102 ANIMAL BEHAVIOUR 



(that is, is exposed to some physical agency, touch, chemical 

 action, heat, etc.). 



Irritability is just a special case of physical inter-relationship 

 among natural things : one billiard ball impinges on another 

 and causes it to move ; the rays of the sun heat up stones, or 

 dry up the water of pools ; a lightning flash may set dry timber 

 afire and so on. In its immediate nature the stimulation of the 

 irritable substance of an organism is no more than this. 



The outer surface of the animal body is especially irritable 

 and exhibits responses to simple contact with other bodies, to 

 changes of temperature, to contact with specific chemical sub- 

 stances, to electric currents and charges, to light, etc. It is 

 proper to think of the whole integument of the animal body 

 as being capable of stimulation by all known physical agencies, 

 but the degree to which a part of the body, or integument, may 

 be irritable to some agencies may be relatively great, while the 

 same part may only be irritable to other agencies in an 

 infinitesimal degree. 



In general there is a " threshold " of stimulation. This 

 means that when the stimulus falls below a certain intensity, or 

 is '' sub-liminal," the part of the body exposed does not display 

 an observable response. But some parts of the skin, or irritable 

 surface, are diff"erentiated so that they respond to very feeble 

 stimuli of some particular physical kind, while they do not 

 exhibit observable response to other physical agencies even when 

 the latter are relatively very intense. Thus the threshold is 

 lowered, in respect of some specific physical agency, in these 

 differentiated parts and it is raised for all, or most other physical 

 agencies. For instance, the retina in the human eye is very 

 sensitive to light but is shielded from most other stimuli ; the 

 skin of the cheeks is very sensitive to heat, but the skin over 

 other parts of the body may be relatively insensitive ; the skin 

 of the face may be " burned " by chloroform but not so that of 

 the bare hands, and so on. Thus general irritability rises to 

 " peak value " in the regions of differentiation. Such differ- 

 entiated regions are receptor organs, or " organs of sensation." 



39^. The Classes of Receptors. Receptors are therefore 

 parts, or organs, that are differentiated, or have structure and 

 special properties in the above sense. They occur everywhere 

 in the animal body, but predominantly in the skin. They may 



