Johnston, MorpJiology of the Head. 227 



rami. This is probably true of the dorsal anlage ;/, v, w, x 

 described by Braus in Spinax; of two ganglia in Acanthias des-^ 

 cribed by Hoffmann; an of and uncertain number of the anlages 

 said by Dohrn to be present in all his selachian embryos. In 

 the sheep embryo the hypoglossal ganglion lies over the caudal 

 part of the vagus ganglion and its roots enter the oblongata 

 dorsal to and independent from the roots of the vagus. It is 

 evident from our present point of view that these dorsal roots 

 of occipital nerves are composed of general cutaneous elements 

 alone. As in the cyclostomes, the visceral elements have been 

 collected into the vagus. There is therefore no difficulty arising 

 from the presence of dorsal roots in those segments which 

 possess ventral rami from the vagus. In fact, the presence of 

 a dorsal general cutaneous root is to be expected to fill out the 

 complement of functional divisions, and only its absence re- 

 quires a special explanation. Where it is absent, as it com- 

 monly is from a few segments following the vagus root, two ex- 

 planations are possible. One is that the general cutaneous 

 fibers have been collected into the vagus, which I regard as 

 very improbable since there is no cause for it, mechanical or 

 other. The true explanation probably is that as a result of the 

 reduction of myotomes behind the ear and shifting forward of 

 cervical myotomes, the cutaneous area of the original occipital 

 region has been greatly reduced and the general cutaneous 

 nerve roots have aborted in consequence. The general cu- 

 taneous component of the vagus supplies the skin of this region. 

 It may be added here that, as is indicated in Fig. 10, I believe 

 that the shifting of the vagus root described in a previous sec- 

 tion has resulted in gnathostomes in the absorption by the 

 vagus root of the general cutaneous root of the segment next 

 following the proper vagus segment. This is an entirely differ- 

 ent matter from the process of collecting the visceral nerves. 

 It is rendered probable not only by the consideration of em- 

 bryonic conditions but by two other facts: namely, the large 

 area of skin supplied by the dorsal rami of the vagus between 

 the vagus and first spinal nerve; and the existence of two dis- 

 tinct dorsal rami of the vagus (114, 46) with general cutaneous 



