When the complexity of the human body is considered, it seems surprising that outgrowths of tissues in the form of tumors are of so infrequent occurrence. Is there somewhere in the nervous cen tres a working model of the body, which by an unknown force is so controlled that there is departure from the normal shape without an assignable cause? 6. There can be but little doubt that some varieties of tumors, such as papil loma and cancers, are due to infecting agents received into the body from with out. At the present time no such organ ism has, however, been demonstrated.
Irritation alone cannot be the cause of cancerous tumors, or else they would occur more frequently. Parasites fre quently cause cell proliferation inde pendent of inflammatory conditions, as shown by the lymphoma.ta of typhoid and the enlarged spleen of malaria, a proliferation analogous in general feat ures to cancer; they produce metastases as does cancer; they often select special cells for their habitat, as blood-cells in malaria, multinuclear leprosy cells in leprosy, and giant cells in tuberculosis, while the hypothetical cancer parasite may equally well choose epithelium. Finally, certain parasites, coccidia, cause in the bile-ducts of rabbits an increase of epithelial and fibrous tissue closely resembling malignant adenoma of the rectum. C. F. Martin (Montreal Med. Jour., Feb., '96).
In reviewing the present state of knowledge as to the etiology of cancer the following propositions are advanced: That in malignant growths of men and animals certain extraneous bodies may be found within the protoplasm of the cell. These bodies are identical morpho logically with the so-called coccidia found by various authors in the cells of epitheliomata and sarcomata. They are blastomycetes, and resist acids and al kalies just as these parasites do. They are found in malignant growths only and in no other pathological growth. They are generally seen at the growing edge of a new growth, not in the centre, and only exceptionally in the cell-nuclei. They react to special methods of stain ing, and may be obtained in pure cult ures from the malignant neoplasms of men and animals. The lower maimnalia (guinea-pigs, rabbits) are more sus ceptible to inoculation with these blasto mycetes than the higher (dogs). Thus,
while certain blastomyeetes in the lower classes give rise to generalized lesions, in the higher manimalia the lesions are lo calized. These lesions have essentially nco-prodnctive and non-inflammatory characters. The question of the etiology of cancer can be resolved only when the hlastomyeetes isolated from malignant tumors of animals of one species repro duce in animals of the same species the same malignant growths. Roncali (Supp. al Policlin., Oct. 31, '96).
All the so-called parasites which have, up to the present, been described as oc curring in cancers are susceptible of ex planation as degenerations of one or all of the constituents of tumor-cells or as metamorphosis of their secretion. H. J. Stiles (Lancet, Feb. 27, '97).
The following deductions may be made from personal observations and experi ments:— 1. That in cancers there are certain intracellular bodies (which may also be found rarely outside the cells) which are neither parts of the cell structure nor any known degenerative change, and which are only found in cancer; and that they are only found at the periphery of the growing parts of a cancer, and not in the degenerated parts, and that these bodies have distinctive microchemical re actions.
2. That there are certain cancers, which occur rarely, in which these bodies are present in enormous numbers.
3. That by the use of appropriate means these bodies can be isolated and cultivated outside the body.
4. That these cultures, when intro duced into certain animals, can cause death, with the production of tumors, so far of endothelial origin; and that pure cultures can be made from these tumors which, if inoculated into suitable ani mals, will produce again similar growths. H. G. Plimmer (Practitioner, Apr., '99).
Tumors may be considered as being derived:— l. From embryonic cell "rests," which have for a shorter or longer period re mained latent in one or other tissue and then have taken upon themselves a rapid proliferation, leading to tumor-formation.
2. From the mother-cells of a tissue which, remaining undifferentiated, but capable of active proliferations through out life, now assume excessive prolifera tive powers, their daughter-cells retaining to a greater or less extent the characters and the properties of the mother-cells.