Morbid conditions include three distinct classes: those due to
1. Injuries
2. Infections
3. Diseases
Injuries. To this class belong all the processes due to physical agencies and it includes besides traumatism, the effects of heat and cold, of chemicals, of light and of electricity.
Infections. These may be either local or general. The reaction might occur at the point of entrance of the bacteria, or constitutional symptoms may evidence their invasion into the blood, or the absorption of their toxic products.
Many conditions in this class are linked closely with those in the following class:
Diseases. Here are classified (a) new growths, both benign and malignant; (b) changes due to age and environment, and (c) diseases not belonging in either of the above classes. These latter are generally known as idiopathic or spontaneous in their etiology. These terms, however, often indicate only a limit of knowledge as to their true etiology.
The Tissues. The tissues of the body, though apparently so different and varying so decidedly in their functions, are in many respects similar.
Every tissue is composed of two parts: the cellular elements and the intercellular substance. These are called cells and stroma. Upon the first of these depends the vitality and function of the part, while its density, shape and general physical properties are determined by the second. Likewise along the same lines of reason, all of our organs have two separate areas of tissue: the parenchymatous and the interstitial. The first contains the functioning and the second the supporting elements.