xvii THE NKRVOL'S SVs I IN! ,85 



cuts across the spinal cord, or so damages it thut a certain 

 length of it is more or less completely severed from, or at least 

 can no longer conduct impulses from the brain. If the injury 

 is in the middle of the back, the man will be unable to move 

 his legs or any part of his body which is supplied by nerves 

 which arise from the cord below the level of the injury ; all 

 parts below the level of the injury will be " paralysed." He 

 will also be unable to feel the prick of a pin or a hot coal 

 applied to his legs or to any part below the injury ; that is, 

 there is a loss of sensation in all parts below. It is one 

 of the functions of the spinal cord to transmit up to the 

 brain the impulses it receives by the sensory nerves ; when 

 the cord is severed from the brain these impulses can no 

 longer pass upwards. Again, when a man wishes to move 

 a leg or other part, impulses are started in the brain and 

 transmitted from the brain downwards along the spinal cord, 

 and cause motor impulses to pass out along the anterior roots 

 of the spinal nerves going to the appropriate muscles ; 

 when the cord is severed from the brain these impulses can 

 no longer pass downwards. A man whose spinal cord is injured 

 at the level of the first rib still goes on breathing, but by means 

 of the diaphragm only, since impulses from the respiratory 

 centre of the spinal bulb cannot pas.s down to the intercostal 

 nerves, which are branches of the spinal nerves of the thoracic 

 region below tin- injury, but they still can pass to the diaphragm 

 by the phrenic nerves, which branch off from the spinal nerves 

 of the cervical region above the injury. 



Reflex Action. If the soles of the feet of a man whose 

 >l>inal (<ml is injured anywhere above the sacral region be 

 tickled, it often happens that his legs will be suddenly drawn 

 up, though the man cannot feel the tickling, and cannot of his 

 own will draw up his legs. How is this brought about ? 

 The tickling causes sensory impulses to pass up the sensory 

 fibres of the nerves along their posterior roots into the 

 spinal cord. . These impulses so act on the grey matter of 

 the cord that they cause new impulses, motor impulses, to 

 ari>e, and these pass from the grey matter of the anterior 

 horn by the anterior roots into the nerves going to the muscles 

 of the leg, and so the muscles contract. These movements 

 a iv produced without the action of the will or brain, for all 



