Peripheral Nervous System in Human Embryo 295 



and at intervals further forward the motor nerves of the eye ; the lateral 

 column furnishes visceral motor fibres to the dorsal spinal roots and to 

 the vagus (spinal accessory), glossopharyngeal, facial and trigeminal 

 nerves. In the 10 mm. embryo these neuroblast columns are longitudin- 

 ally continuous from the spinal cord into the vagus region, and there is 

 present a continuous series of median (ventral) and lateral rootlets. As 

 other structures develop the columns become interrupted and particularly 

 the lateral column; thus in the nucleus ambiguus of the adult we have 

 a broken series of discrete nuclei. 



The motor neuroblasts of the vagus point dorso-lateralward and form 

 rootlets which emerge just ventral to the entrance of the sensory roots. 

 After emergence they turn forward and form a common trunk which 

 in the spinal region lies between the dorsal roots and the side of the 

 spinal cord. The more caudal rootlets are devoid of sensory fibres; but 

 as we go forward we meet with ganglionated rootlets. In the vagus as 

 in the glossopharyngeal there are the ganglia of the roots and the 

 ganglion of the trunk (ganglion nodosum). The ganglia of the roots 

 represent a series diminishing in the caudal direction, as shown in 

 Plate I. In the adult the more caudal ganglia usually disappear except 

 for traces of scattered ganglion cells found occasionally on the rootlets 

 of the spinal accessory division. These more caudal vagus ganglia are 

 not to be mistaken for the Froriep ganglion, which represents a persis- 

 tent precervical ganglion. In the one case we have a series diminishing 

 from the head toward the tail, and in the other it is in the opposite 

 direction. Owing to the tendency to regression on the part of more 

 caudal of the vagus root ganglia the vagus complex, as was shown in a 

 former paper (Streeter, '04), becomes differentiated into a fore part or 

 vagus division which is predominantly sensory, and a back part or 

 accessory division which is almost wholly motor. In the 10 mm. embryo 

 there is no division between the two parts. 



On entering the wall of the neural tube the sensory fibres immediately 

 unite in a longitudinal tract continuous with similar fibres from the 

 facial and glossopharyngeal thus completing the formation of the tractus 

 solitarius. In Plate III is shown a cross section of the neural tube in 

 the vagus region, and on it is indicated the position of the tractus soli- 

 tarius. The marginal or reticular zone of the alar plate in this region 

 is mostly made up of the longitudinal fibres of this fasciculus, and directly 

 ventral lies the similar group of fibres belonging to the trigeminal 

 nerve. The relation of the two is best shown in Plate II. 



