144 NERVOUS SYSTEM. 



What is the influence of the vagus upon the heart ? 



There are numerous branches to the cardiac. plexus from the 

 trunk of the vagus and from its inferior laryngeal branch. Stim- 

 ulation of the pneumogastric nerve diminishes the frequency, or, 

 if strong, entirely stops the heart in diastole. The nerve is there- 

 fore regarded as having an inhibitory action. This is an unusual 

 eifect, for in other cases the stimulation of nerves going to muscles 

 causes contraction : the heart, however, becomes flaccid under the 

 influence of the stimulated vagus. 



What is the origin of the spinal accessory or eleventh cranial 



nerve? 



It is twofold. One root arises in the gray matter of the medulla 

 near the nucleus for the vagus, while the others arise from the 

 lateral tract of the cord as low as the fifth or sixth cervical vertebra, 

 and pass up between the anterior and posterior spinal nerve-roots to 

 join the medullary (or accessory) portion at its emergence from the 

 medulla. The united nerve passes out through the jugular foramen 

 with the glosso-pharyngeal and pneumogastric nerves. 



How is the nerve distributed? 



Soon after leaving the skull it again divides, the medullary root 

 joining the trunk of the pneumogastric, while the spinal root sup- 

 plies the sterno-mastoid and trapezius muscles. 



What is the function of the spinal accessory nerve ? 



The nerve is a motor to all intents, though it has some sensory 

 fibres, as is shown by the pain caused by pinching it. 



(1) The anastomotic branch, which joins the pneumogastric, is 

 apparently largely given off in the recurrent laryngeal nerve, but 

 its section does not produce the same effect upon the larynx as 

 section of the trunk of the vagus or of its inferior laryngeal 

 branch. There is paralysis of the voice, but not of the move- 

 ments of the glottis for respiration. There are probably some fibres 

 of this nerve also given off to the cardiac plexus. 



(2) The muscular branch supplies the sterno-mastoid and trape- 

 zius muscles, but these muscles are also supplied by the cervical 

 spinal nerves, and their action is not paralyzed by the section of 

 this branch of the spinal accessory. It is found, however, that 

 the relation of these muscles to respiration is impaired by isolation 

 from this nerve ; that is, when the breath is held in any violent 

 exertion, as straining or pushing, or when a loud cry is uttered, 



