George L. Streeter 1 1 1 



In the lower fishes the cranial and .spinal elements are clearh' separated 

 and their territories do not overlap; a line may be drawn oral to which 

 all the nerves are cranial and caudal to which all are spinal. In the 

 phylogenesis, owing to a caudal encroachment of the skull into the spinal 

 region, the more oral of the spinal nerves become included in the head 

 region and have special foramina of exit. Those that are thus assimi- 

 lated by the selachii have been styled by Fiirbringer as occipital nerves, 

 and those assimilated in addition later by the holocephali are called 

 occipito-spinal nerves. With this assimilation, however, the spinal and 

 cranial elements are still discretel}^ separated by a transverse line of 

 demarcation. There is no actual overlapping of the two until we come to 

 the sauropsida. Here and in all higher vertebrates, accompanying the 

 conversion of certain vagus gill muscles into the trapezius and sterno- 

 cleido-mastoideus, the cranial elements (/. e. vagus complex) make a 

 caudal invasion into the spinal region, in such a manner that the acces- 

 sory portion of the vagus is found wedging itself in between the ventral 

 and dorsal spinal roots mesial to the ganglia, gaining attachments to the 

 cord just ventral to those of the dorsal spinal roots. 



In the human embryo the different stages of this invasion cannot be 

 demonstrated. Either the early steps are not repeated in the embryo- 

 logical history of higher types, or it may be, as McMurrich, 03, suggests, 

 that the derivation of such structures cannot be demonstrated ontogeneti- 

 cally because the phylogenetic stages occur while the structures are still 

 in an undifferentiated state. In the embryos studied, as soon as the 

 nerve elements can be distinguished, they have their final relative posi- 

 tion, and the accessorius is found extending well down into the cervical 

 region. Its caudal end is indicated hy " E " in Figs. 1, 2, and 3. The 

 vagus-accessory anlage is, in all three instances, about of the same size. 

 Some variation exists in the extent of overlapping ,of the cranial and 

 spinal parts, as is evidenced by the variation in distance between the 

 ganglion jugulare and the first cervical ganglion. It is doubtless a 

 variation of the individual, and is of the same character as the variation 

 occurring in the distance over which the accessory nerve extends into the 

 cervical region of the adult. In Pig. 14 is shown the wedge-like invasion 

 of the cranial nerve elements into the spinal territory. The figure is a 

 diagrammatic profile reconstruction of an embryo one month old. The 

 gill arches, vertebral skeleton and muscular apparatus, and spinal and 

 cranial nervous systems are plotted out wdth view to a comparison of their 

 relative positions. It shows clearly the impossibility of drawing any 

 transverse lino tlirmigli tlio body, m-al to which everything would be 

 cranial, and caudal to which everything would belong to the spinal 



