Cutaneous sense organs are common, including those sensitive to pain, temperature, pressure, and stretching of the skin. Although pain and temperature receptors are best known on the heads of reptiles, these receptors are not confined there. The mechanoreceptors that detect touch, pressure, and stretch are present over the body, especially within the hinges of scales. Receptors that detect infrared radiation (heat) are also of dermal and epidermal origin. In boas and pythons, these receptors are associated with the lips. In pitvipers such as rattlesnakes, a membrane containing heat receptors is stretched across the inside of each pit approximately 0.04-0.08 in (1-2 mm) below the external meatus. The geometry of the bilateral pits is such that their receptive fields overlap, allowing stereoscopic infrared detection. The nerves of the pits project to the same brain areas as do the eyes, giving rise to images containing elements from the visible part of the spectrum as well as the infrared part. When a pitviper is in the process of striking a mouse, the snake's mouth is wide open with fangs erect, so that the pits and eyes are oriented up rather than straight ahead toward the prey. It turns out that in the roof of the mouth near the fangs are additional infrared sensitive receptors that appear to take over guidance of the strike during these final moments.
Reptiles also possess proprioceptors associated with muscles, tendons, ligaments, and joints. Proprioceptors report the positions of body components to the brain, allowing the brain to orchestrate posture and movement. Another class of internal receptor contains taste buds, which are located in the lining of the mouth and on the tongue. In reptiles with slender, forked tongues specialized for conveying nonvolatile chemicals to the vomeronasal organs, lingual taste buds are generally absent, but taste buds may be present elsewhere in the mouth.
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