What is most evident in an alive being is its anatomy, that is the shape of its body and organs,
which make him (her) to seem to all the living beings of the same species and differentiate to all
the beings of other species. This shape is not, on the other hand, invariable: it varies markedly
since the newborn until the aged.
Shape and volume of bodies and organs are kept by definite tissues which conform the frame and
wrapping and therefore have a great mechanical strength.
Bones and skin are different types of those tissues, called conjunctive and their mechanical
properties are due to one protein: collagen. 
This is the widest spread animal protein; it is found in skin, tendons, ligaments, cartilages and
membranes as, for instance, those which are in the basis of superficial tissues or epithelium or
those which filtrate blood at renal level. It is found as well, with special properties, in cornea, the
transparent cover of eye-globe, and in heart valves, where they ensure the rithmic opening and
closing of the heart cavities.
This protein is mainly found in the cutaneous system of the mammals.
Nowadays, kidneys, heart, cardiac valves, a cornea, etc., can be replaced but, facing a superficial injury, we can not substitute the lacking skin for another skin; it can only be adequately covered until healing.