It has already been stated that sarcomas and cancers are the tumours which regularly present a malignant tendency, but on rare occasions other tumours also do so. Cartilaginous tumours are not infrequently malignant. Next to them mucous-tissue tumours most frequently become malignant, but even fibrous tumours have been observed to do so, and cases of colloid goitre have been recorded in which secondary tumours occurred. So that malignancy is not confined to sarcomas and cancers. On the other hand, tumours having the structure of sarcomas may remain local to the end.
It has been usual to believe that malignancy is dependent on peculiarities of the tumour itself, and it is true that when a simple tumour, as sometimes happens, assumes malignant characters, it usually assumes the structure of a sarcoma or cancer. An attempt has been made, based on the views brought forward by Thiersch as to the causation of cancer (see under Cancer), to account for the difference between a simple and a malignant tumour. In the case of a simple tumour the normal tissues are able to prevent the tumour-tissue from penetrating into them, but in the case of malignant tumours the tissues have become weakened and are unable to form a barrier to their extension. When a simple tumour becomes sarcomatous, it is because the tissues around have acquired a peculiar weakness. When pieces of a tumour are transported to a distance, a struggle, as it were, occurs between the tendency of the tissue of the tumour to grow, and that of the normal tissues to prevent its growth. In the case of a simple tumour the tissues assume the upper hand, and cause the absorption of the tumour-tissue. In the case of malignant tumours, however, the tissues are too weak to accomplish this, and the secondary tumour develops. This theory is not sufficient to explain all the facts, and we must suppose as well a special power of growth in the tumour. No doubt the tissues differ very greatly in their power of restraining the growth of secondary tumours. In the liver, for instance, cancers grow very freely, and attain large dimensions, but looking to a cancerous liver, it is difficult to believe that there is not a special activity in the growing tissue of the tumours.
Tumours are exposed to the same pathological processes as normal tissues, and in a higher degree. Thus we meet with fatty degeneration, especially in quickly growing tumours; calcareous infiltration in structures which are obsolete; haemorrhages, principally in superficial rapidly growing tumours, where the bloodvessels are ill-formed; and necrosis mostly in tumours near the surface, and thus exposed to mechanical irritation, the consequences of such necrosis being ulceration with, it may be, suppuration, haemorrhage, or decomposition.
Conditions like those last named will seriously affeot the organism as a whole, and they will occur most frequently in the case of malignant tumours, which grow quickly and rapidly come to the surface. Malignant tumours also affect the organism as a whole by the readiness with which they extend to or produce secondary tumours in important organs, and secondary cancers frequently also induce inflammations as in the case of the peritoneum or pleura.
It is clear, therefore, that malignant tumours, especially by bleeding, by ulcerating and sloughing, by invading important parts, by producing inflammations, etc., have a tendency to deteriorate the system, producing anaemia and general weakness. If growing quickly they also tend to emaciate by using up the nutritive material of the body. A simple tumour, if it happens to be at the surface and exposed to mechanical violence, may also ulcerate and produce serious constitutional results, but this is exceptional. To these conditions of the body as a whole the name Cachexia is often given. Sometimes also the term Dyscrasia or Diathesis is used with the meaning that there is some peculiar condition of the system preceding and inducing the formation of malignant tumours, but of this there is no evidence, and the changes in the general condition are always secondary.
Tumours have sometimes been classified and named according to their clinical characters, whether innocent or malignant, the term cancer being used to include malignant growths in general. A true system of classification will take into account the origin, structure, and mode of growth of tumours, and such a classification will be found ultimately to correspond with clinical facts.
In classifying tumours according to their origin we may assume that they take origin in tissues of their own nature. This is at least admitted in the case of simple tumours; a fatty tumour, for example, originates in adipose tissue. In the case of cancers, whose more important elements are epithelial, Virchow asserted that they originated from connective tissue. This was opposed by Thiersch, who showed that in the case of epithelial cancer the cells originate from existing . epithelium. This view has been amplified by Waldeyer, who, in a very elaborate series of investigations, has shown that in their very various seats, cancers are in their origin connected with the epithelia structures. This has led to a reference to embryology in the classification of tumours. An attempt has been made to distinguish those arising from the various layers of the embryo, the tumours of meso-blastic being separated from those of hypoblastic or epiblastic origin. It is very difficult to carry out this distinction absolutely. The epiblast gives rise to much besides epithelium, as does also the hypoblast, and there seems even now considerable doubt whether the serous cavities are hypoblastic or mesoblastic, and whether, in conse quence, the layer of cells covering these cavities is endothelium or epithelium. The attempt to carry out rigidly this mode of classification has led to great difficulties in the nomenclature and placing of some tumours. Thus we have tumours of the pleura and peritoneum, which have the structure of cancers, but German authors regard the endothelium of these cavities as belonging to the connective tissue, and many of them would place such tumours amongst those arising from connective tissue, and separate them from the cancers.
The structure and mode of growth have to be taken into account as well as the origin of tumours. It has been indicated above that in respect to structure and mode of growth tumours may be typical or atypical. It will be convenient to divide tumours, in the first instance, on this basis into two great groups, the typical and the atypical. In the further subdivision of these groups the relation of the tissue to the corresponding normal tissue is taken into consideration, and we may have both typical and atypical tumours referable to the same normal tissue.
The grouping of tumours into typical and atypical forms will nearly correspond with the clinical distinction of innocent and malignant, but it must not be forgotten that the distinction is not absolute, and that typical tumours sometimes assume the characters of the atypical.
In naming tumours the structure is chiefly taken into account. The typical tumours are named by adding the usual suffix -oma to the name of the tissue. The atypical tumours have special names which are variously derived. Thus, Sarcoma is a name originally used in a very indefinite way, but by Virchow applied to atypical connective-tissue tumours. Carcinoma also had at one time a wide and somewhat indefinite significance, but is now limited to atypical tumours whose more important structure is epithelium.
A very great advance was made in our knowledge of tumours by the publication of Virchow's classical work, "Die krankhaften Geschwiilste." From this work many of the following illustrations are borrowed by the kindness of the author.
J. Muller, Bau und Form der krankhaften Geschwiilste, 1888; Virchow, Die krankhaften Geschwiilste, 1864-67; Paget, Lectures on Tumours, 1852, Surgical Pathology, 3rd ed., 1870, Path, trans., xxv., p. 319; Lucke, in Pitha and Billroth's Handbuch, vol. ii., part 2, 1869; Cohnheim, Allg. Path., 2nd ed., vol. i., 1882; Broca, Traite des Tumeurs; Butlin, Art. Tumors, Internat. Encycl. of Surg., vol. iv., 1884; Zahn, Congres med. internat. de Geneve, 1878; Leopold, Virch. Arch., lxxxv., p. 283; Baker, St. Barth. Hosp. Rep., vol. ii., p. 129; Cripps, do., vol. xiv., p. 287; Snow, Clin, notes on Canter, 1883; Thiersch, Epithelialkrebs, 1865; Waldeyer, Volkmann's Sammlung, No. 33, 1872, Virch. Arch., xli. and iv.; Coats, Path, trans., xxxviii., p. 399, 1887.