If I hit my head on a blunt object it invariably produces a large lump within a few minutes, yet when I hit, say, my thigh or my hand with the same force, if any swelling occurs at all it is very minor. So why the difference?
• The lump you feel is a bruise. Bruising results from the force of the impact damaging tiny blood vessels or capillaries. Blood escapes into the surrounding tissues to produce swelling. It is this blood that is responsible for the purple colour of visible bruises such as a black eye.
Blows to the head produce such a lump because there is only a thin layer of soft tissue between the skin and skull to cushion the blow. Because the thinner tissues of the scalp dissipate less of the blow, more of the energy translates to capillary damage, leading to profuse bruising.
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The thigh, in contrast, has much thicker deformable muscle between the skin and the bone so does not bruise so easily. Even if you hit your thigh harder to produce bruising comparable to that on your scalp, the swelling on your thigh would be less. This is because on your thigh, the leaking blood can perfuse more or less in all directions. Conversely, on your head, the much thinner soft tissue restricts sideways perfusion. This, together with the non-deformable skull, means that tissue distension is predominantly outward, making the lump bigger, with the greater pressure beneath the skin making it feel firmer and more painful.
Also, because the thigh bone is essentially a rod, an oblique blow to the thigh may transfer the shock pattern so that it dissipates almost entirely through the softer fat and muscle, reducing the size of the bruise.
Inadvertent knocks to the hands typically happen with the arm swinging freely, so the energy of the impact translates mainly to movement of the hand or arm about the joint rather than tissue damage. Deliberate raps on a solid surface with even the soft parts of the hand, however, are quite a different story – as anyone who has tried attracting the attention of a neighbour whose front door lacks a knocker will know.
It is for the same reason that blows to the head with a blunt object can break the skin. Hair provides little protection. Similar considerations account for the pain frequently accompanying skin damage after knocking one’s shin or stubbing a toe.
“Because of the hard skull and thinness of overlying tissue, blows to the head can break the skin”
Len Winokur, Leeds, UK