APPLIED PHYSIOLOGY 



Diagram of the origin of nerves in 

 the spinal cord. 



tinguished. These spinal nerves supply the whole body 



below the neck. 



502. Action of nerve cells in voluntary motion. The 



spinal cells do/ not originate impulses or act of their own 



accord, but they act only when 

 ordered to do so by the brain 

 or when the cells of the body 

 express a need of protection, 

 nutrition, or rest. When a 



a motor nerve root from the front part person wishes to lift his hand, 



of the nerve. -L v i i 



b sensory nerve root from the back hlS bram SGnds an Order to 



part of the cord. these nerve cells, and they in 



c gray matter of the cord. j ., -, 



d white matter. turn send th e order over the 



nerves to the muscles which 



move the hand. In this way the mind sends all orders 

 for voluntary motion. For each muscle there is a sepa- 

 rate group of spinal cells. If these cells are injured or 

 destroyed, a person cannot move his muscles voluntarily. 



a tack pricking the hand. 



d motor nerve. 



Diagram of reflex action. 



b sensory nerve. c nerve cell in the spinal cord. 



e muscle moving the hand. 



503. Reflex action. The spinal _clie- * also send out 

 orders in response to impulses brought by sensory nerves. 

 Motor impulses sent in response to influences brought by 



