The Gonome is at the heavily advanced state of mutation and barely human anymore, what follows is just my explanation not necessarily true to cannon the headcrab could have increased the victims production of keratin the tough protein that fingernails are made of which produces these large bone like protrusions that break through the skin forming the spikes and "teeth" on the chest I will however break the ribs before release as to allow for the range of movement the gonomes chest jaws would require.
Thanks for the comments and suggestions.
nrd2001
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