+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. The physical conditions of the interstitial tissue or the intercellular substance vary greatly in density. _Blood_ is a tissue, the intercellular constituent of which is fluid, and as we consider more dense tissues, we encounter all degrees of density of the framework or intercellular substance, until with the additional presence of calcareous elements, we conceive the hardness of bone and dentine. Tissues as a whole, however, are not solid; there are spaces in the supporting structure to admit of the passage of arteries, veins, nerves, and lymphatics. Abnormal conditions arise in the various parts of the tissues. Certain diseases affect the parenchymatous tissue in an organ more than the interstitial tissue and again others affect the blood vessels particularly. There may be _hypertrophy_, in which the entire organ or part becomes larger as a whole, the active cells and stroma sharing alike in the process, or there may be an _hyperplasia_, in which the active cells of the part proliferate abnormally. When the interstitial tissue alone develops excessively it is known as an _infiltration_. Under such circumstances the parenchymatous cells often undergo what is termed _pressure atrophy_; they are diminished by squeezing. _Atrophy_ of the part or organ, from whatever source, signifies its diminution in size; its function is, of course, either impaired or suspended as the process goes on. The efforts on the part of the organism as a whole to combat or repair injury, bacterial invasion and disease are directed along definite lines. The study of these functions does not rightly come within the scope of these pages, belonging to physiology, but must be considered here, if only in brief for the purpose of a good understanding of the processes at work in surgical conditions. As surgeons, the functions which concern us most are the 1. Nervous 2. Circulatory 3. Lymphatic +THE NERVOUS SYSTEM+