The malignant tumors, on the contrary, differ in their structure from the normal tissues of the body. Their growth is rapid and infiltrating rather than slow and concentric. Such tumors usually have a predominance of cells and thin walled blood-vessels. The former may be little else than nuclei enveloped in an easily destructible protoplasm, or they may be composed of multi-nucleated masses of protoplasm, and are then known as giant-cells. The most malignant tumors are those which tend to become generalized as well as to spread locally. They recur locally, and appear in the nearest lymph-glands and at remote parts of the body. The disturbances produced by the malignant tumors depend less upon their mechanical relations than upon their tendency to destroy tissues and disturb functions. With their presence and progress in vital organs there is associated, from their manner of growth, a destruction of the cells of such organs, as the kidneys and liver, the lungs and heart. When they are seated in the spleen and lymphatic glands, a disturbance in the blood-making process must be associated. Their occurrence in the alimentary canal opposes the admission, digestion, and expulsion of its contents, and produces disturbances varying as to the seat and peculiarities of the tumor. The progress of the malignant tumor is often associated with ulceration, watery discharges, and hemorrhage. The frequent coexistence of emaciation, weakness, anæmia, and a yellowish discoloration of the skin forms a group of disturbances which, included under the name "cachexia," have long been prominent as significant of malignant tumors. At the present day this cachexia is regarded rather as the result than the cause of the tumor, whereas formerly the reverse was the case. The modern classification of tumors is based chiefly on their structure, in part upon their method of origin, and in part upon their cause. With the observation of the similarity of appearances in the flesh of which the external and internal neoplasms are composed, the suggestion readily presented itself to regard the external tumors and the internal growths as similar in character. External forms, physical characteristics, clinical peculiarities, all proved insufficient as a means of identifying the two, and the step was a short one which led to the minute study of the flesh of the tumor and a comparison of its resemblances and differences. This comparison obviously included a knowledge of the structure and peculiarities of normal tissues. As histological studies advanced, so did the pursuit of pathological histology, and the tumors which were once designated as encephaloid, mastoid, pancreatoid, or nephroid, from real {115} or fancied resemblances to certain organs of the body, became analyzed into their microscopic rather than macroscopic characteristics.