The bicuspid (mitral) valve complex of the human heart consists of functional units which include the valve leaflets, chordae tendineae and the papillary muscles. The mechanical properties of these functional units depend to a large extent on the link between the muscle and the valve. This link is usually arranged in a branching network of avascular tendinous chordae composed of collagen and elastic fibres, which transmit contractions of the papillary muscle to the valve leaflets. In order to perform their function efficiently, the chordae have to possess a high degree of elasticity, as well as considerable strength and endurance. Human chordae tendineae originating from the left ventricles were obtained from 7 embalmed cadavers and 6 postmortem subjects of various ages. Samples washed in saline were fixed or postfixed in 9 % formol saline. Observations were made by illuminating the chordae along their axes. The reflected images originating from the superficial collagenous layers of the relaxed chordae showed a striped pattern 11 µm in width. Scanning electron and light microscopy of the chordae confirmed an undulating pattern of collagen fibrils arranged in bundles of planar waves in register and around the entire circumference of the chorda. The dimensions of the waves correlated with those of the striped reflected pattern. The observed undulating arrangement of the collagen fibrils appears to produce an inherent built-in elasticity which is likely to be of considerable advantage for a tissue which is under continuous repetitive stress. The chordae were covered by endocardium composed of a superficial layer of smooth squamous endothelial cells and an underlying dense layer of elastic fibres. It is suggested that the relaxed striped chordae, consisting of undulating collagen fibrils, straighten when the chordae become stretched by papillary muscle contraction, thereby mitigating the peak stress developed during muscle contraction. On relaxation the elastic tissue tends to return the collagen to its wavy configuration. It is also suggested that the regular wavy pattern of collagen seen in young individuals gradually changes with age by elongation of the wave pattern which eventually becomes randomised. In addition, with increasing age, substantial cushions of connective tissue appear below endocardium while the dense collagenous core has a reduced cross-sectional area which may lead to stretching and eventual rupture of the chordae.

Sl'~DIA!n' III illfallts IIlld pz/;lets sulijected to periods of circulatory arrest at ;!()GC there u.'as close corrclatiol1 bet1£'eell duratioll of arrest alld de/a)' in return of c/ectroel1ceplza/ographic actit'ity. Post mortem studies demonstrated histological lTidelIce of brain damage in patients after circulatory arrest at 20°C. Similar histolor;ical changes lt'ere demonstrated in piglets, including some wlzo had complete I,'.E.(;. and clillical rcc07'ery fro/ll circl/latory arrest. In a pre\'ious stud~' of the effects of circulator~' arrest at :WcC in the infant pig (Fisk et al. ] 974), we found lesions in the cerebral cortex of pigs subjected to circulatory arrest for a period of one hour at 20°C and then allowed to survivc for 1 0 da~'s before being killed. These lesions were not found in pigs subjected to continuous perfusion at 20°C or at ;)72C. \\'e suggested tha~ the delay in return of electrocncephalogram (E.E.G.) activity after such circulatory arrest may be a useful index of the degree of ischaemic cerebral insult. BreclmtT et al. (HI;)~I) showed an eXjlonential relationship between the period of circulatory arrest at :~OcC in patients undergoing open-heart surgny and the time taken for return of continuous E.E.G. activity. They suggested that this provided a guide to the severity of the ischaemic insult of the brain. In this pillwr we report on E.E.G. activity and postmortem histological examination of the brain in infants SUbjected to circulatory arrest for open-heart surgery, and on the results of a study of infant pigs subjectcd to various periods of circulatory arrest at 20°C. \\'c have attempted to "correlate the period of arrE'st and the associated delay in return of E.E.C. activity with the presence of detectable histological changes in the cerebral cortex in surviving piglets.

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