CONNECTIONS AND FUNCTIONS OF CRANIAL NERVES 469 



Afferent functions : 



Regulate respiration. Stimulation of central end may quicken 

 respiration and promote inspiration, or may inhibit inspiration. 

 Stimulation of central end of superior laryngeal branch causes 

 stoppage of inspiration, expiration, cough. 

 Depressor and pressor (from heart to vaso-motor centre). 

 Reflex inhibition of heart. 



Its afferent fibres arise from cells in the ganglia on the trunk of 

 the vagus, namely, the jugular ganglion and the ganglion trunci vagi. 

 The spinal accessory nerve arises partly in connection with the vagus, 

 partly by a series of roots from the lateral region of the spinal cord as low 

 as the sixth cervical segment. The spinal portion of the nerve is purely 

 motor and supplies fibres to the sterno-mastoid and trapezius muscles. 



The twelfth or hypoglossal nerve arises from a collection of large 

 multipolar cells in the floor of the fourth ventricle at its lower end close 

 to the middle line. The nerve-trunk issues from the ventral part of 

 the medulla in the groove between the anterior pyramid and the 

 olivary body. The hypoglossal is purely motor in function, supplying 

 the muscles of the tongue, the extrinsic muscles of the larynx, as well 

 as those moving the hyoid bone. 



Since the integrity of the nuclei of the cranial nerves is a necessary 

 condition for the carrying out of various reflex acts in which these 

 nerves are involved, the grey matter of the fourth ventricle and aque- 

 duct is often spoken of as if it were cut up into a series of centres distinct 

 for every act. The chief of these are the respiratory and the vase- 

 motor centres. Other centres that may be enumerated are : 



Centres for movements of intrinsic and extrinsic ocular muscles. 



Cardiac inhibition. 



Mastication, deglutition. 



Sucking. 



Convulsive (connected with respiratory). 



Vomiting. 



Diabetic (connected with vaso-motor). 



Salivary. 



Centres of phonation and articulation. 



We shall have to consider the action of these centres more fully in 

 treating of the several functions of the body. It must be remembered, 

 however, that when a dozen or more centres are enumerated as being 

 situated in the fourth ventricle, it is not meant that we can anatomi- 

 cally distinguish a group of cells for each act or group of actions 

 named. When we say that a part of the nervous system is a centre 

 for any action, we merely mean that this part forms a necessary link, or 

 meeting of the ways, in the complicated directing of nerve impulses 

 that takes place in every co-ordinated act. 



