hand, a stimulus seldom acts upon the body apart from all other stimuli, so 

 that the normal reaction to one stimulus may interfere with the normal re- 

 action to one or more other stimuli. A dog about to seize a piece of meat 

 which he spies might be stopped in his tracks by a simultaneous loud 

 sound. 



The impressions which an organism receives through various stimuli 

 that do not immediately set up the normal reflex seem to register somehow 

 in the brain cells. In this way some of the experiences influence the animal's 

 later activities. It is in some such way that we are capable of learnifig 

 from experience; the delayed or obstructed reaction gives the organism an 

 opportunity to react in one of several ways, and the way ''selected" seems 

 to depend upon previous experience. It is probably in the delayed reaction 

 that the organism makes a beginning at control — control of its own actions, 

 and so in the end control of its environment. 



How Do the Nerves Make the Other Organs Work? 



Nerve Impulse If an efferent nerve that is connected with a gland is 

 detached and applied to a muscle, it can act in its new position to stimulate 

 the muscle. This kind of transposition has been repeatedly carried out in 

 experiments. The results show that the nerve in such cases acts merely as 

 a transmitter of energy or of a stimulus. The nerve apparently has no in- 

 fluence upon the character of the response. An electrical disturbance ap- 

 plied to a motor nerve brings about contraction of the muscle. A me- 

 chanical stimulus, such as that in the statocyst of a lobster (see page 285), 

 brings about movements corresponding to the position of the mova- 

 ble sand grains — that is, to the particular nerve endings that are being 

 stimulated. 



How does disturbance at one end of a neuron bring about a change at 

 the further end, sometimes many inches away ? The transmission is accom- 

 panied by chemical and electrical changes. Perhaps it is a chemical dis- 

 turbance which passes from point to point through the length of the neuron. 

 Or the transmission may be a simple electrical impulse, such as passes 

 through a wire. But apparently it is neither of these. Nor is it like the 

 transmission of a shove through a billiard cue. Nerve transmission seems 

 to be peculiar to protoplasm. And changes in the protoplasm itself take 

 place in the process. 



Voluntary and Involuntary Muscles In the simplest animals the whole 

 protoplasm takes part in receiving a stimulus and in reacting to it. In our 

 bodies, movements are brought about by the contraction of muscles, which 

 make up the "flesh" in all larger animals (see illustration, p. 296). Through 

 the striated skeletal muscles the animal moves about, grasps, gets and chews 



292 



