THE AUTONOMIC NERVOUS SYSTEM 193 



Their immediate control is vested in certain centers located in 

 the medulla. This location for the centers insures that they shall 

 never be wholly free from sensory stimulation, for no matter how 

 quiet 'the surroundings of the Body may be the processes going 

 on within it give rise to sensory stimuli, and, as we have seen, 

 whatever impulses are aroused are sure to pass through the brain 

 stem. Detailed consideration of the various centers is not neces- 

 sary here as each will be treated in connection with the vital 

 process with which it is related. 



The Autonomic or Sympathetic System. This system is 

 treated as a distinct portion of the nervous system because to a 

 rather special physiological function it adds peculiar anatomical 

 relationships. In spite of its anatomical and physiological pe- 

 culiarities, however, it forms an integral part of the whole nervous 

 system, and interacts with other parts as completely as though 

 nothing distinguished it from them. Its old 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 name autonomic, by which it is at present known, 

 signifies a mechanism not under voluntary control, and in thus 

 emphasizing an important feature of the system, constitutes a more | 

 satisfactory designation. The special physiological function of the | 

 autonomic system may be stated in a sentence : it forms the efferent j 

 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 neu- 

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

 matter and whose axons extend directly to the muscles. The 

 autonomic system differs from the motor system to skeletal mus- 

 cles 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. 147) 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. 



