Function of the Heterotopic Bone n the Second Thoracic Vertebra in Shrews

Heterotopic bones are bones that are dissociated from the rest of the body skeleton in vertebrates. Through the examination of double stained specimens of seven of the eight shrew species in Taiwan, a needle-shaped heterotopic bone was found over the second thoracic vertebra（T2）in three Crocidura spp. and Suncus murinus, but not in Episoriculus fumidus, Chodsigoa sodalis or Anourosorex yamashinai. In this study, I used C. tanakae and C. shantungensis who have T2 heterotopic bone and Episoriculus fumidus who lacks the bone as subjects and conducted myological analysis, behavioral observation, head and body structure analysis, and statics analysis for muscle force affected by T2 heterotopic bone, in order to understand the functions of this bone in shrews.My results revealed that the splenius muscle is the major muscle that originates from the T2 heterotopic bone and inserts to the lateral half of the lambdoidal ridge of skull. The center of gravity of skulls of Crocidura spp. was farther away from occipital region than that of E. fumidus and was farther away from T2 fulcrum than that of E. fumidus in “Head Stretch Forward” and “Head Search” movements. Benefiting from the mechanical advantage enhanced by the T2 heterotopic bone for splenius muscle, Crocidura spp. can perform higher frequency of movements associated with the head. They can also stretch more of their cervical and thoracic vertebra in these movements than E. fumidus.