214 ANIMAL PHYSIOLOGY. 
the nerve alone and not the skin be stimulated, there may be a 
spasm only and not adaptive movement. 3. Nervous impulses, 
when very abundant, may pass along unaccustomed or less ac- 
customed paths (experiments 4 and 5), This is sometimes spoken 
of as the radiation of nervous impulses, 
The sixth experiment is very important, for it shows that 
the result varies far more with the condition of the nervous 
centers (cells) than the stimulus, the part excited, or any other 
factor. 
Automatism.—But, seeing that these central cells have such 
independence and controlling power, the question arises, Are 
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 Amcba. 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 physiologists, reserving our own 
criticisms 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 food, 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 
action 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 doubtful. 3. Later in- 
vestigations render the automaticity of the heart more uncer- 
