Hip bone is easily located through the skin. The iliac crest shows clear marks of the attachment of the three abdominal wall muscles. The ischium forms the lower and back part of the hip bone and is located below the ilium and behind the pubis. The ischium is the strongest of the three regions that form the hip bone. It is divisible into three portions: the body, the superior ramus, and the inferior ramus. The body forms approximately one-third of the acetabulum. The ischium forms a large swelling, the tuberosity of the ischium, also referred to colloquially as the "sit bone". When
Bone A bird's beak is primarily made of bone as projections of the mandibles which are covered in keratin. A deer's antlers are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed. The extinct predatory fish "Dunkleosteus" had sharp edges of hard exposed bone along its jaws. Many animals possess an exoskeleton that is not made of bone. These include insects and crustaceans. The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine