Scientific Proceedings (26). 



innocuous the introduction of the various toxic materials employed. 

 It is very probable that in animals debilitated by old age, malnu- 

 trition and disease, the aorta is no longer normal and is probably 

 the seat of minute microscopic lesions such as stretching of the 

 elastica and insufficiency of the muscular fibers. In such animals 

 various toxins introduced into the circulation, especially those 

 tending to raise blood-pressure, will bring out this latent disposition 

 and cause arterial necrosis, calcification and aneurysmatic dilatation. 



27 (283) 



On the influence of various substances, applied directly 

 to the medulla oblongata, upon the respiratory 

 rhythm in frogs. 



By T. BRAILSFORD ROBERTSON. 



[From the Rudolph Spreckles Physiological Laboratory of the 

 University of California."] 



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 C0 2 tension of the blood supplied to the brain at 

 first accelerates and later, if the increase in C0 2 tension be suffi- 

 ciently 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. Accord- 

 ingly 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 



1 Maxwell : Journ. of Biol. Ckem., 1906, ii, p. 183. 



