A finger is a limb of the human body and a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates. Normally humans have five digits, termed phalanges, on each hand (exceptions are polydactyly, oligodactyly and digit loss). The first digit is the thumb, followed by index finger, middle finger, ring finger, and little finger or pinky. Some other languages use the same generic term for all five digits of a hand.
English dictionaries describe finger as meaning either one of the five digits including the thumb, or one of the four excluding the thumb (in which case they are numbered from 1 to 4 starting with the index finger closest to the thumb). Linguistically, it appears that the original sense was to include the thumb as a finger: the word is derived from *penkwe-ros[citation needed] (also rendered as *penqrós[citation needed]) which was, in the inferred Proto-Indo-European language, a suffixed form of *penkwe (or *penqe), "five", which has[citation needed] given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve or flow from concepts of fiveness.
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.
The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.