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: theparenchymatous 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.

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