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Experiments were carried out in cats anaesthetized with chloralose, (a) to examine the effect on blood pressure, heart rate and respiratory frequency produced by topical application of leptazol, nicotine and sodium pentobarbitone to the ventral surface of the medulla at an area around the rootlets of the XII cranial nerve, and (b) to study the role of this area in some cardiovascular reflexes. Leptazol applied uni- or bilaterally to this area produced hypotension, bradycardia and bradypnoea. The area from which leptazol produced these effects was localized 3-6 mm lateral to the mid line and 5-9 mm caudal to the lower border of the trapezoid bodies. When comparing the effects of leptazol and nicotine applied to this area it was found that in concentrations that produced similar falls in arterial blood pressure and heart rate leptazol produced a much stronger bradypnoea than nicotine. The hypotension produced by leptazol was mainly due to inhibition of sympathetic vasomotor tone since it was little affected by section of the vagi and by atropine given intravenously. Bilateral application of sodium pentobarbitone produced a small hypertension, tachycardia and pronounced tachypnoea. Unilateral application of sodium pentobarbitone had no effect by itself but inhibited the effects of leptazol applied to the same site. Cardiovascular reflexes produced by sinus nerve stimulation, by increased sinus pressure or by injections of veratridine into a vein or into the left ventricle of the heart were potentiated by topical application of leptazol to the ventral surface and depressed by the topical application of sodium pentobarbitone. The chemoreceptor reflex, produced by retrograde injections of lobeline into the lingual artery, was partially affected by topical application of sodium pentobarbitone: the evoked bradycardia was attenuated but the tachypnoea and hypertension were not affected. These results suggest that this medullary area on the ventral surface of the medulla plays an important role in normal cardiovascular regulation.

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