Skull of a horse with myelofibrosis. Myelofibrosis is a disorder of the bone marrow that leads to the marrow being replaced by fibrous tissue. This causes a reduction in the amount of marrow and results in blood components being produced in other organs such as the liver and spleen. As a result these organs become enlarged. Photographed in the museum of the National Veterinary School of Alfort, Maisons-Alfort, France.