Anterior Process of the Malleus in Rodents

Abstract

A LARGE anterior process of the malleus (processus gracilis, folianus, or longus of various authors) is known to occur in Monotremata, Marsupialia, Insectivora, Edentata, and Chiroptera1,2. The process is much reduced in higher mammals2. In an investigation of the tympanic region of some mammals it was found that most Egyptian rodents possess a large anterior process of the malleus which in adult individuals completely fuses with the tympanic bone. The process is perforated for the passage of the chorda tympani and may project with a free extremity on the ventral surface of the skull between the bulla and the base of the squamosal. A very long and broad process has been found in Mus musculus (Fig. la), Rattus rattus, R. norvegicus, Acomys cahirinus, A. russatus and Arvicanthis niloticus (Murinae). The process is short and broad in Gerbillus pyramidum (Fig. 16), G. gerbillus, Meriones libycus, Pachyuromys duprasi and Psammomys obesus (Gerbillinae). It is long and narrow in Allactaga tetradactyla, adze-shaped in Jaculus jaculus and J. orientalis (Dipodinee), and is much reduced in Oryctolagus cuniculus (Leporidae).