THE CRANIAL NERVES. 231 



The spinal portion has its origin in nerve-cells located in the 

 lateral gray matter of the spinal cord as far down as the fifth cer- 

 vical nerve. From this origin the nerve-fibers pass to the surface of 

 the cord to emerge between the ventral and dorsal roots in from 

 six to eight filaments, after which they unite to form a well-defined 

 nerve. It then passes into the cranial cavity through the foramen 

 magnum and unites with the medullary portion. 



Distribution. After the union the common trunk emerges from 

 the cranial cavity through the jugular foramen and after sending 

 branches to 'the pneumogastric and receiving others in turn from the 

 pneumogastric as well as from the upper cervical nerves it divides 

 into two branches viz. : 



1. An internal or anastomotic branch which soon enters the trunk 

 of the pneumogastric nerve. The fibers of this branch are ulti- 

 mately distributed to some of the pharyngeal muscles ; to all of 

 the muscles of the larynx by way of the laryngeal branches of the 

 vagus nerve, and, according to most authorities, to the heart. 



2. An external branch consisting chiefly of the accessory fibers 

 from the spinal cord. It is distributed to the sterno-cleido-mastoid 

 and trapezius muscles. 



Properties. At its origin it is a purely motor nerve, but in its 

 course it exhibits some sensibility, due to the presence of anasto- 

 mosing fibers. 



Destruction of the medullary root e. g., tearing it from its at- 

 tachment by means of forceps, impairs the action of the muscles of 

 deglutition and destroys the power of producing vocal sounds from 

 paralysis of the laryngeal muscles, without, however, interfering 

 with the respiratory movements of the larynx, these being controlled 

 by other motor nerves. The normal rate of movement of the 

 heart is increased by destruction of the medullary root. 



Irritation of the external branch throws the trapezius and sterno- 

 mastoid muscles into convulsive movements, though section of the 

 nerve does not produce complete paralysis, as they are also supplied 

 with motor influence from the cervical nerves. The sternomastoid 

 and trapezius muscles perform movements antagonistic to those of 

 respiration, fixing the head, neck, and upper part of the thorax, and 

 delaying the expiratory movement during the acts of pushing, pulling, 

 straining, etc., and in the production of a prolonged vocal sound, as 

 in singing. When the external branch alone is divided, in animals, 



