THE SPINAL CORD 279 



sensory nerves are reflex acts. Reflex action is designed 

 to protect the body from injury and to supply its needs. 

 Most acts of the cord are reflex. When the finger touches 

 a hot object, the sensory nerves carry the sensation to the 

 cells of the spinal cord and to the brain. Before the sen- 

 sation reaches the brain, the spinal cord sends out an order 

 for the muscles to move the finger away from the heat. 

 The brain becomes conscious of the burn and of the move- 

 ment of the finger at about the same time. In this way 

 the spinal cord protects the body against all kinds of 

 injuries. 



504. Reflex action in relation to nutrition. Digestion 

 is mainly a reflex act. Motor impulses for glands to pro^ 

 duce the digestive juices are sent out from the spinal cord 

 when the sensory nerves bring word that food is present 

 in the stomach or intestine. Peristalsis is also a reflex act 

 dependent upon the presence of food. The sensory nerves 

 also carry to the spinal cells news of the temperature of 

 a part and of its need of more or of less blood, and, in 

 response, the cells send out motor impulses for the arteries 

 to change their size. The heart is also somewhat affected 

 in a reflex way. The sensations of exertion and fatigue 

 are carried to the spinal cells, which send out orders for 

 more rapid heart beats. Fear, joy, anger, and sorrow all 

 affect the heart in a reflex way. 



The growth of each separate cell is controlled by the same set 

 of spinal cells that produce motion in a part. Muscle cells, especially, 

 need the constant stimulus of the spinal cells to keep them growing, for 

 otherwise they slowly waste away and become weak. The spinal cord 

 is continually overseeing the nutrition and growth of cells, and if it 

 were to cease its oversight, their death would soon take place. 



505. Reflex action in habitual movements. The reflex 

 action of the cord aids in performing simple movements of 



