202 NERVOUS SYSTEM OF VERTEBRATES. 



terms ill- adapted to the comparative description of the nervous 

 system of vertebrates have come into use. The nerve is named 

 the facial from the distribution of its larger branches in man, but 

 the comparative anatomy shows that this distribution has been 

 secondarily acquired and only in higher vertebrates. The facial 

 nerve is really the nerve of the hyoid segment. 



As in the case of the somatic motor nuclei the course of the 

 neurites of the visceral motor cells is not always simple. Not 

 only do the fibers run longitudinally in the brain for longer or 

 shorter distances before going out in their nerve roots, but there 

 have occurred shiftings of segmental relations which are at first 

 sight difficult to understand. In the case of the trigeminus, a 

 part of the nucleus of origin of its motor component lies in all 

 vertebrates caudal to the root, and the fibers from this part of the 

 nucleus run forward lateral to, or in the lateral part of, the fasci- 

 culus longitudinalis medialis to join the remainder of the root. 

 In the case of the facialis the entire nucleus always lies caudal to 

 the plane of exit of the root and the fibers run forward as a dis- 

 tinct bundle in the fasciculus longitudinaHs mediaUs and turn 

 laterad to form the root. In man this is known as the geniculated 

 root of the facialis. The roots and nuclei of the IX and X nerves 

 are arranged in much the same way as those of the V ner\^e. This 

 condition is more pronounced in the vagus and the fibers do not 

 all unite into one root but the vagus has a number of motor as 

 well as of sensory rootlets. This is explained by the fact that 

 the vagus has gathered into it all the branchial nerves of the 

 segments following that of the glossopharyngeus. The roots 

 of the more caudal branchial nerves have gradually shifted forward 

 until they have united with that of the vagus, but the union is 

 not complete in any class of vertebrates. In some fishes the number 

 of rootlets approaches twenty and they are scattered for a con- 

 siderable distance along the side of the medulla oblongata (Figs. 

 2, 3, 7, 12). The fibers of the most caudal of these motor rootlets 

 in fishes go to supply certain muscles connected with the pectoral 

 arch which are homologized with a part of the trapezius muscu- 

 lature in mammals. In higher vertebrates the nuclei of these 

 more caudal rootlets apparently extend farther back in the spinal 



