THE CRANIAL NERVES. 559 



followed by impairment of deglutition and a loss of the power of 

 producing vocal sounds on account of paralysis of the constrictor 

 muscles of the larynx. According to some authorities, there is also an 

 acceleration of the heart's action from a loss of inhibitor influences. 



Stimulation of the external branch gives rise to contraction of 

 the sterno-cleido-mastoid and trapezius muscles, though division of 

 the branch does not give rise to complete paralysis, as they are sup- 

 plied with motor fibers also from the cervical nerves. In consequence 

 of division of the external branch animals experience extreme short- 

 ness of breath during exercise, from a want of coordination of the 

 muscles of the fore-limbs and the muscles of respiration. 



Functions. The spinal accessory nerve transmits nerve impulses 

 outward which influence the movements of deglutition, and the 

 vocal movements of the larynx, which inhibit the action of the heart 

 and which control respiratory movements associated with sustained 

 or prolonged muscle efforts. 



TWELFTH PAIR. THE HYPOGLOSSAL. 



The twelfth cranial nerve, the hypoglossal, consists of peripher- 

 ally coursing nerve-fibers which serve to connect the nerve-cells 

 from which they arise with the musculature of the tongue. 



Origin. The axons composing the hypoglossal nerve arise from 

 a collection of nerve-cells situated beneath the floor of the fourth 

 ventricle. This nucleus is elongated and extends from the medullary 

 striae downward as far as the lower border of the olivary body. It is 

 located ventro-laterally to the spinal canal. After leaving the cells 

 of the nucleus the axons pass forward and outward toward the surface 

 of the medulla, from which they emerge in ten or twelve small bundles 

 or filaments in the groove between the olivary body and the anterior 

 pyramid. Beyond this point they unite to form a common trunk. 



Distribution. The common trunk thus formed passes out of 

 the cranial cavity through the anterior condyloid foramen. In its 

 course it receives filaments from the first and second cervical nerves, 

 the sympathetic and vagus. It is finally distributed to the intrinsic 

 muscles of the tongue and to the genio-hyo-glossus, hyo-glossus, and 

 stylo-hyoid muscles. Branches derived from the cervical plexus 

 pass to muscles which elevate and depress the hyoid bone. 



Cortical Connections. The hypoglossal nerve nuclei are con- 

 nected with nerve-cells in the lower third of the general motor area 

 around the inferior termination of the fissure of Rolando by axons 

 which descend in the pyramidal tract. 



Properties. Faradization of the nerve gives rise to convulsive 

 movements of the muscles to which it is distributed. Division of the 

 nerve is followed by a loss of motion and an interference with deglu- 



