THE BEGINNINGS OF LIFE. 37 



stimulus calls into play the contractility of a muscle, 

 and so gives rise to a motor act. 



As we have already seen in respect to the muscle, 

 that its contractility lasts for a varying period after the 

 death of the animal, so is it in the case of the nerve. 

 This, after the death of the animal, is still capable of 

 transmitting a stimulus a fact which is shown by its 

 power (when stimulated) of calling into action the muscle 

 to which it is distributed. The precise length of time 

 during which the property survives increases also in pro- 

 portion as the animal is low in the scale of organization. 

 Again, there are many experiments of the most striking 

 kind on record which show the complete dependence of 

 the nervous system upon a due supply of arterial blood. 

 Without this all nerve-functions soon cease in parts 

 thus cut off from their stores of potential energy. The 

 experiments of many observers have shown that, when 

 the posterior part of the body of a mammalian animal 

 has been cut off from its blood supply by ligature of 

 the abdominal aorta, the complete insensibility and 

 disappearance of all reflex^ excitability which soon 

 supervenes, may be made to cease in the course of a 

 few minutes by the removal of the ligature from the 

 main artery. The renewal of the circulation of the 

 blood through the grey matter of the spinal cord re- 

 stores to this and to the paralysed parts generally their 



1 That is to say, the ability to give rise to movements, in response to 

 external stimuli, through the intervention of lower nerve-centres, in- 

 dependently of the action of Will or volition. 



