And then there were four: Anatomical observations on the pollical palmar interosseous muscle in humans

Although anatomists generally agree upon the presence of four interosseous muscles in the human hand, the number and identity of the palmar interosseous muscles remains contentious. Recent studies suggest that a majority of human hands possess four palmar interossei, yet most contemporary texts suggest the presence of only three. The pollical palmar interosseous muscle (PPIM), associated with the first digit, has been alternatively interpreted as a distinct muscle, part of another hand muscle, or nonexistent. We examined 45 hands from 23 human cadavers to investigate the prevalence of this muscle and found it to occur in varying degrees of expression in 91% of specimens. We also tested the hypothesis that the PPIM forms the smaller part of a "parallel muscle combination" and is therefore ideally suited to act as a proprioceptive organ. Results do not show a significantly higher density of muscle spindles in the PPIM relative to the adjacent adductor pollicis, provisionally refuting this hypothesis. The presence of the PPIM, observed in the majority of hands from several populations, indicates that it should be regularly included in mainstream anatomy texts and atlases.