Frequently used in both OT and NT, but rarely in the physiological sense, since the Hebrews were unaware of its function; but its importance was recognized, and so it acquired a metaphorical sense for what is dearest and best—a ‘clean heart’ (Ps. 51: 10; Ezek. 36: 26). The heart was also regarded as the seat of emotions (1 Kgs. 8: 38), of memory (1 Kgs. 4: 29), and of wisdom (NRSV prefers ‘mind’ instead of ‘heart’: 1 Kgs. 3: 12), and the new covenant will be written in the hearts of the people (Jer. 31: 31–4).
Similarly in the NT, Jesus is ‘gentle and humble in heart’ (Matt. 11: 29). The ‘pure in heart’ (Matt. 5: 8) are those without an accusing conscience; for ‘God searches the heart’ (Rom. 8: 27) and being ‘cut to the heart’ is the prelude to conversion (Acts 2: 37). In Paul's analysis of human nature the heart is neutral: it may be evil (Rom. 1: 21), but it may also be good (2 Cor. 1: 22), and serves sometimes to refer to the whole human personality.
Modern researchers suggest that the region of the brain which deals with memory and emotions can be so disturbed that it destabilizes the rhythm of the heart—hence the expression ‘a broken heart’ (e.g. Ps. 51: 17) is more than a mere metaphor.