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