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