i 5 4 ORGANIZATION OF THE HEAD v. 9 



for muscles of the hyoid arch and sensory ones for the skin of that 

 region. 



This nerve is obviously the branchial nerve to the spiracle; we can 

 safely say that the facial and abducens are the dorsal and ventral roots 

 of the third or hyoid segment. The auditory nerve is included as part 

 of the dorsal root of the third somite because the auditory sac is 

 formed by sinking in of a portion of the ectoderm within the territory 

 of the facial nerve. The labyrinth still communicates with the surface 

 of the head in the adult dogfish by a canal, the aquaeductus vestibuli. 

 The nerve that innervates the auditory sac, whatever complexities it 

 may acquire, is to be regarded morphologically as a portion of the 

 dorsal root of the hyoid segment. 



The segmental nature of the structures in the pro-otic region can 

 therefore be made out without serious difficulty. The disturbance 

 introduced by the auditory capsule makes the segmental arrangement 

 of the more posterior region of the head somewhat confused. The 

 series of dorsal roots is uninterrupted; the ninth (glossopharyngeal) 

 nerve is the dorsal root of the fourth segment of the series and runs 

 out through the cartilage of the auditory capsule. The dorsal roots of 

 the succeeding segments are then fused to form that very puzzling 

 nerve the vagus. The branches it sends to the gills are clearly typical 

 branchial nerves, but why should they all come off together from the 

 medulla oblongata, and if there is any advantage in this union, why 

 is the ninth nerve not also so incorporated? Above all, why does 

 the vagus send two branches far outside the segments of its origin, the 

 lateral line branch carrying fibres to the organs right to the tip of the 

 tail and the visceral branch fibres to the heart, stomach, and probably 

 small intestine? 



Evidently these 'wanderings', from which the vagus gets its name, 

 began very long ago. The nerve reaches as far back in cyclostomes as 

 in any other vertebrates. It is easy to understand that if visceral 

 functions are to be directed from the medulla oblongata there is an 

 advantage in having sensory impulses sent direct to that region of the 

 brain and motor impulses sent out direct to the viscera. It may be 

 that these advantages allowed the centralization of these visceral 

 functions, while the need for serial contraction of the swimming 

 muscles led to the retention of the segmental arrangement of the 

 spinal cord. It is an interesting thought that but for the swimming 

 habits of our ancestors our nervous system might by now consist of a 

 central ganglion with nerves passing from it direct to all the organs. 

 Indeed we are tending in that direction, as the spinal cord shortens 



