228 Journal of Comparative Neurology and Psychology. 



distribution. The nerve which has been absorbed in gnatho- 

 stomes is the one in cyclostomes which Hatschek calls 

 "Vagusanhang" and which Koltzoff calls the first spinal. 



If the vagus nerve really became a collector of the visceral 

 sensory and motor components in all the following branchial seg- 

 ments in primitive vertebrates, the condition of these compon- 

 ents in gnathostomes is clear. The number of gills has been 

 reduced and the ramus branchio-intestinalis has consequently 

 lost the corresponding ganglia and rami. It may be suggested 

 here in view of the evidence that a much larger number of gills 

 once existed (Sec. 3), that the ramus intestinalis itself would be re- 

 garded as a trunk which owed its formation in the first instance 

 to the collecting process above described. In other words the 

 ramus intestinalis is the collector of all branchial nerves which 

 may at one time have existed behind the present gill region. 

 In accordance with this view the oesophagus and stomach, in- 

 nervated by the r. intestinalis, would be included in the primi- 

 tive gill region. Kingsley's argument (74) that the stomach is 

 formed by a rapid expansion of a part of the alimentary canal 

 immediately following the gills is perhaps to be carried one step 

 further by adding: a region which had earlier been reduced by 

 the loss of more caudally situated gills. The effect of the pres- 

 ent suggestion would be to regard the distribution of the ramus 

 intestinalis as a measure of the primitive gill region. 



The formation of the spinal accessory nerve should be ex- 

 plained in this connection. The visceral motor nuclei are en- 

 larged in the oblongata and this enlargement continues into the 

 spinal cord where it gives rise to the roots of the accessory 

 nerve. This nerve innervates the trapezius musculature which 

 is formed from the lateral mesoderm, and the nerve is homolo- 

 gous with the visceral motor components in the vagus. Its 

 attachment to the vagus is due to processes similar to those 

 which brought about the union of the branchial nerves with the 

 vagus. V It has been "collected" and probably at a time when 

 the pectoral arch was a gill arch. The position of the roots 

 offers no difficulty, since the true vagus roots in some cases have 

 a similar position ; that is, more or less spread out along the 



