19© COMPARATIVE MORPHOLOGY OF VERTEBRATES 



lingual branch to the taste organs. In the amniotes the conditions 

 are much the same, although the cleft is closed. The posttrematic 

 branch, here called the pharyngeal nerve, is much the larger. 



In the lampreys, elasmobranchs and some ganoids a dorsal nerve is given 

 oflf from the glossopharyngeal near the petrosal ganglion (fig. 189, d) which 

 recalls that of a spinal nerve. It is somatic sensory, supplying the skin on the 

 upper side of the head. In the mammals the auricular nerve is similar to this 

 in its general relations. In the ichthyopsida the ninth is connected with the 

 fifth nerve by a trunk called Jacobson's commissure (fig. 189, j). 



X. The vagus nerve (fig. 189) is apparently a composite of 

 several metameric nerves like the ninth, with the addition, in the 

 ichthyopsida of a part of the lateralis system. The nerve differs 

 considerably in the ichthyopsida and the amniotes, so, for conven- 

 ience of description the ichthyopsidan condition will be considered 

 first. 



In all vertebrates which respire by gills the vagus arises by a 

 number of rootlets and has two closely associated ganglia, an anterior 

 lateralis and a posterior jugular, the latter containing both somatic 

 and visceral sensory cells. From the jugular ganglion a single 

 branchio-intestinal nerve arises which sends branches to each gill 

 cleft behind the first, these varying in number from four in most 

 forms to six in the lower sharks and lampreys and an uncertain num- 

 ber (see p. 255) in the myxinoid Bdellostoma. In lampreys and 

 elasmobranchs each of these branchial nerves bears an epibranchial 

 ganglion, but in the teleosts these are fused in the main trunk. Be- 

 yond the ganglion each branchial nerve divides above the gill cleft 

 into pre- and posttrematic branches like those of the ninth. Beyond 

 the nerve going to the last cleft the trunk continues as the intestinal 

 nerve (pneumogastric of human anatomy) to the heart, stomach and 

 air bladder (when present) . 



The lateralis part of the nerves separates from the rest and follows 

 the lateral-line organs of the body back to the tail, running in the 

 partition between the epi- and hypaxial muscles, sometimes near the 

 vertebral column, sometimes just beneath the skin, and sending small 

 nerves to the lateral sense organs. Like the lateralis nerves of the 

 seventh, that of the vagus occurs in all ichthyopsida which have gills, 

 but in the anura and caducibranch urodeles lateral-line organs and 

 nerves are lost at the time of metamorphosis, though they are retained 

 throughout life in the perennibranchs. 



In all vertebrates branches of the intestinal nerve enter into close relations 



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