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The opposability of the thumb to the finger tips is one of the unique attributes that distinguishes man from all other primates and mammals. The normal human infant is born with the thumb flexed into the palm. This position is more or less retained until the third or fourth month of postnatal life. Voluntary grasping movements then arise to individualize the thumb's activity. The thumb maintains a larger cortical representation in the motor area of the cortex than the other digits. It is extremely sensitive to functional disturbance and therefore of great value in differentiating between the various types of paralysis that involve the nervous system from the brain to the upper extremity.
Patients who have recovered from the acute manifestations of a neurological lesion above the midcervical level usually display characteristic alteration of the thumb position and may readily be screened into groups for special studies or indicated dispensary,
Fay T. THE THUMB AS A CLINICAL AID IN DIAGNOSTIC SCREENING OF PARALYSIS. JAMA. 1954;155(8):729-732. doi:10.1001/jama.1954.03690260021006