530 COMPARATIVE ANATOMY 



fibers connect with the otic ganglion and, by postganglionic fibers, with 

 the parotid gland. The special visceral efferent fibers innervate the 

 stylopharyngeus muscle. In cyclostomes, fishes, and urodeles the 

 glossopharyngeus forks over the first gill slit to form post- and pre- 

 trematic branches. With the disappearance of gills in land amphibia, 

 the glossopharyngeal nerve does not lose its post- and pre-trematic 

 branches. The latter, which is the larger, persists as the pharyngeal 

 nerve. 



X. Vagus. The vagus or pneumogastric is a mked nerve, which has 

 the widest and most diverse distribution of any of the cranial nerves. 

 It arises by an extended series of fine roots from the medulla posterior to 

 the roots of the glossopharyngeus. 



The sensory components arise in the ganglion jugulare and ganglion 

 nodosum and are distributed to the larynx, trachea, lungs, esophagus, 

 stomach, intestine, and gall bladder. Their central connexions are with 

 the ala cinerea in the floor of the medulla. The vagus in cyclostomes, 

 fishes and urodeles receives special somatic afferent fibers from the lateral 

 line organs of the trunk and tail. 



Some of the motor fibers come from the nucleus ambiguus and some 

 from the dorsal motor nucleus of the vagus near the nucleus ambiguus. 

 They innervate the muscles of the larynx, esophagus, stomach, small 

 intestine, and part of the large intestine. Inhibitory fibers pass to the 

 heart and secretory fibers to the gastric glands and pancreas. 



The lateral line components of the vagus are lost in the land amphibia 

 and appear in none of the higher vertebrates. 



XI. Accessorius. The spinal accessory, purely motor, combines 

 characteristics of cranial and spinal motor nerves. Some of its fibers come 

 from the vagus and ambiguus nuclei and join vagus fibers in the innerva- 

 tion of thoracic and abdominal viscera. Another group of fibers of 

 similar medullary origin innervate striated muscles of the pharynx and 

 larynx. A third set of somatic motor fibers innervate sternocleidomastoid 

 and trapezius muscles. The accessory nerve of amniotes is in part a 

 somatic motor spinal nerve which has been added to the ten cranial nerves 

 of anamnia as a result of the union of cervical vertebrae with the occipital 

 region of the skull, and in part visceral motor like the vagus with which 

 it is associated. 



XII. Hypoglossus. The hypoglossal is a somatic motor nerve which 

 supplies the tongue muscles. Its roots of origin lie posterior to those of 

 the abducens between the pyramid and the olive. The nucleus is medial 

 and ventral to that of the vagus, and in series with the spinal division 

 of the accessory. Like the accessory, the hypoglossus is a spinal nerve 

 which has become cranial as the result of the extension of the occipital 

 region of the skull. 



