So, is the constant current provided by the voltage source in the top right?

In the "old days" Diodes were commonly drawn within a circle but so were Transistors. It was most probably accepted that way because we were transitioning from Vacuum (Valves) Tubes.I was going to say a diode, but then I saw the whole diagram. After seeing this I'm even more annoyed that they taught us at school that a diode symbol includes a circle just like that. (I knew better fortunately)