Truck Repair: smoke, motor valve, blowby

Question
my question is on a early 90's model detroit 60 350hp engine there is a metal tube that runs from the top of the motor valve cover etc. that winds around and points straight down, what is the pupose for this because I have recently noticed light smoke comming from this from time to time?
dont know if it has always done this but I have just noticed.

Answer
next time you go up a hill in a car, look under the cab at the big truck.. if its new and nice nothing wil be there or little. as it wears the amount of smoke and fumes increases.
worn engines its like a volcano on a long hill, its blowby, and trucks have so much if they didnt allow it out it would pressurize the engine and blow out all the seals.