鼩鼱第二胸椎異位骨之功能

Abstract

異位骨為游離於中軸及四肢骨以外的骨頭。由台灣尖鼠科動物之透明骨骼標本發現，台灣長尾麝鼩、小麝鼩、台灣灰麝鼩及家鼩之第二胸椎（T2）上方有一游離的尖長形異位骨，而台灣短尾鼩、細尾長尾鼩及台灣長尾鼩則不具有此骨。本研究選擇有T2異位骨的灰麝鼩、小麝鼩及無T2異位骨的長尾鼩為對象，透過肌肉解剖、頭部基本形值測量、行為觀察、骨骼結構相對位置及力學分析等方法，探討此骨可能之功能。 實驗結果顯示，麝鼩類之夾肌與異位骨直接相連，而無異位骨的台灣長尾鼩，其夾肌則與中線的項韌帶相連，兩類鼩鼱的夾肌皆連到頭骨兩側的人字脊上，因此影響頭部動作。頭部形值測量的結果顯示麝鼩類頭重心在頭部的位置較長尾鼩偏前端，行為觀察的結果顯示在「頭部探索」、「攀爬」、「落地」、「跳躍」、「站立」及「挖掘」等六項可能與頭部動作相關的行為類別中，「頭部探索」行為發生的頻率遠高於其他五者，且麝鼩類展現「頭向前」、「頭向前且抬升」、「頭向上」及「頭搜尋」動作的頻率高於長尾鼩，而在次數比例上，除了「頭搜尋」動作在三物種中皆佔最高比例（長尾鼩54.6±7.1%、灰麝鼩45.6±8.2%、小麝鼩48.3±10.7%）外，灰麝鼩及小麝鼩展現第二高比例的動作是「頭向前延展」（分別為19.8±8.9%及17.9±8.9%），但長尾鼩卻是「頭左右擺動」（21.1±6.3%）。 針對各種頭部動作進行骨骼結構相對位置及力學分析，發現麝鼩類在表現「頭向前」及「頭搜尋」時，頭重心遠離支點（即T2）的程度大於長尾鼩，且在「頭向前」動作時頭頸、頸胸椎夾角可延展的角度大於長尾鼩。力學分析發現，異位骨的存在使麝鼩夾肌的機械效率顯著提高近2倍，使其在大部份頭部動作的表現顯著優於長尾鼩，而有助於麝鼩類克服其原本較偏前端的頭部重心，並展現出頻率較高且延展角度較大的頭部動作。

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.