THE CEREBELLUM, MEDULLA AND MIDBRAIN 165 



The Sympathetic System. This system is treated as a distinct 

 portion of the nervous system because to a rather special physio- 

 logical function it adds peculiar anatomical relationships. In 

 spite of its anatomical and physiological peculiarities, however, 

 it forms an integral part of the whole nervous system, and inter- 

 acts with other parts as completely as though nothing distin- 

 guished it from them. Its name has no present significance, 

 having been given to it in the erroneous belief that its function is 

 to bring remote organs into sympathy with each other. The 

 special physiological function of the sympathetic system may be 

 stated in a sentence: it forms the efferent connection between the 

 central nervous system and all the smooth muscles and glands 

 of the body, and the heart. 



It will be recalled that the skeletal muscles have motor con- 

 nection with the central nervous system by means of motor 

 neurons, structures whose cell-bodies lie in the ventral horns of 

 gray matter and whose axons extend directly to the muscles. 

 The sympathetic system differs from the motor system to skeletal 

 muscles in that each pathway from the central nervous system 

 to a smooth muscle or to a gland is made up of a succession of 

 two neurons. The first neuron has its cell-body in the ventral 

 horn of gray matter; its axon passes out by way of the ventral 

 root of the spinal nerve and the communicating branch (see 

 p. 126) to one of the sympathetic ganglia where it forms synaptic 

 connection with the second neuron of the chain. This neuron sends 

 its axon back over the communicating branch to the spinal nerve 

 along which it passes to its destination in a smooth muscle or a 

 gland. Because of their positions with regard to sympathetic 

 ganglia the first and second neurons are known respectively as 

 pre-ganglionic and post-ganglionic neurons. The latter present 

 the anatomical peculiarity of being for the most part devoid of 

 myelin sheaths; nerve-trunks made up of post-ganglionic fibers 

 can therefore be distinguished from other nerve-trunks by their 

 gray color. 



The structures innervated by the sympathetic system perform 

 their functions by acting to a considerable extent in groups 

 together; not individually as do skeletal muscles. To enable them 

 to be stimulated in groups single sympathetic pathways com- 

 monly involve numerous end structures, This is accomplished 



