You need to use the connector in order to assemble the head and the spinal column. For more rigidity, a beam was added between the head and the support (it crosses the cervical vertebrae).
Human skull: https://www.printables.com/fr/model/607248-human-skull
C1-C7 spine model: https://www.printables.com/fr/model/610193-flexible-c1-c7-spine-model
The author marked this model as their own original creation.