212 COMPARATIVE PHYSIOLOGY. 



these, or are there any such cells, capable of originating im- 

 pulses in nerves wholly independent of any stimulus from 

 without ? In other words, have the nerve-centers any true 

 automatism ? Apparently this quality is manifested by uni- 

 cellular organisms of the rank of Amoeba. Has it been lost, or 

 has it become a special characteristic developed to a high degree 

 in nerve-cells ? 



We shall present the facts and the opinions based on them 

 as held by the majority of pliysiologists, reserving our own 

 cijticisms for another occasion : 1. The medulla oblongata is 

 supposed to be the seat of numerous small groups of cells, to a 

 large extent independent of each other, that are constantly 

 sending out nervous impulses which, proceeding to certain sets 

 of muscles, maintain them in rhythmical action. One of the 

 best known of these centers is the respiratory. 2. The poste- 

 rior lymph hearts of the frog are supplied by nerves (tenth 

 pair), which are connected, of course, with the spinal cord. 

 When these nerves are cut, the hearts for a time cease to beat, 

 but later resume their action. 3. The heart beats after all its 

 nerves are cut, and it is removed from the body, for many hours, 

 in cold-blooded animals. 4. The contractions of the intestine 

 take place in the absence of foqd, and in an isolated piece of 

 the gut. The intestine, it will be remembered, is abundantly 

 supplied with nerve-elements. 5. In a portion of the ureters, 

 from which it is believed nerve-cells are absent, rhythmical ac- 

 tion takes place. 



Conclusions. — 1. Whether the action of the respiratory and 

 similar centers could continue in the absence of all stimuli can 

 not be considered as determined. 2. That there are regular 

 rhythmical discharges from the spinal nerve-cells along the 

 nerves to the lymph hearts seems also doubtfbl. 3. Later in- 

 vestigations render the automaticity of the heart more uncer- 

 tain than ever, so that the result stated above (3) must not be 

 interpreted too rigidly. 



Similar doubts hang about the other CEises of apparent au- 

 tomatism. 



As regards the various comparatively isolated collections of 

 cells known as ganglia, the evidence, so far as it goes, is against 

 their possessing either automatic or reflex action ; and new 

 views of their nature will be presented in due course. 



Nervous Inhibition. — If the pneumogastric nerve passing 

 from the medulla to the heart of vertebrates be divided and the 



