THEORIES AND PROBLEMS OF CANCER 679 



developed and the power of undergoing differentiation least 

 perfectly. 



There are many observations which support this theory ; we 

 may, I think, dispense with the details of the clinical facts that 

 bear upon it — they are so well known that it is not necessary to 

 do more than point out the general behaviour of malignant 

 growths. In the earlier stages they cause no symptoms at all ; 

 the symptoms are always secondary and due to pressure upon 

 or invasion of some part of the body. Pain exists only as a 

 response to this pressure or invasion. The growth of the 

 tumour is more marked than the destruction of already existing 

 tissues and the destruction is probably entirely mechanical, that 

 is to say, it is due to such causes as pressure and the obstruction 

 of the blood supply brought about by the mere presence of the 

 tumour cells, not by any intrinsic destructive properties of the 

 cells themselves. Clinically, in fact, the cells of a malignant 

 growth act as parasites. 



There are several other facts of a general nature which point 

 to the same conclusion. Malignant tumours never have any 

 nerve supply. Though in the rare primary tumours occurring 

 in nerve sheaths a nerve may be involved at an early stage, 

 there are no nerves belonging to the tumour. This further 

 accentuates the suggestion that the controlling influence of the 

 nervous system as a whole in vertebrates modifies and in- 

 fluences in some way the somatic co-ordination. Malignant 

 growths are common only in those tissues in which cell 

 proliferation takes place normally throughout life and are 

 frequent in a tissue in proportion to the amount of prolifera- 

 tion usual in the cells forming it. Cancer is generally of more 

 rapid growth in young persons than in old. The age at which 

 cancer most commonly occurs is that at which physiological 

 cell proliferation is becoming less active and therefore cell 

 divisions produced by some unusual stimulus must involve a 

 greater departure from the normal course of events than is 

 the case in younger tissues. Cancer apparently cannot occur 

 except in tissues in which multiplication of the cells is taking 

 place. After the human embryo is fully formed, the nerve cells 

 never proliferate and malignant growths originating in nerve 

 cells are unknown. 



As we have already seen, the body of a multicellular 

 organism originates from a single cell, the fertilised ovum. The 



