278 C. W. PRENTISS 



THEORETICAL DISCUSSION 



It seems evident from the different dissections we have made 

 that the hypoglossal nerve first develops as several ventral 

 spinal roots (5 to 6 in number) which arise independently, lie 

 parallel to each other and are in series with spinal nerves. Later 

 these independent roots unite to form the trunk of the hypo- 

 glossal nerve. Comparing the earlier stages with the later, it 

 would seem that the more anterior roots atrophy and this is in 

 harmony with the observations of Bremer ('08). 



Allowing that the hypoglossal is a composite of ventral spinal 

 roots, then we should expect to find their ganglia, if present, 

 between the first cervical and jugular ganglia. We do find a 

 chain of ganglia occupying this very position, but they are rudi- 

 mentary, appear late and soon show retrogressive changes. 

 They arise from the same neural crest as do the spinal ganglia 

 and root ganglia of the vagus and glossopharyngeal. They form 

 a continuous series, but show variations in form characteristic 

 of all rudimentary structures. As far as their early development 

 is concerned they cannot be divided into a pre-cervical and a 

 cerebral group, nor is there an overlapping of spinal and cerebral 

 ganglia, according to the theory of Froriep. 



Objections have been raised as to whether these ganglia were 

 really homologous with spinal ganglia. The points made have 

 been: (1) Difference in form, these rudiments rarely resemb- 

 ling spinal ganglia; (2) their frequent connection with the vagus 

 rather than with the hypoglossal; (3) their lack of segmental 

 arrangement; (4) their many variations and irregularities of 

 form . 



As to their form, in the sheep Froriep found it similar to that 

 of the cervical ganglia. In the pig where they are best developed 

 they are usually spindle-shaped, but broader forms, resembling 

 spinal ganglia, have occasionally been observed. In many mam- 

 mals too, including man, the first cervical ganglion loses its 

 typical form and may become vestigial. In the pig the first 

 cervical is smaller than the other spinal ganglia and develops 



