46 Physiology. 



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 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, there- 

 fore, are in the left half of the brain, and the right half of 

 the brain controls the left hand. 



Nervous System compared to a Telegraph System. - 



It is convenient to compare the nervous system to a tele- 

 graph system. Nerve impulses pass along the nerve fibers 

 as electric currents travel along the wires. The ganglions, 

 which receive and send impulses, are similar to the offices 

 which receive and send out electric currents. But there is 

 one important difference : in telegraphing, the same wire 

 is used, both for sending and receiving messages ; while 

 in the nervous system there are two sets of fibers, one for 

 incoming impulses and another for outgoing impulses. 



Harmony in Muscle Action. In throwing a stone a 

 number of muscles are used. Each muscle shortens under 

 the influence of a nerve impulse started by the brain and 

 brought by a motor nerve. If any muscle shortens an 

 instant too soon, or a little too strongly, the stone goes to 

 one side. In a tune on a piano we know that the right 

 keys must be struck; that each must be struck at the 

 right time, with the proper degree of force, and held for 

 the right length of time, or we have discord instead of 

 harmony. What the player is to the instrument, the brain 

 is to the body. 



Effect of Alcohol on the Nerves. Alcohol may, and 

 often does, cause inflammation of the nerves. This dis- 

 ease may be brought on by the so-called " moderate " 



