The reflex respiratory effects on cats of breathing through a tube.

1. In anaesthetized cats, breathing air through an external dead space (a tube, 20-40 ml.) stimulated ventilation in excess of that expected from the end-tidal gases. 2. The stimulatory effect remained after bilateral vagotomy. 3. The stimulatory effect was reduced or abolished after bilateral section of the carotid sinus nerves. 4. The stimulatory effect was abolished by bilateral section of both the vagi and the sinus nerves. 5. Recording from chemoreceptor afferents in the sinus nerve showed that, although the tube increased mean discharge frequency, this increase was insufficient to account for the stimulation of ventilation observed. 6. Some feature of chemoreceptor discharge other than its mean level, possibly related to its wave form, may explain the effect.