The normal coloring matter of the red blood corpuscles
   of vertebrate animals. It is composed of hematin and globulin, and is
   also called haematoglobulin. In arterial blood, it is always combined
   with oxygen, and is then called oxyhemoglobin. It crystallizes under
   different forms from different animals, and when crystallized, is
   called haematocrystallin. See Blood crystal, under Blood.