Pathological conditions affect an organ in one of two ways: as an increase or as a decrease in vital activity. Hence such conditions may be divided into two classes: the retrogressive and the progressive disorders. It is obvious that each of these processes involves functional as well as anatomical differences and abnormalities. Between the two processes seems to stand the condition of arrest of development, due to me chanical causes or to interrupted vascular sup ply. If development begins, but is arrested, the condition is termed aphasia. If the arrest is par tial, the condition is termed hypoplasia. If an organ is wholly undeveloped, the condition is termed ayenesia.
The pathologist's attention is fixed largely upon the nature of anatomical changes found, and the explanation of their occurrence. These changes are termed 'lesions,' and upon more or less ac curate idea of their nature and extent depend the success of measures taken for the relief of dis ease. Having established a conception of the lesions present, the investigator turns to their cause. The inquiry as to causation is embraced in the branch of pathology termed etiology. This department includes not only the study of vege table organisms (bacteria) and animal organ isms (such as plasmodia), and mineral toxins, and their agency in producing lesions, but also the study of reaction following the activity of the agent. This reaction depends, in some meas ure, upon the developmental disorder present, or the inherited dispositions of the tissues as to resistance or susceptibility. Inherited condi
tions are largely speculative and inferential, and this factor of etiology is of less importance in the analysis of disease. The essential feature of the whole matter is the adequate comprehension of the abnormal expression of cellular life.
There are, then, three subdivisions of pathology: etiology, or the study of causes of disease; mor bid anatomy, or the study of structural changes in disease; and morbid physiology, or the study of disturbances of function, including morbid chemical action.
Pathology naturally separates into two great divisions: (1) genera] pathology, or the study of morbid conditions which are common to sev eral different diseases; (2) special pathology, or the study of individual diseases. To illustrate, fever is a morbid condition which enters into a number of diseases• and therefore belongs to the realm of general pathology; while the study of the lesions of typhoid fever, an individual dis ease, belongs to special pathology. The study of the gross and microscopic anatomy of diseased tissues is termed pathological histology. The study of the lesions in diseases referred to the surgeon for treatment and of the lesions post operative conditions constitutes surgical pathol ogy.