io6 ANIMAL BEHAVIOUR 



in the air inhaled is far too small to admit of a purely chemical 

 test. Perhaps the detection of electro-magnetic radiation by the 

 grid of a thermionic valve that is included in a wireless receiver 

 involves smaller quantities of energy than in most physical tests, 

 yet it is probable that the analogous organic process — the 

 affection of a synapse by a nervous impulse (see Section \id) 

 is far more delicate. 



The '' reception," in the strict sense, involves the nerve- 

 terminations in the sense-organ, and there are many examples 

 of bare and simple reception in this sense. But in the higher 

 animal these conditions are superadded to such simple reception. 



/. The accessory parts of the sense-organs amplify^ block ^ 

 analyse, or otherwise modify the energies that fall upon them. 

 Thus taste-buds on the tongue amplify the chemical changes 

 that are the basis of the sensation of taste : we do not taste 

 many things on the lips. These receptors also analyse, so that 

 there are many different kinds and degrees of taste. So also 

 with smell. The accessory parts of the eye act so as to set up 

 an image on the retina such that there are parts in this image 

 which have '' one-to-one " correspondences with the parts of 

 the environment that are in the field of view of the eyes. The 

 organ of Corti in the internal ear analyses the total body of 

 sound that is conveyed into the perilymph surrounding it so 

 that a multitude of vibrations of different frequencies are received 

 separately by the nerve-terminations. And so, on the other 

 hand, small changes of temperature do not affect the retina, nor 

 the pain spots in the skin, nor the pressure receptors in the joints, 

 etc. The multitudinous agencies acting on the animal body are 

 therefore partially isolated from each other, minimized or magni- 

 fied and are received, to some extent, apart from each other. 



a. The receptors are localized in the animal body. In many 

 animals all the external parts may be equally irritable, or generally 

 receptive — this is probably so in the case of an Amoeba, for 

 instance. But in the amoeba, and to a less extent in many other 

 animals there is no very pronounced orientation of the bodily 

 parts. In the vertebrate animals, however, the body typically 

 moves so that the cranial extremity precedes and so there are 

 right and left sides. It moves on limbs so that there are upper 

 and lower surfaces. There is paired symmetry in most organs 

 (right and left limbs, lungs, kidneys, etc.). Therefore the 



