In addition to the dental characters, rodents have other anatomical features in common. The bones of the lower arm, the radius and ulna, are distinct, and the elbow joint permits free motion of the forearm. The hand usually has five fingers, though the thumb may be vestigial or absent. The toes number three to five. The stomach is variable, ranging from a simple sac to a complex, ruminantlike organ, as in the lemmings.
Rodents have a generalized, rather than specialized, type of brain and placentation.
The tails of the members of some families break off readily when the animals are caught by the tail, enabling them to escape. A partial replacement of the lost portion of the tail then grows. The skin of the tail may also break readily and slip off beyond the break, leaving the flesh and bone exposed. The animal later amputates the exposed portion of the tail with its teeth, and the end heals. The Johns Hopkins University Press.