The Senses and the Nervous System 1 1 1 



prey in parts of the sea where light does not reach. The 

 lateral-line canal, therefore, operates on the principle of a 

 hearing organ, but functions more as an accessor)^ organ of 

 touch. Lacking the abilit)- to imagine just what its messages 

 mean to the fish's brain, we are probably explaining its nature 

 to ourselves as well as we can if we say that it is an organ 

 which combines the senses of hearing and touch. 



XtTY recently another activity of the lateral line has been 

 discovered. It helps the fish to recognize heat and cold. The 

 fish's situation in this regard is difficult for us to imagine, for 

 it is normally at the same temperature as the water in which 

 it lives, whereas we are normally at 98 degrees while the 

 air may be at no degrees or below zero. We are designed 

 to operate at a constant temperature, and when we suffer 

 from cold or heat it is a warning that the temperature out- 

 side our bodies is attempting to invade the inside, and that 

 we had better make efforts either to get warm or to cool off. 

 The fish's sufferings from cold or heat, if any, are obviously 

 different, since he makes no such efforts. When the water 

 is warm, he is warm, and when the water is cold, he is cold. 

 Such discomfort as he feels, if any, is caused by whatever 

 difficulty his insides may have in carr\-ing on their work at 

 high or low temperatures. However, occasion sometimes 

 offers for the fish to go abruptly from colder into warmer 

 water or z'ice versa. If he does this, he is in the condition 

 to which we are accustomed, with his insides at a different 

 temperature from the outside. Lacking our control mecha- 

 nism, his insides then change temperature rapidly, and this 

 is bad for him. The hormonal machinery which helps to 

 adjust his metabolic rate to temperature conditions takes time 

 to get into action. This is why many fish which can stand 

 considerable extremes of temperature when the change is 

 gradual die when the change is sudden. To prevent unwit- 



