IBremeR, Aberrant Roots. 



627 



The discussion of the significance of these aberrant roots seems 

 to me to fall into two parts; first, the question of the homology of 

 the cranial nerves with the spinal nerves; and second, the relation 

 of the different components of these nerves. Most writers agree 

 that the cranial nerves must be serially homologous with the spinal 

 nerves, though with many components lost; and no great difficulty 

 is experienced in adopting the idea that the anatomical nerves, 

 known as the vagus, accessory, and hypoglossal, are really the 

 separated components of several segmental nerves. Froriep 

 (1901.2), Streeter (1904. i) and a few others oppose this view, 



Fig. 6. Graphic reconstruction of chick embryo, 25.0 mm. (H. E. C. series 516, sections 312 to 

 332.) X 50. a, small anterior ventral root, pointing toward anterior foramen; has., branch of basilar 

 artery; cart., cartilaginous base of skull, pierced by paramedian foramina for the roots of the hypo- 

 glossal nerve. The anterior foramen is empty, probably because the nerve root, a, has degenerated; 

 d.r., dorsal ramus. 



substituting a theory that the cranial nerves represent a secondary 

 system of nerves, connected with the higher, more complex func- 

 tions of the muscles innervated, characterized by a lateral^ instead 

 of a ventral, motor root, and taking the place of the spinal nerves 

 in the brain. Streeter does not mention the nerves anterior 

 to the glossopharyngeal, and so does not explain the abducens, 

 or the other eye-muscle nerves. This secondary system is con- 

 sidered to have grown backward over the spinal nerves, making 

 a wedge-shaped area of cranial system, reaching down the cord 

 as far as the lowest roots of the accessory nerve, overlying an area 

 of the spinal system, which extends as far headward as the anterior 



