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