NERVOUS SYSTEM 233 



resemblance can be traced between man and other Vertebrates. 

 It is possible to distinguish in various parts nerve- fibres as the 

 conducting apparatus and nerve-cells as the physiological 

 centres ; the latter do not consist of merely individual cells but 

 possess a much more complex structure. 



The peripheral nerves may be divided into three classes : 



1. Centrifugal, or efferent, fibres, running from the brain, 

 or spinal cord, carrying motor, secretory, trophic or inhibitory 

 impulses. 



2. Centripetal, or afferent, fibres, nerves of sensation, or 

 reflex nerves. 



3. Inter-central, or commissural, fibres, linking together the 

 ganglionic centres. 



From the brain there spring direct twelve pairs of nerves, 

 known as the cranial nerves, of which the majority are purely sen- 

 sory nerves, the remainder are mixed motor and sensory nerves. 



Thirty pairs of nerves arise from the spinal cord, each single 

 nerve originating in an anterior motor and a posterior sensory 

 root, which unite and give off posterior branches to the skin 

 and muscles of the neck and back, whilst the anterior branches, 

 consisting of mixed motor and sensory nerves, form four 

 plexuses, namely, the cervical plexus supplying the skin and 

 muscles of the neck, the brachial plexus, for the skin and 

 muscles of the upper extremity, the lumbar plexus distributed 

 to the pelvis and anterior surface of the thigh, and lastly, the 

 sacral plexus which gives off various cutaneous nerves in 

 addition to the great sciatic nerve, the motor and sensory nerve 

 to the outer surface of the thigh and the leg as a whole. 



The fibres of the sympathetic system fulfil the special function 

 of innervating the unstriped muscles generally and the striped 

 muscle of the heart, while its ganglia are the centres for the 

 movements of the viscera (heart, stomach, intestines and the 

 abdominal glands). 



The spinal cord is a central organ, not a mere conducting 

 path between the brain and peripheral nerves, for in it originate 

 the reflexes which are constantly controlled by the brain. The 

 communicating tracts between the brain and the spinal centres 

 which by reflex impulses contribute to the performance of 

 certain co-ordinate movements, exist not only in man but to a 



