Johnston, MorpJiology of the Head. 207 



It is either a dorsal ramus alone or has incorporated in it what- 

 ever vestige of a ventral ramus exists. 



The visceral elements which should be found in a typical 

 head segment are absent from this one. The visceral apparatus 

 is represented in the most primitive forms by very much re- 

 duced and transient structures (see sections on the mesoderm 

 and on the anterior end of the head, above), and the splanchnic 

 sensory and motor nerves with their appropiate central nuclei 

 are wholly unknown in this segment. 



In the central nervous system, as has been shown elsewhere 

 (69, 70), the caudal part of the tectum opticum and the nucleus 

 of N. Ill represent respectively the somatic sensory and the 

 somatic motor divisions of this segment. A further treatment 

 of the sensory center will be found below in the sections on the 

 lateral line system and on the visual organs (Sees. 10, 13). 



8. Segment 6. 



Neuromere vi, N. V, N. IV, Somite 2, mandibular arch, sympathetic ganglion. 



In the last section reference has been made to the struc- 

 tures found in this segment, and part of the evidence for their 

 relations given. For the bringing together of Nn. IV and V in 

 one segment we have a long line of investigations including 

 V. WijHE (i 19), Hoffmann (58, 59), Platt (106), Neal {j6, 97), 

 and KoLTZOFF (76). For the relations of somite 2 and the 

 mandibular arch respectively to these two nerves abundant evi- 

 dence is found in the same papers. It must be noticed that N. 

 V contains both general cutaneous components for the skin of 

 this segment and viscero-motor fibers for the muscles of the 

 mandibular arch. These muscles are formed from the lateral 

 mesoderm and the nucleus of origin of the motor V belongs to 

 the lateral motor column in the oblongata {70). There are 

 found, then, all the elements ot a typical segment except the 

 splanchnic sensory nerve component and its center. The only 

 vestige of this component found in any vertebrate is the sympa- 

 thetic ganglion which will be treated below in the general sec- 

 tion on the sympathetic. The small entodermal area belonging 

 to this segment, if indeed there is any, is supplied by com- 



