What is the tube that protrudes from the firewall into the engine bay for? It has two places for what looks like the heater hoses to attach to. My 51 3100 has the heater hoses going through the firewall via the grommets and into the heater, should one of them go to the heather then to this thing sticking out of the firewall and then one out to the water pump? What does it do?

Thanks in advance for your help.