PATHOLOGY, the branch of medical science which deals with the causes of disease, the anatomical, physiological and func tional changes which these causes induce and the methods whereby the body combats both causes and changes. Used alone, the term refers to man and animals; for vegetable pathology or phyto pathology, see PLANTS : Pathology.
The study of pathology began about 10o years ago and it was in the last 3o years of the 19th century that its importance began to be recognized by definite and separate academic provision for its prosecution. At first it consisted mainly in investigation of the changes found in the human body after death and their correlation with the signs of disease which had been observed during life. The large progress made along these lines soon led to its expansion in a variety of directions until pathology came to include almost anything which had to do with disease, and its field included aetiology, pathogenesis, morbid anatomy, microscopic histology, parasitology, functional changes, chemical alterations, and indeed any topic, except diagnosis and treatment, which was open to fairly accurate study. The contemporary progress of medicine and surgery showed also that laboratory methods for the investigation of sick persons afforded valuable aids in diagnosis; this large subject of "clinical pathology" is properly recognized as belonging to clinical medicine rather than to pathology. This very diverse assemblage of studies forms together a consistent whole, but the increase of knowledge in some directions has been so great that bacteriology, immunology, protozoology, pathological chemistry and other branches have been progressively separated as special ized topics. (See BACTERIA AND DISEASE; IMMUNITY; PARASITIC DISEASES, etc.) In the residuum, apart from a tendency to as similate pathology to the other biological sciences and a consider able increase in the attention paid to functional as distinct from anatomical changes, no ideas or methods of primary importance have emerged in late years to cause any great change in general outlook, and workers have mostly been engaged on the elaboration and consolidation of the science. Pathology in this restricted sense may be taken to mean the study of the reactions of living organisms to injury.
of diet), electricity, radiation energy and other factors which are still indefinite. The injury may immediately cause gross altera tions in structure, and it is known that the physical injury of a cell often entails radical changes in the biological and chemical characters of its protoplasm which depend in some way on its architecture as a whole. Or the injury may be anatomically per ceptible though unaccompanied by any large structural changes. Or it may, like electric shocks, cause no visible change and yet lead to a complete and permanent cessation of the activities which constitute what we know as life. Or, finally, it may cause no ana tomical changes recognizable by present methods and be put in evidence only by greater or smaller alterations in function.
The obvious anatomical changes in injured tissues with which the early students of pathology mostly concerned themselves are not as a rule the direct and immediate consequence of the injury. They result partly from alterations which take place in cells of ter death by the process known as autolysis and partly from the activ ity of the uninjured and healthy cells in the neighbourhood and elsewhere in the body which constitutes the response or reaction.