Final answer:
Ventral flexion to lower the head to the ground is mainly caused by the flexibility in the neck due to the extensive range of motion provided by the cervical vertebrae and the action of neck muscles.
Step-by-step explanation:
Ventral flexion to lower the head to the ground is caused primarily by flexibility in the neck, which consists of the cervical vertebrae. The cervical vertebral column provides a wide range of motion for flexion, extension, lateral flexion, and rotation, thanks to the specific orientation of its articular processes. The major muscles involved are the neck muscles, including the sternocleidomastoid, which when contracting bilaterally can cause the head to flex or extend. Movement in the thoracic or lumbar regions is more limited for flexion with the thorax having a restricted range due to the rib cage, and while the lumbar spine allows for some flexion and lateral flexion, its primary movements do not typically contribute to lowering the head to the ground.