Opposable Thumb Facts
The human thumb is opposable and prehensile, making it the most unique digit of the hand, responsible for hand functions, grasp, manual dexterity, and fine motor skills
The human thumb, although similar in structure to the other fingers, is opposable and prehensile, making it the most unique digit of the hand and conferring on human beings features such as grasp, manual dexterity, and fine motor skills.
From simply picking up an object to fine skills such as writing and the invention or development of tools, the opposability of the thumb is responsible for most of the hand functions.
The Opposable Thumb and Prehensile Thumb
The thumb can be rotated so that it comes to face the palm and can touch all the other fingers. This property, called the opposability of the thumb, is a unique feature seen not only in human beings but all primates.
Prehensility is the property that makes an organ or appendage suitable for grasping. Organs like hands, claws, tails, and even tongues and noses are prehensile in various species. In primates, the presence of an opposable thumb makes the hand prehensile.
Structure of the Thumb – The Thumb Joint and Bones
The basic bony structure of the thumb is similar to the other fingers. It is made of:
- One metacarpal bone – the bone in the palm which connects the thumb with the wrist
- Two phalanges (singular phalanx) – the short finger bones which actually make up the thumb
The thumb differs from the other fingers in that:
- Its metacarpal is shorter
- It has only two phalanges while all the other fingers have three
The carpometacarpal joint of the thumb, the joint between the metacarpal bone of the thumb and the carpus which connects the metacarpals to the wrist, is a saddle joint. Such a joint allows the metacarpal of the thumb to rotate and is one of the most important factors responsible for opposability.
The muscles of the thumb are:
- Abductor pollicis longus and brevis
- Extensor pollicis longus and brevis
- Flexor pollicis longus and brevis
- Adductor pollicis
- Opponens pollicis
Of these, the opponens pollicis is responsible for opposability. The bulk of these muscles are actually in the palm and forearm and only their tendons lie in the thumb itself. Some of these muscles form the bulge on the palm close to the thumb called the thenar eminence.
The thumb muscles are controlled by all the 3 major nerves of the hand – ulnar nerve, radial nerve, and median nerve.
Movements of the Thumb
The thumb is capable of the following movements:
- Flexion – bending inwards
- Extension – bending or stretching outwards
- Abduction – away from the palm
- Adduction – towards the palm, touching its side
- Opposition – rotation to face the palm and touch the other fingers
- Circumduction – complete rotation
These movements enable grasp required for hand functions like clutching and holding and fine motor skills like writing that require the tip of the thumb to touch the tips of the other fingers.
Evolution of the Opposable Thumb
Opposability of the thumb is thought to have developed in Homo habilis, may have been used for food gathering, and also contributed to the development and use of tools. Research now indicates that HACNS1 is a gene enhancer with a role in the evolution of the opposable thumb.
The opposable and prehensile thumb is responsible for hand functions, manual dexterity, and fine motor skills, all hallmarks of the human species.
Romanes, G.J. “Cunningham’s Manual of Practical Anatomy” (Oxord University Press; 15th Edition, 2003)