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1. The blood supply to the medulla was determined by the injection of indian ink via the vertebral arteries. Virtually the whole medulla was supplied by penetrating vessels from the ventral surface. The highest density of small arterioles and venules was found close to the roots of XII and on the ventrolateral surface.

2. The pH of extracellular fluid (pH_e.c.f.) was measured with pH microelectrodes of tip size 1-3 µm in cortex and medulla in seventeen cats, anaesthetized with pentobarbitone or a chloralose—urethane mixture. Parallel measurements were made of the pH of c.s.f. and plasma, the DC potential between plasma and brain and ventilation or phrenic nerve discharge.

3. In the majority of tests under steady conditions, the pH of e.c.f. was found to be lower than that of c.s.f. by between 0·03 and 0·08 units. No systematic pH gradients could be found to a depth of 5 mm beneath the surface of either medulla or cortex.

4. When plasma P_CO2 was altered, pH_e.c.f. changed with a latent period and speed of response related to the density of blood vessels. In vascular areas of the medulla and in the cortex, the latent period of 4 sec and the change of pH_e.c.f. coincided with changes in ventilation. Changes in pH_c.s.f. over the same areas were invariably slower.

5. CO₂ buffering capacities were in the order plasma > e.c.f. > c.s.f. Typical values were respectively, -2·2, -2·1 and -1·6.

6. The pH of e.c.f. was unaffected by the intravenous injection of H+ and only slowly by the injection of HCO₃-. Only up to a depth of 1 mm beneath the surface was pH_e.c.f. affected by superfusion of mock c.s.f. in the range 6·8-8·0 units. This response had a latent period of 2-3 min and was complete in 15 min.

7. The pH of e.c.f. fell with hypoxia after a latent period of > 1 min and if all vasosensory nerves had been cut, pH_e.c.f. was markedly affected by changes of blood pressure.

8. These results indicate that even under steady conditions, the pH of e.c.f. and c.s.f. is not identical, that pH_e.c.f. is more obviously affected by changes in P_{a, CO2} than pH_c.s.f. and that putative H+ sensors which drive respiratory neurones are likely to be similarly affected.

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