You need to use the connector in order to assemble the head and the spinal column (you'll find it on the display stand page).
Human skull: https://www.printables.com/fr/model/607248-human-skull
Display stand: https://www.printables.com/fr/model/610234-human-skull-stand
The author remixed this model.
This C1-C7 spine model is assembled using 1.75mm filament unstead of 2.85mm and the scale is 1.4 smaller than the original model allowing it to be adapted to the human skull. The vertebrae are not numbered on the global file.