Bremer, Aberrant Roots. 629 



chick, and in this connection it is interesting to note that Froriep 

 (1886. i) states that the occipital bone of the chick is made of five 

 fused vertebrae, instead of four as found in mammals. The rela- 

 tively large eye of turtle and chick embryos probably is corre- 

 lated with the common double root of the abducens in these forms. 

 The duration of these vestigial roots is short; they have not been 

 found in human embryos of more than 30.0 mm. in length, at 

 which time the muscles are fairly well laid down; and I have not 

 seen them in the adult, though the possibility of an anomaly of 

 the nerves in this region should not be overlooked. As for the fate 

 of the laterally directed fibers, if they arise with roots of the hypo- 

 glossal nerve, they may occassionally be retained to form the 

 small recurrent twigs described in adult anatomy as running to 

 the dura of the anterior condyloid foramen, or to the wall of the 

 jugular vein. These tw^igs usually appear late, but in fig. 7, taken 

 from a section of a 29.0 mm. human embryo, a small branch from 

 the hypoglossal nerve is seen going to the wall of the jugular vein, 

 and its position, just without the foramen, and its lateral course, 

 suggest that it may be one of the lateral aberrants being converted 

 into one of the recurrent twigs of the adult. 



It seems possible that by a more extended study of these vesti- 

 gial roots we might arrive at a solution of the vexed question of 

 the number of neural segments in the head, at least as far forward 

 as the abducens. I have not seen any traces of similar aberrant 

 roots in the more anterior parts of the head, in conjunction with 

 the trochlear and motor oculi nerves, but, as I have not made an 

 extensive examination of this region, I am not prepared to say 

 that such aberrant roots may not exist. 



In regard to the relations of the different components of the 

 cranial nerves, I wish to advance a theory which rests partly on 

 the distribution of, and the course pursued by, these aberrant 

 roots, is supported by many facts long known, but differs in some 

 respects from former theories of the components of the nerves, 

 and seeks to reconcile facts which are not in accord with these 

 theories. Johnston (1906), following Gaskell (1886), divides 

 the nerves into four components, which he calls the somatic 

 efferent or motor, the visceral efferent or motor, the somatic 

 afferent or sensory, and the visceral afferent or sensory. Of the 

 afferent or sensory components I shall say nothing further. The 

 somatic motor fibers arise from cells arranged in groups in the 



