194 NERVOUS SYSTEM OF VERTEBRATES. 



The number and segmental position of the roots represented 

 by the hypoglossus varies in different vertebrates, owing to the 

 difference in the number of gills and to the extent caudally of the 

 branchial apparatus. The tongue musculature arises from 

 myotomes situated immediately behind the gills, and in fishes 

 and higher vertebrates possessing five or four gills it is probable 

 that one or two more anterior myotomes may enter into the tongue 

 muscles than in forms like the cyclostomes where there are seven 

 or more gills. In all higher vertebrates the hypoglossal roots 

 include the first ventral roots following the eye muscle nerves. 

 In cyclostomes, however, and to a less extent in selachians, a 

 number of myotomes are preserved anterior to those which contrib- 

 ute to the tongue muscles and to supply these muscles ventral 

 nerves are present anterior to the hypoglossal roots. In Petro- 

 myzon all the myotomes behind the ear form permanent muscles, 

 while the first myotome to contribute to the tongue muscles is 

 the seventh behind the ear (myotome lo). However, from one 

 to three nerves are absent in different species of Petromyzon 

 in spite of the presence of permanent muscles formed from their 

 myotomes, and there remain from three to five nerves anterior 

 to those which enter into the formation of the hypoglossus. The 

 most anterior ventral root present in Petromyzon dorsatus belongs 

 to the same segment as the vagus (Fig. 51). The segment of the 

 glossopharyngeus also has a somatic motor nerve in Bdellostoma, 

 so that, as noted above, the series of somatic motor roots in this 

 animal is complete from the segment of the glossopharyngeus 

 backward. 



The eye muscles are developed from the first, second and 

 third somites. From the first somite are derived the rectus superior 

 and inferior, the rectus intemus and the obliquus inferior. From 

 the second somite comes the obliquus superior, and from the third 

 comes the rectus extemus. The nerve which innervates the rectus 

 externus muscle is the VI or abducens. It is the ventral motor 

 nerve of the third somite and is comparable in every way with a 

 ventral spinal nerve. In Petromyzon there is no VI nerve. The 

 muscle which is usually regarded as the homologue of the rectus 

 externus is innervated by a branch from the trigeminus ganghon 



