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