ORGANS OF THE SENSORI-MOTOR SYSTEM 103 



be simple, or provided with accessory parts. They may receive 

 stimuli originating in the substance of the bodily parts, or in 

 the cavities of the body, or from things outside the body but in 

 contact with the latter, or from things situated at very great 

 distances from the body. We thus make classes of receptors. 



i. The Distance-Receptors . These are the visual organs^ or 

 " eyes," which are affected by radiation of a certain, very limited 

 range of frequency, originating perhaps in the most remote 

 parts of the universe ; the auditory organs which are affected 

 by the vibrations of material bodies, such as sound-waves in 

 the atmosphere or in water : such vibrations have much lower 

 frequency than has light-radiation ; temperature-organs in the 

 skin which are affected by radiation of lower frequency than 

 that of light- waves. The distance-receptors place the organism 

 " in touch " with physical events that occur far away. 



ii. The Near- Receptors. Such are touch, or tactile organs in 

 the skin and elsewhere — these are affected by contact with 

 material things ; Taste-organs in the tongue and palate, chemical 

 receptors in the skin of lower animals, olfactory organs in the 

 mucous membranes of nasal cavities — these three kinds of 

 receptors are affected by some chemical (soluble) substances 

 that come into actual contact with them and set up chemical 

 reactions in addition to mere contact ; temperature organs in the 

 skin and elsewhere which are affected by actual contact with 

 hot or cold material objects. 



Hi. Intero- Receptors. The walls of the cavities of the 

 animal body (of course we refer particularly to the body of the 

 mammal) are irritable, that is, they contain receptors. These 

 internal cavities are the alimentary canal (but we have already 

 considered the mouth and pharynx) ; the pericardial and pleural 

 cavities ; perhaps the cavities of the heart and blood- 

 vessels ; the bladder, ureters, urethra and vagina. General 

 sensation, and pain, may be the results of stimulation of these 

 receptors. 



iv. The proprio-ceptors. In most of the cases already con- 

 sidered the receptors are stimulated by things and agencies that 

 are really outside the bodily tissues, but in the cases of the proprio- 

 ceptors this is not so. There are receptors in the muscles and 

 joints which are stimulated by variations of tension (of a muscle). 



