Fig. 2. The effects of chronic hypercapnia (CHC) and olfactory denervation (OD) on (A,B) fictive breathing frequency (fictive breaths min^-1), (C,D) the number of fictive episodes min^-1 and (E,F) the number of fictive breaths per episode recorded from in vitro brainstem-spinal cord preparations taken from control (A,C,E) and CHC (B,D,F) toads. Open symbols represent preparations taken from animals with the olfactory nerves intact (OI). Filled symbols represent preparations taken from animals in which the olfactory nerves were cut (OD) prior to CHC. The data are plotted as the mean ± 1 s.e.m. Letters (a-c) represent significant differences between ventilatory parameters measured at different artificial cerebral spinal fluid (aCSF) pH levels within any group. A number sign (#) represents a difference between the OI and OD groups. A plus sign (+) represents a difference between the control and CHC groups (comparing panel A with B, C with D and E with F).