Samples of human and Macaca mularta cranial bone have been tested quasistatically in tension, compression, simple shear, and torsion. The results of these experiments have been analyzed, taking into account observed anisotrophies and varying structures. Statistical correlations of properties have been made with density and a model proposed that summarizes these results. The cranial bones appear to be transversely isotropic and they are generally much stronger and stiffer in the transverse or tangent to the skull direction in comparison to the radial direction. The structure of the dip& region was found to be highly variable and this strongly influenced many of the mechanical responses. The model, however. explains much of the observed variation.

SUMMARY1. We studied the influences of phase of respiration and breathing frequency upon human sinus node responses to arterial baroreceptor stimulation.2. Carotid baroreceptors were stimulated with brief (0.6 sec), moderate (30 mmHg) neck suction during early, mid, and late inspiration or expiration at usual breathing rates, or, during early inspiration and expiration at breathing rates of 3, 6, 12, and 24 breaths/min.3. Baroreceptor stimuli applied during early and mid inspiration and late expiration provoked only minor sinus node inhibition; stimuli begun during late inspiration and early expiration provoked maximum sinus node inhibition.4. At breathing rates of 3, 6 and 12 breaths/min, expiratory baroreflex responses were significantly greater than inspiratory responses; at 24 breaths/min, however, inspiratory and expiratory baroreceptor stimuli produced comparable degrees of sinus node inhibition. 5. Our results delineate an important central biological rhythm in normal man: human baroreflex responsiveness oscillates continuously during normal, quiet respiration. The phase shift of baroreflex responsiveness on respiration suggests that this interaction cannot be ascribed simply to gating synchronous with central inspiratory neurone activity. Regularization ofheart rate during rapid breathing is associated with loss of the differential inspiratory-expiratory baroreflex responsiveness which is present at usual breathing rates.

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