104 ANIMAL BEHAVIOUR 



or by pressure (on the surfaces of a joint), or by chemical changes 

 in the tissues themselves. The membranous labyrinth in the 

 internal ear of a vertebrate animal has receptors that are stimu- 

 lated by changes in bodily posture. In most invertebrate animals 

 there are receptors contained in organs called otocysts, or stato- 

 cysts, and these are also stimulated by changes of posture. 

 Thus the agencies that affect the proprio-ceptors are those of 

 the animal body itself and not of the outer environment. 



396. The Nature of a Receptor Organ. Essentially, and 

 in its most simple form, a receptor consists of one or more 

 nerve-terminations (see Sections 12, /, g). But, as a rule, the 

 organ has accessory parts. Thus the essential visual receptors in 

 a vertebrate are the rods and cones in the retina : each of these 

 is connected with a chain of neurones (see Section 40^) which 

 is the peripheral part of the optic nerve tract. But they are 

 disposed, in the eye, as one surface of a membrane or retina, 

 and the ball of the eye, with lens, pupil, iris, etc., constitute 

 the accessory visual apparatus and are instrumental in causing 

 an image to fall on the retina in the same way as the photo- 

 grapher's camera causes an image to fall on the sensitive plate. 

 In a statocyst, as we find it in, say, many molluscs there is a 

 sac which contains a little rounded stone. There are receptors, 

 or nerve-terminations, on the internal wall of the sac. Changes 

 in the posture of the animal cause the little stone, or otolith, 

 to press now upon one side of the sac and again on some other 

 side, and so on. 



Artificial accessory parts. Such apparatus as spectacles, tele- 

 scopes, microscopes, telephones, ear trumpets, spectroscopes, 

 cameras, microphones, wireless receivers, thermometers, etc., 

 are contrivances that extend the range, sensitivity, etc., of the 

 natural accessories of receptor organs. Thus normally a very 

 small quantity of light enters the human eye and so the stimu- 

 lation of the retina may fall below the threshold when a distant 

 star is looked at. But a telescope is a contrivance that interposes 

 a large lens between the distant object and the eye, so that a 

 much greater quantity of light is made to fall on the retina. 

 Thus a subliminal stimulus can be magnified in intensity so as 

 to affect the receptors in the retina. 



In the case of a wireless receiver electro-magnetic waves of 

 very high frequency are concerned. We have no receptors that 



