Chapter X.

The strong features of this veteran of research are shown in the portrait, Fig. 50, which represents him at the age of seventy.

In 1847 he was called to the University of Würzburg, where he remained to the time of his death. From 1850 to 1900, scarcely a year passed without some important contribution from Von Koelliker extending the knowledge of histology. His famous text-book on the structure of the tissues (_Handbuch der Gewebelehre_) passed through six editions from 1852 to 1893, the final edition of it being worked over and brought up to date by this extraordinary man after he had passed the age of seventy-five. By workers in biology this will be recognized as a colossal task. In the second volume of the last edition of this work, which appeared in 1893, he went completely over the ground of the vast accumulation of information regarding the nervous system which an army of gifted and energetic workers had produced. This was all thoroughly digested, and his histological work brought down to date.

Schultze.--The fine observations of Max Schultze (1825-1874) may also be grouped with those of the histologists. We shall have occasion to speak of him more particularly in the chapter on Protoplasm. He did memorable service for general biology in establishing the protoplasm doctrine, but many of his scientific memoirs are in the line of normal histology; as, those on the structure of the olfactory membrane, on the retina of the eye, the muscle elements, the nerves, etc., etc.

Normal Histology and Pathology.--But histology has two phases: the investigation of the tissues in health, which is called normal histology; and the study of the tissues in disease and under abnormal conditions of development, which is designated pathological histology. The latter division, on account of its importance to the medical man, has been extensively cultivated, and the development of pathological study has greatly extended the knowledge of the tissues and has had its influence upon the progress of normal histology. Goodsir, in England, and Henle, in Germany, entered the field of pathological histology, both doing work of historical importance. They were soon followed by Virchow, whose eminence as a man and a scientist has made his name familiar to people in general.

Virchow.--Rudolph Virchow (1821-1903), for many years a professor in the University of Berlin, was a notable man in biological science and also as a member of the German parliament. He assisted in molding the cell-theory into better form, and in 1858 published a work on _Cellular Pathology_, which applied the cell-theory to diseased tissues. It is to be remembered that Bichat was a medical man, intensely interested in pathological, or diseased, tissues, and we see in Virchow the one who especially extended Bichat's work on the side of abnormal histology. Virchow's name is associated also with the beginning of the idea of germinal continuity, which is the basis of biological ideas regarding heredity (see, further, Chapter XV).

Leydig.--Franz Leydig (Fig. 52) was early in the field as a histologist with his handbook (_Lehrbuch der Histologie des Menschen und der Thiere_) published in 1857. He applied histology especially to the tissues of insects in 1864 and subsequent years, an account of which has already been given in Chapter V.

Cajal as Histologist.--Ramon y Cajal, professor in the University of Madrid, is a histologist whose work in a special field of research is of world-wide renown. His investigations into the microscopic texture of the nervous system and sense-organs have in large part cleared up the questions of the complicated relations between the nervous elements. In company with other European investigators he visited the United States in 1899 on the invitation of Clark University, where his lectures were a feature of the celebration of the tenth anniversary of that university. Besides receiving many honors in previous years, in 1906 he was awarded, in conjunction with the Italian histologist Golgi, one of the Nobel prizes in recognition of his notable investigations. Golgi invented the staining methods that Ramon y Cajal has applied so extensively and so successfully to the histology of the nervous system.

These men in particular may be remembered as the investigators who expanded the work of Bichat on the tissues: Schwann, for disclosing the microscopic elements of animal tissues and founding the cell-theory; Koelliker, as the typical histologist after the analysis of tissues into their elementary parts; Virchow, as extending the cell-idea to abnormal histology; Leydig, for applying histology to the lower animals; and Ramon y Cajal, for investigations into the histology of the nervous system.

Text-Books of Histology.--Besides the works mentioned, the text-books of Frey, Stricker, Ranvier, Klein, Schäfer, and others represent a period in the general introduction of histology to students between 1859 and 1885. But these excellent text-books have been largely superseded by the more recent ones of Stöhr, Boem-Davidoff, Piersol, Szymonowicz, and others. The number of living investigators in histology is enormous; and their work in the subject of cell-structure and in the department of embryology now overlaps.

In pathological histology may be observed an illustration of the application of biological studies to medicine. While no attempt is made to give an account of these practical applications, they are of too great importance to go unmentioned. Histological methods are in constant use in clinical diagnosis, as in blood counts, the study of inflammations, of the action of phagocytes, and of all manner of abnormal growths.

In attempting to trace the beginning of a definite foundation for the work on the structure of tissues, we go back to Bichat rather than to Leeuwenhoek, as Richardson has proposed. Bichat was the first to give a scientific basis for histology founded on extensive observations, since all earlier observers gave only separated accounts of the structure of particular tissues.