Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 295 



Branches. Each spinal nerve trunk contains sensory and motor nerve-cell 

 processes from the respective roots. After a spinal nerve trunk emerges from 

 the vertebral column it divides into several branches, one supplying the 

 muscles and the skin of the back, another the sides of the body, still another 

 branch contains fibers of spinal and autonomic nerve cells (Fig. 16.14). The 

 size of the spinal nerves depends upon the functional demand in the area 

 supplied; in man the largest ones extend into the legs. 



Plexuses. Different nerves may join and form a plexus in which their 

 fibers are bound together (Fig. 16.12). As a result, one nerve that reaches 

 a muscle may contain fibers of several nerves and all of them may stimulate 

 the muscle. 



Cranial Nerves. Most persons are more regularly aware of cranial than of 

 spinal nerves since the former are in control of smiles and toothaches as well 

 as of sight and hearing (Fig. 16.13 and Table 16.1). 



Autonomic Nervous System 



The autonomic (involuntary) part of the nervous system is largely in con- 

 trol of internal organs that are more or less continuously active, such as the 

 alimentary canal, blood vessels, lungs, and heart (Fig. 16.15). The activity 

 of most of these is essential to life. Each one, the heart for example, is in- 

 nervated by nerves carrying impulses that have antagonistic effects; impulses 

 via one nerve hasten its activity, those in the other slow it. In the autonomic 

 nervous system there is an almost total absence of voluntary control. The 

 movements of the heart cannot be slowed by willpower as the tongue can 

 be halted. In this system neither afferent (sensory) nor efferent (motor) fibers 

 are directly connected with the higher centers in the cerebral cortex. Thus 

 stimulation of sensory autonomic fibers does not result in any conscious 

 sensation such as that which results from impulses carried to the brain by 

 the fibers of peripheral nerves. We do not feel the dust in our lungs or food 

 entering the stomach. 



In general the autonomic nervous system is one of multiple reflexes and 

 adjustments beyond the direct control of the individual, a great insurance 

 of safety in crises when voluntary action often fails. It is entirely absent 

 in the invertebrates but becomes progressively more elaborate in the verte- 

 brates, especially in mammals. The whole system was formerly called the 

 sympathetic system. It is now divided into two parts, the sympathetic and 

 parasympathetic systems. Of the double sets of autonomic fibers whose 

 impulses have antagonistic effects on various internal organs, one set is in a 

 sympathetic and one in a parasympathetic nerve (Table 16.2, Fig. 16.15). 

 In the autonomic system, two neurons always make up the efferent or motor 

 pathway of an impulse, a contrast to the single neuron in the motor pathway 

 of the ordinary reflex arc. 



