neck, head
頸 centers on the neck as the body part connecting head and torso; the head meaning is a natural extension referring to the area above the neck.
頸 combines 巠 (which likely contributes the sound) with 頁 (head), pointing to the neck as the part below the head.
The left side 巠 looks like a vertical passage, and the right side 頁 is a head. Picture the neck as the passage connecting the head to the body.
For ケイ, imagine a key (ケイ) unlocking the connection between head and body: the neck is the key passage.
neck
carotid artery
cervical canal
neckband; round upright collar (on some traditional Japanese clothing)
internal carotid artery
inseparable friendship
external carotid artery
Collar of the Supreme Order of the Chrysanthemum
(uterine) cervix
cervical cancer
cervical canal
cervical incompetence; cervical insufficiency
cervix; uterine cervix; cervix uteri
longing for someone or something; waiting for someone of great talent to arrive; stretching one's neck and standing on tiptoes
nape; back of the neck
torticollis; wryneck; stiff neck
cervical dystonia; spasmodic torticollis
bombardier beetle (any insect of subfamily Brachininae)
common carotid artery
head and neck carcinoma; cancer of the head and neck