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