136 ANIMAL SENSE ORGANS 



Receptors or Sense Organs are made up in the higher 

 animals of groups of cells in which specialization has 

 taken the form of intensifying the property of irritability 

 at the expense of other fundamental properties of proto- 

 plasm. The cells of the sense organs carry on, of course, 

 regular basic metabolism, but have no functional metab- 

 olism so far as we know except such as is involved in that 

 alteration in the state of the protoplasm which results 

 from the stinmlus, and after the disturbance has passed 

 on to other parts of the body the activity of the sense 

 organ comes to an end until it is stimulated again. 



Classes of Sense Organs. — The common method of 

 classifying the sense organs is in accordance with the 

 kinds of impressions to which they are particularly 

 susceptible. In accordance with this scheme, we find 

 that in all the animals except the very simplest we have 

 two main classes: (a) internal; (b) external. As the 

 names imply, the internal sense organs are located inside 

 the body and are acted upon by stimuli arising within 

 the body; the external receptors, on the other hand, lie 

 on or near the surface and are acted upon by stimuli 

 which are exterior to the body. These external stimuli 

 may be of various kinds. In the main, they consist either 

 of changes in the environment immediately in contact 

 with the body or in changes in the environment at a dis- 

 tance which in some way are able to impress the sense 

 organs. Those receptors which respond to immediately 

 adjacent changes are called contact receptors. For ex- 

 ample, the organs of touch are affected only by agents 

 which come against the body. Those receptors which are 

 affected by disturbances at a distance are called distance 

 sense organs. The organs of sight and hearing are illus- 

 trations of these. 



Internal Sense Organs. — We recognize in the higher 

 animals at least five kinrls of internal sense organs. Some 

 of these are known to be present also in many of the lower 

 animals; with regard to others we are not quite so sure. 



