The marrow is lymphoid tissue consisting of a connective-tissue reticulum in which are numerous capillaries and venous vessels. The marrow-cells are large and round, and contain clear nuclei of vesicular character. Besides these there are many eosinophilic cells, endothelioid cells, fat cells, nucleated and non-nucleated red blood-corpuscles, giant-cells, and cells containing erythrocytes and pigment.

In early life the marrow of the long bones is reddish, but the color finally changes to yellow, on account of the increase of the fat.

Anemia

In pernicious anemia the fatty tissue disappears and the marrow returns to its early condition. The color, normally yellow, becomes a darker red than usual. A few myelocytes and many nucleated red cells are present. The change begins at the epiphyses, and extends toward the center of the shaft.

In myelogenous leukemia the marrow is rather gray in color, and scattered throughout may be seen small pale areas which consist mainly of myelocytes. These areas may be so numerous as to give an appearance of suppuration. Numerous nucleated red blood cells are also present. The myelocytes in myelogenous leukemia are supposed to be formed in the bone-marrow.

In lymphatic leukemia the changes are slight.

In typhoid fever the marrow contains areas composed of many lymphoid cells, large phagocytes, and foci of necrosis. The lesions are similar to those found in the other lymphatic tissues in typhoid.

Atrophy occurs in old age and marasmic conditions. The fat is absorbed, and the number of cells decreases.

Hypertrophy is the term applied to the changes that take place in anemic conditions.

Degenerations

Fatty infiltration occurs normally up to about the sixteenth year. It may be developed excessively in cases of general obesity and in conditions of ill nutrition.

Mucoid degeneration is sometimes seen.

Fatty degeneration occurs in severe infections.

Necrosis may be part of inflammatory conditions.

Pigmentation occurs in the marrow in cases of malaria, or in conditions causing hemolysis. External pigments, as an-thracosis, may be deposited in the marrow by the blood.

Inflammation or osteomyelitis occurs in the various severe infectious diseases, as typhoid fever, smallpox, etc. The marrow becomes redder than normal, punctate hemorrhages occur, focal necroses and cellular infiltration of the blood-vessel walls are present, and also granular degeneration of the cells. At times the marrow may be distinctly purulent. The specific diseases and tumors are dealt with in the chapter on Diseases of the Bones.