Final answer:
The primary union of a wound is when edges can be brought close together, allowing efficient healing, whereas tunneling indicates a wound has extensions beneath the surface.
Step-by-step explanation:
The term 'primary union' refers to a condition of a wound where the edges are close enough to be brought together, potentially requiring fastening. This scenario permits a faster and more comprehensive healing process. In contrast, when wounds have extensions under the skin in a lateral or 'tunneling direction,' it poses a challenge for healing.
Such wounds may extend beneath the intact surface, making it harder to treat and heal. This type of extension can be particularly troublesome in cases of infection, as the disease or infection may spread along these tunnels, which was seen in the case of Mark mentioned in the Clinical Focus narrative.
The primary union is the ideal healing scenario, where a wound is neatly closed, and the risks of infection are minimized. The contrast to primary union can be 'secondary healing', where the wound must heal from the base upwards due to larger gaps or tissue loss.