The head represents a rational mind. Which is a mind of indivduality without authority. Leading the mind to have their own thoughts without the prescence of an authority type, tradition, or culture.
The could is the act of thinking. It is an act of doubting. Such as the people he saw outside but one could doubt they were men.
This also leads to, "I think therefore I am.". He is saying if we think we exist. The only way to know you exist is by being able to think.
Descartes had the phulosophical idea "Cogito ergosum.", in latin or, "I think therefore, I am.". Which explains, I think therefore I exist. This is important because he says the only way we know we exist is by thinking. Descartes also says that we, humans, are only alive to think, but he has also said authority should not influence the was you think. Yet he is an authority in the world of phyosophicaology.