The covers enclose a cartridge extension mechanism capable of extending the pen cartridge from the housing upon {TAIL ~ application} of a pressure to the respective covers, and retracting the pen cartridge into the housing upon removal of the pressure from {HEAD ~ the covers} 
The covers enclose a cartridge extension mechanism capable of extending the pen cartridge from the housing upon { TAIL ~ application } of a pressure to the respective covers , and retracting the pen cartridge into the housing upon removal of the pressure from { HEAD ~ the covers }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD