<P> The intrinsic muscle groups are the thenar and hypothenar muscles (thenar referring to the thumb, hypothenar to the small finger), the dorsal and palmar interossei muscles (between the metacarpal bones) and the lumbrical muscles . The lumbricals arise from the deep flexor (and are special because they have no bony origin) and insert on the dorsal extensor hood mechanism . </P> <P> Aside from the genitals, the fingertips possess the highest concentration of touch receptors and thermoreceptors among all areas of the human skin, making them extremely sensitive to temperature, pressure, vibration, texture and moisture . Recent studies suggest fingers can feel nano - scale wrinkles on a seemingly smooth surface, a level of sensitivity not previously recorded . This makes the fingers commonly used sensory probes to ascertain properties of objects encountered in the world, making them prone to injury . </P> <P> The pulp of a finger is the fleshy mass on the palmar aspect of the extremity of the finger . </P> <P> Although a common phenomenon, the underlying functions and mechanism of fingertip wrinkling following immersion in water are relatively unexplored . Originally it was assumed that the wrinkles were simply the result of the skin swelling in water, but it is now understood that the furrows are caused by the blood vessels constricting due to signalling by the sympathetic nervous system in response to water exposure . One hypothesis for why this occurs, the "rain tread" hypothesis, posits that the wrinkles may help the fingers grip things when wet, possibly being an adaption from a time when humans dealt with rain and dew in forested primate habitats . A 2013 study supporting this hypothesis found that the wrinkled fingertips provided better handling of wet objects but gave no advantage for handling dry objects . However, a 2014 study attempting to reproduce these results was unable to demonstrate any improvement of handling wet objects with wrinkled fingertips . </P>

The part between your thumb and index finger