Question #15: What is the difference between an arm and a leg?

Why is it that we say dogs have four legs, humans have 2 legs and two arms and that octopi have 8 arms?

This is actually a very simple concept. Legs are used, exclusively, for walking, running and standing. Quadrupeds, those animals that walk on all four limbs, have four legs. Insects, like grasshoppers, have six legs and arachnids, like spiders, have eight legs.

Humans and some other animals such as apes and birds have two legs and two arms. We use two of our limbs for walking and two for other purposes. In humans and apes we use our arms for grabbing and manipulating things in our environment. Birds use their arms, we call them wings, for flight.

We call the appendages of octopi and squid arms because like humans they use these appendages for grasping and manipulating the environment. So, octopi have eight arms and squid have ten arms.

Then there are some odd animals like praying mantis. What do we do now? They appear to have four walking legs and the two forward appendages that are not always used for walking but catching prey. Praying mantis use their front appendages for walking when they are not catching prey so these are legs. Praying mantis therefore have six legs.

BUT WAIT.... it gets more complex.

When it comes to animals with internal skeletons anatomy takes over the definition of what is an arm and what is a leg. The definition is based on both joints and bones. Lets take a look at some ideas that define arms and legs via skeletal anatomy.

Shoulder and hip joints.
Shoulder and hip joints are very different. Shoulder joints attach front or fore limbs to the rest of the body allowing those limbs to move. Shoulder joints involve the humerus, upper arm bone, and the shoulder blades, as well as other bones which vary depending on the animal. Hip joints attach the lower or rear limbs to the body and involve the pelvis and femur.

The shoulder is a synovial ball and socket joint. This means that the joint has a sack of liquid, called a bursa filled with synovial fluid, around the joint area which allows it to glide more easily. (Like lubricating parts in a machine to help them move freely) Ball and socket joints allow for full circular movement, try it with your own shoulder (or hip as we will talk about soon)

Shoulder joint of a human (thanks to Brown University)

Quadrupeds, those animals with four legs or appendages for walking have restricted shoulder joints, which means they can not move as freely. If you are around a dog or cat you quickly notice their forelimbs, front legs, tend to only move from front to back and rarely directly out to the side. This is due to the restriction of the joint and the position of the bones involved. Compare the human shoulder above to the quadruped shoulder below.

Horse anatomy

There are two things to notice. The positions of the humerus and scapula, shoulder blade, are different. In humans, our shoulder blade is next to/behind the shoulder joint. In horses, the scapula is stacked on top of the humerus. This stacking offers greater weight bearing ability, necessary for carrying a body, but less mobility. Want more about the shoulder check out this site.

What about Pelvis and Femur?

The hip joint is also a synovial ball and socket joint formed by the head, top protrusion, of the femur sitting in a cup shaped cavity in the pelvis. Hip joints tend to be a more freely moving joint in all animals, both bipeds like humans and quadrupeds like horses. However, the same stacking effect happens for weight bearing. The hips of a quadruped are narrower and sit on top of the joint while the hips of a biped are wider and the joint is out to the side. More on the human hip joints can be found here. Compare the human and horse again below.

Human hip joint. In cross section to see the head of the femur sitting in the joint.

Horse skeleton showing the hip joint.

What is an arm and a leg?
There are two different answers.

1. Arms and legs defined by function. Legs are for walking and arms are for grasping and specialized tasks.

2. Arms and legs defined by anatomy of animals with internal skeletal systems. Arms have a shoulder joint where the shoulder blade, scapula, and the humerus met. Arms contain other bones including the radius and ulna, lower arm bones. Legs have the hip joint where the pelvis and femur meet. Legs contain additional bones in the leg, tibia and fibula.

These terms have two definitions, mostly, because not all animals have internal, bony skeletons. If we defined arms and legs solely on the second definition related to bones many animals... insects, cephalopods, crabs and others would need additional anatomical words to describe their appendages.