The block is usually filled with {TAIL ~ a lasing gas} such as a mixture of helium and {HEAD ~ neon} 
The block is usually filled with { TAIL ~ a lasing gas } such as a mixture of helium and { HEAD ~ neon }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD