THE ENCEPHALIC OR CRANIAL NERVES 653 



and other batrachia the vagus contains also accelerator or augmentor fibers 

 derived from the sympathetic; hence stimulation, especially if feeble, may 

 increase the heart's action. 



The Pulmonic Nerves. The pulmonic nerves, given off from the trunk 

 after its entrance into the thorax, do not lend themselves readily to ex- 

 perimentation. Division of both vagi in the neck above the point of exit 

 of the pulmonic branches is followed by a decrease in the frequency of the 

 respiratory acts, with an increase in their depth. At the same time there is 

 a loss of sensibility of the mucous membrane of the trachea and lungs and 

 a paralysis of non-striated muscle-fibers. 



Stimulation of the central end of the divided vagus with weak induced 

 electric currents increases the frequency, but decreases the amplitude, of the 

 respiratory movements. This would indicate that in the physiologic state 

 these nerve-fibers conduct afferent nerve impulses that inhibit the inspiratory 

 discharge and lead to an expiratory movement sooner than would otherwise 

 be the case. If the stimulation be increased in intensity the inspiratory 

 movement gradually so exceeds the expiratory that the inspiratory muscles 

 pass into the condition of tetanus and the chest walls come to rest in the 

 condition of forced inspiration. 



Stimulation of the vagus with strong induced electric currents not infre- 

 quently inhibits the inspiratory movement and increases the expiratory until 

 there is a complete cessation of movement in the condition of expiratory 

 standstill. The effect thus produced is similar to, if not identical with, that 

 produced by stimulation of the superior laryngeal nerve. 



Faradization of the trunks of the pulmonic branches or stimulation of 

 their peripheral terminations in the mucous membrane of the bronchial 

 tubes or alveoli by the inhalation of chemic vapors causes arrest of respira- 

 tory movements, a fall of blood-pressure, and a reflex inhibition of the heart 

 (Brodie). 



The Gastric Nerves. Stimulation of the peripheral end of a divided vagus 

 nerve causes a distinct contraction of the right half of the stomach and 

 secretion from the gastric glands. Division of the nerve abolishes the sen- 

 sibility of the mucous membrane of the stomach, impairs motility, and inter- 

 feres with the secretion of the gastric juice. 



Similar experimentation on the trunk of the vagus has shown that the 

 nerve excites contraction of the upper part of the small intestine and of the 

 gall-bladder, the secretion of the pancreas, the renal circulation, the secretion 

 of urine, etc. 



Functions. The afferent fibers transmit nerve impulses from the area 

 of their distribution to the medulla and thence through cortical connections 

 to the sensor cerebral areas, where they evoke sensations. They therefore 

 endow all parts to which they are distributed with sensibility. 



The efferent fibers transmit impulses outward which excite contraction 

 of the muscle of the lower two-thirds of the esophagus, the stomach, the 

 small intestine, and the gall-bladder, and the muscles of the bronchial tubes; 

 excite secretion from the glands of the stomach, pancreas, and kidney, and 

 exert an inhibitor influence on the activity of the heart. The efferent fibers 

 belong to the autonomic system of nerves and are not connected with the 

 ganglia of the vagus, but with local peripheral ganglia. 



The afferent fibers also assist in the maintenance of certain organic 



