NERVOUS SYSTEM. 35 



Cramp. Cramp is a spasmodic shortening of the 

 muscles, attended with pain. 



Tetanus. Tetanus (or locked jaw) is a spasmodic and 

 continuous shortening of the muscles, causing rigidity of 

 the parts they supply. It is due to the disordered and 

 excessive stimulation of the muscles through the nerves. 



Crossing of the Fibers from the Brain to the Spinal 

 Cord. Both the brain and the spinal cord consist of two 

 lateral halves connected by cross fibers. Each half of the 

 brain is connected with the opposite half of the body. 

 This is accomplished by the crossing of the fibers. The 

 fibers that carry nerve impulses outward are now known 

 to cross as they leave the brain, at the very beginning of 

 the spinal cord, in the part known as the spinal bulb. 

 The sensations arising from touching anything with the 

 right hand, therefore, are in the left half of the brain, and 

 the right half of the brain controls the left hand. 



Voluntary Interference with Reflex Actions. We 



have seen that the jerking of the hand away from a hot 

 object is due to reflex action of the spinal cord. One 

 might, by a powerful effort of the will, keep the hand on 

 an object that is hot enough to burn the skin. One may 

 command the foot to remain quiet when it is tickled ; but 

 as soon as the person is asleep, the same stimulations 

 would be followed by the reflex actions such as we have 

 considered. 



In these cases of interference it is understood that the 

 brain sends a nerve impulse down to the centers of the 

 reflex action, and stops or diminishes their operation. 

 This retarding influence of a group of cells is called inhi- 

 bition. It is not always due to voluntary interference, 

 but may be due to reflex interference, as we may see later. 



