52 VETERINARY PHYSIOLOGY 



directly on the cell protoplasm, e.g. an amoeba, when touched, 

 draws itself together. But, even in these simplest organisms, 

 certain kinds of external conditions will produce one kind 

 of change, while others will produce a different one, as has 

 been shoAvn in considering unilateral stimulation (p. 25). 

 Even among unicellular organisms— e.g. among the 

 infusoria — animals are found in Avhich the cell is differ- 

 entiated into a receiving and a reacting part. Foteriodendron, 

 a little infusorian sitting in a cup-like frame, consists of a 

 long process or cilium extending up from a cell, while a 

 contractile mvoid attaches the cell to the floor of the cup. 

 When the cilium is touched the myoid contracts, and 

 draws the creature into the protection of its covering 

 (fig. 16). 



In multicellular organisms the result is secured by the 

 development at the surface of the body of special structures 

 or Receptors, each kind of which is stimulated or made to 

 undergo a change, more especially by some one kind of 

 external change. 



Thus one kind is specially stimulated by the contact of 

 gross matter, another by the addition of heat, another by its 

 withdrawal ; another kind is specially acted on by the 

 vibration of air called by physicists " sound waves," another 

 by the ethereal " waves of light " ; yet another by the 

 presence of substances suspended in air or in solution in 

 water. 



But before the reaction of the body upon its surroundings 

 can be appropriate, the effects of these various changes must 

 be brought together and harmonised and integrated, and 

 this is secured by the existence of strands of living matter, 

 nerves, which pass from each receptor and lead to a common 

 or to several common receiving arrangements or stations in 

 the central nervous system. 



The combined effect of all the different stimuli from 

 without call into play a series of the other stations in the 

 central nervous system which set in action the great re- 

 acting or effector arrangements, the Muscles. 



It is impossible to study the mode of action of the 

 receptors without also considering the mode of action of the 



