6 4 o TEXT-BOOK OF PHYSIOLOGY 



the muscles of the larynx through the inferior laryngeal nerve, and to the 

 heart according to some authorities. 

 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. 



Cortical Connections. The nucleus of origin of the medullary branch 

 at least, is in relation with nerve-cells in the lower third of the general cerebral 

 motor area, the axons of which descend in the pyramidal tract. 



Properties. Faradization of the medullary portion of the nerve near its 

 origin gives rise to contraction of the muscles to which it is distributed. 

 Destruction of the medullary root is followed by impairment of deglutition 

 from a paralysis of the muscles of the pharynx and palate 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 ac- 

 celeration 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 supplied with motor fibers 

 also from the cervical nerves. In consequence of division of the external 

 branch animals experience extreme shortness of breath during exercise, from 

 a want of coordination of the muscles of the fore limbs and the muscles of 

 respiration. 



Functions. The function of the fibers of the spinal accessory nerve 

 is the transmission of nerve impulses from the cells from which they take 

 their origin to the muscles to which they are distributed. They therefore 

 excite to action some of the muscles of deglutition; the muscles which regu- 

 late the tension of the vocal bands during phonation and the muscles which 

 control the respiratory movements associated with sustained or prolonged 

 muscle efforts. The fibers may also convey nerve impulses which exert 

 an inhibitor influence on the heart. 



THE TWELFTH NERVE. THE HYPOGLOSSAL 



The twelfth cranial nerve, the hypoglossal, consists of peripherally 

 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 collec- 

 tion of nerve-cells situated beneath the floor of the fourth ventricle. This 

 nucleus is elongated and extends from the medullary striae downward as fax 

 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 



