The Temple of the Quetzalcoatl
The Temple of the Quetzalcoatl can be found in Teotihuacan, where it stood from about 150 C.E. to 250 C.E.  If one were to see the temple in its entirety, there would be exactly 365 heads of Quetzalcoatl or Tlaloc, which alternate around the tiers; thus, the temple celebrates the Quetzalcoatl and the passage of time. 
The Temple has more than just these heads, however; it also has many designs of serpents to represent the Quetzalcoatl. In addition, there are images of priests, sacrifices, and the call to people for prayer. No doubt, the temple was create to honor the dragon as well to offer sacrifices to him, despite the fact that he did not require human sacrifices to be made in his name. 
The Temple really is a tribute to "all things that flow": water, time, blood, etc. These things were all valued by the people who worshiped Quetzalcoatl, who represented much of these things to people. 
While the reasoning for having Quetzalcoatl's head carved is obvious, the reasoning for Tlaloc's head is not yet fully known. Many believe that it's there to represent the change of dry and wet seasons, or other such juxtaposition.  There are other connections between Tlaloc and Quetzalcoatl, but many believe the heads are simply a representation of the change or transitions of time. 
Before the temple was built, and when the temple was completed, there were ritual human sacrifices that took place around the temple. The human bodies were found in fetal position, which is believed to be symbolic of the rebirth of Quetzalcoatl, who was believed to have burned himself and flown up to become Venus. 
Even when Teotihuacan was abandoned, the Aztecs still migrated to the temple to offer their prayers to the Quetzalcoalt, and it was considered a renowned pilgrimage. 
For more information on footnotes and references, please see the bibliography.