The Experimental Study of Cancer

NOWADAYS cancer research has many ramifications. These may be summarized as clinical and radiological, pathological, statistical, biochemical, and experimental. This short paper is concerned only with the more important aspects of the experimental study of the disease. The essential nature of cancer is so obscure that it is not possible to estimate the several parts which intrinsic and extrinsic factors play in its origin. Probably both these factors are involved to some degree in every case. Generally speaking, the extrinsic factors are more amenable to experimental investigation, and considerable progress has been made in their elucidation during the last twenty years. Nevertheless , opinion is still divided with regard to certain fundamental aspects of the cancer problem, and it would seem to be impossible to reconcile the two prevailing theories affecting the genesis of the disease. These two theories are conveniently described as the "chronic irritation" theory and the ''virus"' theory respectively. 'T'he "chronic irritation" theory dates from 1770, when Potts recognized anid described chimney-sweeps' cancer and pointed out the etiology of the lesion. It was reasserted by Billroth, who declared that "cancer does not exist without previous chronic inflammation." During the last twenty years, a large number of agencies capable of producing cancer under natural and experimental conditions have been identified, and their variety is extraordinary. To mention only a few of them, there are X-rays, tar, pitch and certain mineral oils, arsenic, carbon-dioxide snow, and burns , all capable of setting up cancer in the skin, while cancer in the urinary bladder can be initiated either by chemical substances derived from the aniline dyes or by the ova of Bilharzia. Fibiger was able to produce cancer of the stomach in a large proportion of rats by feeding them with the larvae of a nematode; Bullock and Curtis have produced sarcoma of the liver in rats by feeding them with the common tape-worm of the cat. So far as is known, there is no property, either chemical or physical, common to all the carcinogenic agencies which have been identified as such. Thus it is assumed that these agencies produce cancer in the long run by determining identical biological changes in the affected cells, which culminate in the progressive growth of these cells. Nothing is known about the biological changes which are alleged to occur in the cells, and the process whereby they are brought about is described as "chronic …