On the influence of various substances,applied directly to the medulla oblongata,upon the respiratory rhythm in frogs

Abstract

Recent experiments by Maxwell 1 have shown that the nerve cells in the cerebral cortex are not stimulated by the ordinary nerve stimulants, such as oxalates, citrates, tartrates, etc. On the other hand we are in the possession of the well-known fact that an increase in the CO₂ tension of the blood supplied to the brain at first accelerates and later, if the increase in CO₂ tension be sufficiently great, inhibits the rhythmic discharge of impulses from the respiratory center or centers. It appeared possible that although the ordinary nerve stimulants do not excite nerve cells yet some other group of substances might be found which does so. Accordingly the following experiments were undertaken.

The roof of the skull, in frogs, was removed by means of a fine pair of bone forceps as far down as the tip of the fourth ventricle; in some instances the membrane covering the floor of the fourth ventricle was removed, in others not. The cerebrum was removed by cutting across the thalamencephalon just in front of the optic lobes; by this means it was found that a more regular respiratory rhythm was obtained, the frog was quieter, and the results were more uniform. The cavity left in the skull by the removal of the cerebrum was then plugged with absorbent cotton and absorbent cotton was placed all round the wound so as to absorb any fluid which might escape from the depression left by the removal of the roof of the skull. With practice the operation could be performed in 3 to 4 minutes, and, if successful, within 3 to 5 minutes after the operation respiration, which at first is suspended, is resumed. After about 5 to 10 minutes, the rhythm of respiration usually becomes constant and, if the frog be left untouched, remains so for over two hours.