THE NERVES AND SPINAL CORD 



a 



A thin slice from the end of a cut 

 nerve ( X 200) . 



a nerve thread. 



b connective tissue binding the 

 threads into a cord. 



call nerves. In the upper part of the arm or leg they are 



as large as a knitting needle, and grow smaller as threads 



are given off to the cells. There 



are more of these nerves in the 



skin than in any other part of 



the body, but yet a thread 



reaches every cell of the body. 



218. Motor nerve messages. 

 Nerves can carry messages 

 both from the mind to the cells, 

 and from the cells to the mind. 

 The mind sends messages to 

 the cells for each to do its own 

 kind of work. Thus it tells the 

 muscle cells to move the arm 



or leg. It tells the salivary glands to make saliva and 

 pour it into the mouth. It tells the liver cells to change 

 digested food to blood. 



The mind also tells every cell in the body how much to 

 eat, and how much oxygen to breathe. The cells can eat 

 and breathe without being told by the mind, but if the 

 nerves do not bring the messages from the brain, the cells 

 do as they please about eating, and sometimes get lazy and 

 hardly eat or breathe at all, but waste away as in a para- 

 lyzed man. 



Nerves which carry messages from the brain to the cells 

 are called motor nerves, for orders to move or change in 

 shape are often sent to the cells. These messages are 

 continually coming and going, and when they stop, life 

 ends at once. 



219. Sensory nerve messages. The cells of the body 

 also send messages of their state and needs. They send 

 messages to the mind whenever anything touches them. 



