886 COMPARATIVE ANATOMY 



lows the lateral line organs of the body back to the tail sometimes close 

 underneath the skin and sometimes close to the vertebral column. 



The lateralis part of the vagus nerve naturally only occurs in gilled 

 animals or those which, like the frog, have gills in their larval stage. In 

 these latter cases the lateral part of the vagus disappears in metamor- 

 phosis. 



There is a nodose (knot-like) ganglion lying on the intestinal nerve. 



In animals possessing lungs, the musculature of the lungs is also 

 supplied by the vagus. The dorsal ramus of the lower fishes is returned 

 as the auricular nerve, and this unites with the auricular branch of the 

 ninth nerve, while sympathetic connections are made at various points 

 with the vagus. 



It is also important to remember that the nerve supply of an organ 

 is considered the best test of its homology, well shown in the instance 

 of the vagus nerve, which arises in the head region but is, nevertheless, 

 distributed to the heart, stomach, and lungs, it being recalled, from our 

 study of embryology that the heart, stomach, and lungs, originally form 

 in the head region of the. embryo. 



XI. The Spinal Accessory Nerve. The e 1 eventh and twelfth cranial 

 nerves occur only in the amniotes, although in the dogfish there are 

 twigs from the vagus which innervate the trapezius muscle, while simi- 

 lar conditions are found in amphibia. In the dogfish and the frog, the 

 brain-center for the eleventh nerve is in the medulla oblongata, while 

 in amniotes the center for this nerve lies both in the medulla and in the 

 spinal cord'; in fact, some of the more posterior rootlets that go to form 

 the spinal accessory nerve leave the cord as far down as the seventh 

 cervical nerve. These rootlets unite into a trunk that runs cephalad 

 into the cranium between the dorsal and ventral roots of the spinal nerve 

 and then leaves the skull close to the vagus. 



The spinal accessory is motor in function, and supplies the trapezius 

 and sternocleidomastoideus muscles which move the shoulder girdle. 



XII. The Hypoglossal Nerve. This is also a purely motor nerve, 

 although during embryological development, ganglia do appear on the 

 dorsal roots in some mammals, but all these disappear again. The root- 

 lets of the nerve are only two or three in number, although there may 

 be more. These unite to form the hypoglossal nerve. The hypoglossal 

 nerve and the more anterior nerves then unite to form the cervical 

 plexus. It is from this plexus that the main trunk goes to the hypo- 

 glossal muscles as well as to the retractors of the tongue, and, in birds, 

 to the syrinx. 



Small occipital nerves leave the skull immediately behind the vagus 

 in many fishes. They pass backward and dorsal to the gill clefts and 

 go forward to supply the muscles at the posterior portion of the head, 

 and the muscles of the pectoral fin. It may be that these occipital nerves 

 are homologues of the amniote hypoglossus. 



