EETARDATION. 281 



ments the best known, it is on these that the current 

 views as to the retardatory nerves are based. It has 

 been explained in § o that the respiratory movements 

 result from the excitement of the nerve-cells of the re- 

 spiratory centre. These movements may be accelerated 

 or retarded, though all the other conditions remain 

 unchanged, if certain nerve-fibres which pass from the 

 mucous membrane of the air-passage to this respira- 

 tory centre are irritated. These retardatory nerves 

 are distinguished from those which pass to the heart 

 by the fact that it is not known whether the latter pass 

 to the muscles of the heart or to the nerve-cells 

 situated in the heart, a doubt which is satisfied in 

 the case of the former by their anatomical arrange- 

 ment. Of the retardatory fibres of the heart it might 

 be supposed that they in some way incapacitate the 

 muscle from contracting ; in the case of the retar- 

 datory nerves of the respiratory system such supposi- 

 tion may be at once rejected, for they are in no way in 

 contact with the respiratory muscles. The only pos- 

 sible explanation is therefore, that the retardatory 

 nerves act on the nerve-cells in which the excitement 

 is generated, thus either preventing the excitement from 

 even coming into existence, or preventing the excite- 

 ment from passing from the nerve-cells in which it is 

 generated to the appropriate motor nerve-cells. For 

 various reasons the latter view has been preferred. It 

 is supposed that the automatically acting ganglion-cells 

 are not directly connected with the appropriate nerve- 

 fibres, but that conducting intermediate apparatus are 

 present between the two, and that these offer a great 

 resistance. This explains both the occurrence of the 

 rhythmic motions and the retardation. The latter, 



