THE AUTONOMIC NERVOUS SYSTEM 



195 



the system through which these processes have their nervous con- 

 trol. In the paragraph dealing with the brain stem the existence 

 therein of reflex " centers" for the various "vital" processes was 

 mentioned. On the afferent side these centers are subject to all 

 sensory stimulations which affect the Body. On the efferent side 

 they act through the autonomic system. 



This reflex mechanism is not subject, to voluntary control except 

 for the single case of the muscle of accommodation of the eye, the 

 ciliary muscle. This muscle is innervated through the autonomic 

 system, but can be voluntarily controlled as completely as any 

 muscle in the Body. When we say that the autonomic system is 

 not under voluntary control we are simply stating in other words 

 that the motor area of the cerebrum is not able to establish con- 

 nection through the pyramidal tracts with the neurons of this 

 system. Since this sytem is outside the control of the motor area 

 all reflexes which affect it must be immediate ones. Only present 

 stimuli can arouse it to activity. When we bear in mind that the 

 proper functioning of the Body requires its vital activities to be 

 adjusted to its immediate circumstances and not to its circum- 

 stances of a week or a year ago, the necessity that autonomic re- 

 flexes be immediate is manifest. 



Grand Divisions of the Autonomic System. Not all parts of 

 the central nervous system give rise to autonomic pre-ganglionic 

 neurons. A group originates in the brain stem. Its fibers are dis- 

 tributed through cranial nerves. These are called cranial auto- 

 nomies. The vagus nerve (p. 152) consists largely of cranial auto- 

 nomic fibers distributed to various organs of the trunk. A second 

 group arises in the thoracic and lumbar regions of the spinal cord. 

 These are distributed through the sympathetic system of the old 

 classification. They are known as thoracico-lumbar autonomies. 

 The third group of fibers arises in the sacral portion of the cord. 

 These are distributed to the pelvic region and constitute the sacral 

 autonomies. 



In a previous paragraph (p. 116) attention was called to the 

 peculiar feature of smooth muscle, shared by heart muscle, of re- 

 quiring double innervation. These tissues must have stimulation 

 to augment their activity and other stimulation to inhibit it. Both 

 sorts of innervation are furnished through the autonomic system. 

 An important feature of the system as a whole is that the opposing 



