98 OF VITAL MOTION. 



the muscular fibres of the tubes in question is asso- 

 ciated with oxygenated air, and the contracted state 

 with air in which the stimulant properties are replaced 

 by others which are directly sedative, we may argue 

 (and with some pretence of reason) that the contrac- 

 tion is not stimulated by the oxygen ; and from the 

 occurrence of contraction, independently of the pre- 

 sence of carbonic acid, we may argue also that this 

 gas is not to be regarded as the efficient cause of this 

 phenomenon. In this case, indeed, as in those with 

 which we are already familiar, the contraction would 

 seem to mark the want, and not the presence of the 

 agent. 



It is not intended to say that some gases have no 

 positive action in inducing contraction. On the con- 

 trary, such a result is not unlikely to follow the con- 

 tinued inspiration of dilute sulphuretted hydrogen ; for 

 it would seem as if the action of this poison was to 

 unsettle the unstable equilibrium of the organic solid, 

 and favour the formation of a similar gas from the 

 decomposed materials, to favour, in other words, a 

 putrefactive change of solids into gaseous matter, 

 which must be attended with the lowering of organic 

 heat. Under these circumstances, therefore, the con- 

 traction of muscular fibre may be the direct conse- 

 quence of the operation of the gas ; but under other 

 circumstances, where the quantity of the poison is 

 larger, and the respiration at once arrested, it is more 



