{HEAD ~ A gap} is defined between {TAIL ~ the first edges} of sufficient dimension to accommodate a biomedical instrument 
is defined between
{ HEAD ~ A gap } is defined between { TAIL ~ the first edges } of sufficient dimension to accommodate a biomedical instrument
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH