NERVOUS SYSTEM OF THE VERTEBRATA. 459 



From the lower commissure springs the lateral nerve, at a point whose 

 relations to the branches of the vagus I have not certainly determined. 



With reference to the dorsal commissure, which is almost certainly 

 derived from the original neural crest, it is to be noted that there is a 

 longish stretch of it between the last branch of the vagus and the first 

 spinal nerve, which is probably the remains of a part of the commissure 

 which connected the posterior branches of the vagus, at a stage in the 

 evolution of the Vertebrata, when the posterior visceral clefts were still 

 present. These branches of the vagus are probably partially preserved in 

 the ramifications of the intestinal stem of the vagus (Gegenbaur). The 

 origin of the ventral commissure, continued as the intestinal branch of the 

 vagus, has not been embryologically worked out. 



The lateral nerve may very probably be a dorsal sensory branch of 

 the vagus, whose extension into the posterior part of the trunk has been 

 due to the gradual backward elongation of the lateral line 1 , causing 

 the nerve supplying it to elongate at the same time (vide Section on lateral 

 line). 



In the Chick the common rudiment for the vagus and glossopharyngeal 

 nerves (Marshall), which has already been spoken of, subsequently divides 

 into two parts, an anterior forming the glossopharyngeal nerve, and a 

 posterior forming the vagus nerve. 



The seventh and auditory nerves. As shewn by Marshall's 

 and my own observations 'there is a common rudiment for the seventh and 

 auditory nerves. This rudiment divides almost at once into two branches. 

 The anterior of these pursues a straight course to the hyoid arch (fig. 

 271 A, VII.} and forms the rudiment of the facial nerve ; the second of the 

 two (fig. 271 A, au.ti), which is the rudiment of the auditory nerve, develops 

 a ganglionic enlargement and, turning backwards, closely hugs the ventral 

 wall of the auditory involution (fig. 272). 



The seventh or facial nerve soon becomes more complicated. It early 

 develops, like the glossopharyngeal and vagus nerves, a branch, which 

 forks over the cleft in front (spiracle), and supplies the mandibular arch 

 (fig. 27 1 B). This branch forms the praespiracular nerve of the adult, and 

 is homologous with the chorda tympani of Mammalia. Besides however 

 giving rise to this typical branch it gives origin, at a very early period, 

 to two other rather remarkable branches ; one of these, arising from its 

 dorsal anterior border, passes forwards to the front part of the head, im- 

 mediately dorsal to the ophthalmic branch of the fifth to be described 

 directly. This nerve is the portio major or superficialis of the nerve usually 

 known as the ramus ophthalmicus superficialis in the adult 2 . 



1 The peculiar distribution of branches of the fifth and seventh nerves to the 

 lateral line, which is not uncommon, is to be explained in the same manner. 



2 The two branches of the ramus ophthalmicus superficialis were spoken of as the 

 ram. opth. superficialis and ram. opth. profundus in my Monograph on Elasmobranch 

 Fishes. The nomenclature in the text is Schwalbe's, which is probably more correct 

 than mine. 



