222 Life and Health 



brain and bring back its orders, the spinal cord is also an 

 independent center for what is called reflex action. By means 

 of its sensory nerves it receives impressions from certain 

 parts of the body, and on its own authority sends back 

 instructions to the muscles by its motor nerves, without con- 

 sulting the brain. This constitutes reflex action, so called 

 because the impulse sent to the spinal cord by certain sen- 

 sory nerves is at once reflected or sent back as a motor 

 impulse to the muscles (Fig. 122). 



< 339. Reflex Action through the Spinal Cord. This reflex 

 action through the spinal cord is a most important matter. 

 This power is possessed only by the gray matter of the 

 cord, the white substance being simply a conductor. 



As already mentioned, certain groups of nerve cells in 

 the medulla oblongata preside over specific functions of 

 animal life. Thus, there are centers for maintaining the 

 action of the heart and the movements of breathing. Low 

 down in the cord also, in the lumbar region, are centers 

 for the control of the various abdominal organs. Numerous 

 other reflex centers are described by physiologists. 

 7^ 340. The Brain as a Reflex Center. The brain, as we 

 have just stated, is the seat of consciousness and intelli- 

 gence. It is also the seat of many reflex, automatic, and 

 coordinating centers. These give rise to certain reflex 

 actions which are as entirely independent of consciousness 

 as are those of the spinal cord. 



These acts take place independently of the will, and 

 often without the consciousness of the individual. Thus, a 

 sudden flash of light causes the eyes to blink, as the result 

 of reflex action. The sudden start of the whole body at 

 some loud noise, the instinctive dodging a threatened blow, 

 and the springing back from sudden danger, are also the 

 results of this reflex action. 



