The architecture of cancellous bone has been studied in macerated human bones. A number of distinct types of architecture can be recognized as follows: Type I consists of a very delicate meshwork of fine rods. Type II is made up of both rods and plates. Sub-type IIa is a meshwork similar to Type I, but a varying proportion of rad are replaced by delicate plates. Sub-type IIb shows the presence of thin but large fenestrated plates with a well marked orientation in preferred planes; these are interconnected by smaller plates and rods. Sub-type IIc is made up of relatively thick and extensive arranged for the most part parallel to one another, the plates being connected to each other by fine rods. Type III is made up entirely of plates. Delicate plates may form a meshwork in which a directional orientation may or may not be apparent (Sub-type IIIa). Elsewhere better defined, larger plates may enclose tubular spaces (Sub-type IIIb). In some areas (where cancellous bone is very dense) small relatively thick plates enclose irregular spaces; the appearance may closely resemble that of a honeycomb when the spaces are small, but elsewhere the spaces may show a definite directional orientation. The wall of the marrow cavity of long bones is seldom smooth. It is characterized by the presence of plates and rods in various configurations. A distinct marrow cavity is seen in the majority of clavicles examined.