Second Metacarpal
Long Bone; part of the appendicular skeleton (limb)
Also known as: Hand Bone, Index Finger Bridge Bone, Palm Bone
Classification
Long Bone; part of the appendicular skeleton (limb)
Dimensions
Generally the longest of the five metacarpals, presenting a slender, shaft-like appearance with a noticeably wide base.
Key Features
It looks like a small barbell with one unusually large, notched end (the base) and one rounded, smooth end (the head). The base is often the most complex feature, having multiple flat surfaces where it locks like a puzzle piece into the small wrist bones.
Similar Bones
Often confused with the third metacarpal or the phalanges (finger bones). You can tell it apart from the third metacarpal because the second is typically longer and lacks the sharp, pointy projection found on the base of the third. It differs from finger bones by being thicker and having a much more complex, square-shaped base compared to the simpler joints of a finger.
Across Species
In many mammals, these bones have specialized significantly. In horses, the side metacarpals like this one have reduced into tiny 'splint bones' to allow for high-speed running on one central toe. In contrast, bats have extremely elongated, thin versions of these bones to support their wing membranes, effectively using their 'hand' bones as the structural ribs of a wing.
Evolutionary History
The metacarpals evolved from the basic structural supports found in the lobe-finned fish. Over millions of years, as early tetrapods moved onto land, these bones became more robust to support weight and eventually diversified to handle complex tasks like climbing, digging, and in the case of primates, precision grasping with the index finger.
Photography Tips
To help with identification, take photos from the back of the hand (the flatter side) and the side profile. Use a clear, soft light that highlights the shape of the joints at either end, and place a common object like a coin or a ruler next to it to show the scale.
Notes
What is the highlighted part?