Human long bones, such as the femur, humerus, ribs, sternum and so on, comprise a tough, dense outer "cortical" (also known as compact) bone, which is wrapped around an inner more open cancellous, trabecular or "spongy" bone that houses the bone marrow.
This cancellous marrow cavity contains thin bony plates that have little structural strength but provide a large surface area upon which grow haematopoietic stem cells, the elements that divide continuously to produce red blood cells, white blood cells and platelets.
But elephants are so heavy that this anatomical arrangement would be impossible in their long bones because they would just not be sufficiently strong to support the animal.
Instead elephant leg bones are made up almost entirely of cortical, compact bone and their chief site of haematopoiesis is in the pelvis.
I found an elephant leg bone when I was wandering around in Zimbabwe a while back. It was like a piece of solid rock!
Chris
- chris - 28th Sep 09