1803 J. Barclay NewAnat. Nomencl. 174The position of the heart in the thorax will be expressed by the two compounds mesio-sinistral and atlanto-sacral. 
1872 Humphry Observ. Myology 107Especially is this the case..in the ‘mesio-dorsal’ part of the lateral muscle. 
mesio-
combining form 
Etymology: mesial + -o- 
 : mesial and 
< mesiobuccal >
< mesiolabial >
< mesioocclusal >
< mesiobuccal >
< mesiolabial >
< mesioocclusal >