Peripheral Nervous System in Human Embryo 293 



ganglia. We still have in the human embryo a trace of these organs 

 in the form of a temporary thickening of the ectoderm directly over 

 the ganglia of these three nerves. 



The 11. facialis is characterized by a large predominance of visceral 

 motor fibres. They make up the bulk of the adult nerve, and in the 

 higher vertebrates they have been made use of to supply the muscles 

 of expression. These motor fibres arise from a group of neuroblasts 

 situated beneath the third rhombic groove. Fibre bundles are assembled 

 and pass directly lateral under the floor of this groove gradually con- 

 verging to form a solid trunk which emerges from the neural tube just 

 median to the acoustic ganglion. The trunk curves backward, as can 

 be seen in Plate I, and breaks up among the cells of the hyoid arch, 

 from which the facial musculature is to be derived. The sensory fibres 

 of this nerve are derived from the geniculate ganglion. From the 

 proximal end of the ganglion the fibres are assembled to form the 

 so-called n. intermedins which enters the alar plate of the neural tube 

 and forms the beginning of the tractus solitarius as shown in Plate 

 III. From the peripheral end of the ganglion the fibres pass down to 

 form the chorda tympani, and finally leave the main trunk of the nerve 

 to enter the mandibular arch, eventually joining the third division of 

 the trigeminal nerve. The great superficial petrosal is another periph- 

 eral derivative of this ganglion which makes its appearance a little 

 later. Though the facial and acoustic nerves are closely united in posi- 

 tion, and connecting fibres are usually present between the two in the 

 adult, it must still be remembered that this is only due to the fact they 

 lie close together. Further than that they have nothing in common, 

 being nerves which belong to entirely different embryological and func- 

 tional classes. The term facial-acoustic complex should only be used in 

 the sense of position. 



In a paper already referred to (Streeter, '08,) it was pointed out 

 that the facial (motor root) and abducens nerves occupy positions in the 

 embryo which are relatively reversed in the adult. The facial at first 

 lies directly under the third rhombic groove, while the abducens is 

 more caudal and is under the fourth rhombic groove. As development 

 progresses the nuclei of these two nerves shift their relative positions, 

 the abducens migrating forward. This migration results in the bending 

 of the motor root of the facial out of its original course and produces 

 the genu facialis which is characteristic of the adult. 



The n. glossopharyngeus forms a more typical visceral nerve than 

 either the facial or vagus. It possesses a ganglion of the root and 



