Bhullar Bhart-Anjan S
Department of Geological Sciences, University of Texas, Austin, Texas, USA.
Anat Rec (Hoboken). 2009 Aug;292(8):1154-61. doi: 10.1002/ar.20955.
The abdominal muscles of lizards and snakes (Squamata) have been the subject of periodic attention from anatomists, embryologists, and systematists. Until now, the presence of a superficial portion of the m. rectus abdominis, named the m. rectus abdominis lateralis, has been considered a key synapomorphy of the clade Autarchoglossa, which includes all extant squamates save Gekkota and Iguania. However, the precise anatomical relations of the m. rectus abdominis lateralis have never been fully investigated. Here, I show that the m. rectus abdominis lateralis is present in Iguania. Its absence in Gekkota represents rare gross anatomical support for recent molecular-structure-based hypotheses of squamate relationships placing geckoes as sister to the remaining squamates. Where present, it is the most superficial trunk muscle, exterior to the m. obliquus externus. The separation of the m. rectus abdominis lateralis from the m. rectus abdominis occurs as the m. obliquus externus aponeurosis and part of the m. obliquus internus aponeurosis emerge superficially to form the outer portion of the rectus sheath. In Autarchoglossa, the contralateral mm. recti abdomines laterales meet at the midline and are attached to the imbricae of the transverse scale rows characteristic of the clade, suggesting developmental, functional, and evolutionary association. Because the m. rectus abdominis lateralis is sometimes continuous with the pectoralis, its exclusive association with the m. rectus abdominis is questionable. It may be a neomorphic layer that is part of the abaxial developmental system, comprising those muscles whose connective tissue is largely derived from lateral plate as opposed to somatic mesoderm.
蜥蜴和蛇（有鳞目）的腹部肌肉一直是解剖学家、胚胎学家和分类学家定期关注的对象。到目前为止，腹直肌的一个浅表部分，即腹直肌外侧肌，一直被认为是自舌亚目分支的一个关键共衍征，自舌亚目包括除壁虎科和鬣蜥科之外的所有现存有鳞目动物。然而，腹直肌外侧肌的确切解剖关系从未得到充分研究。在这里，我表明腹直肌外侧肌存在于鬣蜥科中。它在壁虎科中的缺失为最近基于分子结构的有鳞目关系假说提供了罕见的大体解剖学支持，该假说将壁虎视为其余有鳞目动物的姐妹群。在存在腹直肌外侧肌的地方，它是最浅表的躯干肌肉，位于腹外斜肌之外。腹直肌外侧肌与腹直肌的分离发生在腹外斜肌腱膜和部分腹内斜肌腱膜浅面出现以形成腹直肌鞘外侧部分之时。在自舌亚目中，对侧的腹直肌外侧肌在中线处会合，并附着于该分支特有的横向鳞片行的叠瓦状结构，这表明存在发育、功能和进化上的关联。由于腹直肌外侧肌有时与胸肌连续，其与腹直肌的排他性关联值得怀疑。它可能是一个新形态层，是轴下发育系统的一部分，该系统包括那些结缔组织主要源自侧板而非体壁中胚层的肌肉。
