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