VIII ] 



THE NERVOUS SYSTEM 



103 



The white matter which lies on the outside of the grey matter 

 contains fibres connectmg together the different parts of the 

 central nervous sj'stem. These fibres convey impulses up and 

 down the cord, and one well-marked tract of white matter commg 

 from the cerebral hemispheres is believed to be concerned with 

 the coordination of skilled movements. 



The spinal nerves are given off in pairs at regular intervals 

 from the cord. Each nerve arises by two roots, one dorsal and 

 one ventral. The dorsal root contains only afferent fibres, and 

 each has its ganglion contairdng the nerve cells of the fibres. 

 The ventral root contains efferent fibres. The two roots join 

 one another a short distance from the cord, and form a mixed 

 nerve trunk containing both sensory and motor fibres, the 

 ganglia of the dorsal roots being situated a short distance away 

 from the cord and near the place where the two sets of fibres 

 merge to form a single trunk containing mixed fibres. If the dorsal 

 root is destroyed, the sensations which the nerve fibres normally 



/Spinal Cord 



terior Root 



. Sensory 

 Surface 



'--Sensory nerve fibre 

 Motor nerve fibre 



- Muscle 



Anterior Root 

 Fig. 50. The simple reflex arc. 



convey can no longer be felt. Similarly if the efferent fibres of 

 the ventral root are destroyed, although the sensations conveyed 

 bj^ the fibres of the dorsal root are felt, the muscles in the part 

 of the body concerned (e.g. those of an injured limb) are paralysed 

 and can no longer respond to stimuli by appropriate movements. 

 If the spinal cord is transected as by an animal breaking its 

 back the portion of the cord posterior to the break will be cut 

 off from the higher centres of the brain, and all those parts of the 

 body which are innervated from the isolated region of the cord 

 will be devoid of feeling. Nevertheless, reflex action can still 

 occur in those parts, since afferent impulses can enter the cord 

 and efferent impulses pass out although unaccompanied by 

 sensation. Thus a frog whose brain has been destroyed, if its 

 foot is pinched or irritated by an acid, will withdraw its leg or 



