THE NERVOUS SYSTEM 



219 



Reactions of the kind mentioned are called 

 reflexes. They take place in direct response to 

 some stimulus, without any intention or desire, 

 and they cannot be prevented. Some reflexes 

 are useful to the organism ; probably none are 

 injurious, although we sometimes wish we 

 could control them. For example, a person 

 takes something into his mouth and realizes 

 just after it gets back of the tongue that it is 

 poison ; but he cannot help swallowing it, no 

 matter how much he may wish to. Vojniting, 

 the reverse of swallowing, sometimes takes 

 place against our will. Like other reflexes, it 

 is entirely beyond our control. Reflexes do not 

 always show themselves in movement. When 

 the funny bone is struck, we become aware of 

 a tingling sensation in the palm of the hand. 

 Many reflexes work out through glands, as we 

 have already seen. Some reflexes take place in 

 organs of which we are never conscious unless 

 they are disordered ; examples are movements 

 in the digestive system, breathing, the heart- 

 beat. They go on just as well during sleep as in 

 our waking hours, and in many cases they go 

 on just as well in the absence of the brain or 

 Avhen the connection between the organs and 

 the brain has been cut. No matter how useful 

 such actions or reactions appear to be, reflexes 

 do not represent the desires or intentions of the 

 organism. We do not do these things "on pur- 

 pose." We do them because our nerves are 

 connected in a certain way. 



180. Nerve connections. The reflex rests upon 

 a comparatively simple connection between 

 ( I ) a nerve cell acting as a receptor, or stimulus- 

 receiving structure, and (2) a muscle (or 



ij 



Fig. no. Affer- 

 ent and efferent 

 nerves 



Disturbance of a 

 sense organ S, con- 

 nected with an af- 

 ferent nerve A 

 may set up dis 

 charges in several 

 nerves. There may 

 be a muscle reflex 

 through the effer- 

 ent nerve £j, con- 

 nected with a mus- 

 cle; a gland reflex 

 through the effer- 

 ent nerve £o, that 

 is connected with 

 a gland: and a sen- 

 sation, or a feeling, 

 through a disturb- 

 ance of a brain cell 

 5 by a discharge 

 through the con- 

 nected neuron Ac, 



