586 COMPARATIVE PHYSIOLOGY. 



gether with the vagiis and spinal accessory, constitutes the 

 eighth pair, or rather trio. Functionally, however, they are 

 quite distinct. 



The glosso-pharyngeal arises in the floor of the fourth ven- 

 tricle ahove the nucleus for the vagus. It is a mixed nerve 

 with efferent and afferent fibers : Efferent fibers, furnishing 

 motor fibers to the middle constrictor of the pharynx, stylo- 

 pharyngeus, levator palati, and azygos uvulse. 2. Afferent 

 fibers, which are the paths of sensory impulses from the base 

 of the tongue, the soft palate, the tonsils, the Eustachian tube, 

 tympanum, and anterior portion of the epiglottis. Stimulation 

 of the regions just mentioned gives rise reflexly to the move- 

 ments of swallowing and to refiex secretion of saliva. 



This nerve is also the special nerve of taste to the back of 

 the tongue. 



The Pneumogastrie, Vagus, or Tenth Nerve.— Most of the 

 functions of this nerve have already been considered in previous 

 chapters. 



In some of the lower vertebrates (sharks) the nerve arises 

 by a series of distinct roots, some of which remain separate 

 throughout. This fact explains its peculiarities, anatomical 

 and functional, in the higher vertebrates. In these there have 

 been concentration and blending, so that what seems to be one 

 nerve is really made up of several distinct bundles of fibers, 

 many of which leave the main trunk later. 



It may be regarded as the most complicated nerve-trunk in 

 the body, and the distribution of its fibers is of the most exten- 

 sive character. Following our classification of efferent and 

 afferent, we recognize : 



1. Efferent fibers, which are motor to an extensive tract in 

 the respiratory and alimentary regions. 



Thus the constrictors of the pharynx, certain muscles of the 

 palate, the oesophagus, the stomach, and the intestine, receive 

 an abundant supply from this source. By the laryngeal nerves, 

 probably derived originally from the spinal accessory, the mus- 

 cles of the larynx are innervated. The muscles of the trachea, 

 bronchi, etc., are also supplied by the pneumogastrie. It is 

 probable that vaso-motor fibers derived from the sympathetic 

 run in branches of the vagus. The relations of this nerve to 

 the heart and lungs have already been explained. 



2. Afferent Fibers. — It may be said that afferent impulses 

 from all the regions to which efferent fibers are supplied pass 



