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1. The anatomy of the carotid body and its afferent nerve supply was studied in the duck and a method of denervating the carotid body which ensures a satisfactory post-operative course is described.

2. The effect of denervating the carotid body upon the cardiac response to immersion of the head in water was studied in ten ducks which at the time of the test were unanaesthetized.

3. When the nerves were intact, immersion of the head caused a fall in heart rate after a latent period of between 1 and 9 sec to an average of 24% of the resting rate after 30 sec. Simultaneous measurement of arterial oxygen tension (P_{a, O2}) in the brachiocephalic artery showed a rapid initial fall during the initial 10 sec from control levels, 93-103 mm Hg, to between 42 and 47 mm Hg, followed by a gradual fall of 3-5 mm Hg for each subsequent 30 sec period of submergence.

4. Following carotid body denervation, the latent period before heart rate started to fall was no different from control but the average fall in heart rate was now to 90% of the resting rate and brachiocephalic P_{a. O2} continued to fall steadily during submergence reaching levels of between 10 and 21 mm Hg by the end of the second minute.

5. Stimulation of the central end of branches of the IXth (glossopharyngeal) nerve supplying the glottis caused apnoea and bradycardia.

6. It is concluded that apnoea and bradycardia during submergence in the duck is initiated reflexly from receptors in the nares, pharynx and glottis but that the profound degree of bradycardia and mechanisms which maintain a relatively high P_{a, O2} are regulated by peripheral chemoreceptor activity.

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