The cephalic index is a rating scale that is used to measure the size of the head, which helps determine if it is normal is size (mesocephalic), abnormally small (microcephaly), too large (macrocephaly), too long (dolichocephaly), or too short and broad (brachycephaly). It is calculated by multiplying the maximum width of the head by 100 and dividing that number by the maximum length of the head. The resulting number provides a ratio of the maximum width of the head to the maximum length of the head. A ratio is the relationship of one quantity to another.
The maximum width of the head is defined as the maximum width of the bones that surround the head, above the supramastoid crest. The supramastoid crest is a rough, raised bone that forms the back part of the zygomatic process. The zygomatic process is a bone that attaches to the temporal bone. The temporal bone is a large bone forms the bottom part of the skull and is near the ears. The maximum length of the head is technically defined as the maximum length from the most easily noticed part of the glabella to the most easily noticed point on the back part of the head. The glabella is a flat triangular bone between two rough, raised bones on the forehead, near the eyebrows. The cephalic index is also known as the length-breadth index.
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The cephalic index is used in anthropometry, which is the study pf human body measurements. Cephalic index comes from the Greek word "kephale" meaning "head," the Greek word "ikos" meaning "pertaining to," and the Latin word "index" meaning "that which points out." Put the words together and you get "pertaining to that which points out (the) head."