Fig. 3. Heart rate for Boa constrictor during resting and undisturbed conditions (A), during forced activity (B) and during digestion (C). The figure depicts the untreated control condition (N=8) and the values obtained following autonomic blockade with the ß-adrenergic antagonist propranolol (3 mg kg^–1) and the muscarinic cholinoceptor antagonist atropine (3 mg kg^–1; N=4 for both) and following double autonomic blockade (N=8). The fasting heart rate is also shown for the snakes studied during digestion (C). Values are means + S.E.M. An asterisk indicates a significant difference from the value of untreated and resting animals; a double dagger indicates a significant difference from the value of double-blocked and resting animals.