiltrate the normal tissues, first locally, and then possibly to distant sites via invasion of blood vessels and lymphatics.  There they may form ‘secondary’ deposits.
 
Sir John Percivall Pott (1714-1788). Pott originally planned to study for the clergy but fell under the influence of the notable surgeon Edward Nourse, under whom he instead trained as a surgeon. He described Pott’s Fracture (both bones just above the ankle) after sustaining a leg injury upon falling from a horse on his way to visit an ill patient. Demonstrating his ingenuity, Pott purchased a door from a nearby house to act as a stretcher and summoned two chair-bearers to carry him to hospital. During his long recovery Pott began to record his surgical experiences and subsequently rose to become one of the most influential surgeons in England. He was described as ‘an extremely pleasant man’ and attracted many students. He died in 1788 of pneumonia. His last words were, “My lamp is almost extinguished: I hope it has burned for the benefit of others”.