REFLEXES 55 



them. But we shall follow the usual custom and restrict 

 the term "reflex" to responses the capacity for which is 

 inborn or universally acquired, and to acts which occur 

 as surely when the attention is occupied with other 

 matters as when they are themselves under scrutiny. 



Our stock illustrations of reflexes are apt to be those 

 in which brief and abrupt movements are witnessed. It 

 is well to remind ourselves that adaptive changes of this 

 kind may be gradual and sustained in character. One too 

 easily overlooks the more continuous services of the 

 nervous system. Physiologists apply the word "tonic" 

 to such actions as are mild in degree, but maintained over 

 long periods. Under normal conditions the skeletal mus- 

 cles are incompletely relaxed; we say that the nervous 

 system is subjecting them to a tonic stimulation. This 

 subdued but definite activity of the mechanism is the 

 result of the constant inflow of impulses from various 

 sources; it has the reflex character. The slightly con- 

 tracted state of the muscles is spoken of as a "reflex 

 tonus." We speak often of an "arterial tonus," a sus- 

 tained, moderate contraction of the small blood-vessels 

 for which a particular portion of the nervous system is 

 clearly responsible. We may not be warranted in saying 

 that this tonic condition is equally with the other of a 

 reflex sort, but its variations are so, and they constitute 

 an interesting chapter in physiology that which deals 

 with the vasomotor reactions. 



"Conditioned Reflexes."- -Most reflexes seem highly 

 purposeful in the sense that they are such as to con- 

 tribute to the well-being of the animal. In certain cases 

 the purposeful character cannot be recognized and the 

 reflexes appear bizarre and illogical. Some light was 

 cast on reactions of -this eccentric sort by experiments 

 reported from a Russian laboratory a few years ago. 

 The investigators found it possible to yoke a novel 

 stimulus with one which was familiar, and eventually to 

 confer on the novel stimulus much of the efficiency which 

 at first belonged only to the familiar one. The following 



