Peripheral Nervous Sj'stem in Human Embryo 291 



be made out entering into connection with the cells of the vesicle; the 

 upper group corresponds to the nerves to the superior and lateral ampullae 

 and the utricle, and the lower group the nerves to the saccule and 

 posterior ampulla. The ganglion is somewhat elongated and consists of 

 an upper and lower portion. The upper portion is wholly vestibular, 

 and the lower is partly vestibular and partly cochlear. The cochlear 

 part, or ganglion spiral, is the thickened border of the lower division' 

 of the ganglion of which it is a derivative. The acoustic nerve emerges 

 from the proximal end of the ganglion and enters the brain just lateral 

 to the n. intermedins. The nerve at this time consists almost wholly 

 of vestibular fibres. It is shortly after this that a well defined separate 

 cochlear trunk can be made out connecting the ganglion spirale with 

 the brain. What has been mistaken in younger embryos for a cochlear 

 trunk are the fibres from the lower division of the vestibular ganglion. 

 The acoustic fibres enter the brain wall near its border opposite the 

 third and fourth rhombic grooves, and spread out in the marginal zone 

 just dorsal to the spinal tract of the trigeminal nerve to form the anlage 

 of the tuberculum acusticum. 



Somatic Motor Group. 



The hypoglossal and the three nerves to the extrinsic eye muscles (nn. 

 oculomotorius, trochlearis and abducens) that compose this group are 

 all shown in Plates I and II. As can be seen in Plate II, their nuclei 

 of origin may be considered as a cephalic continuation of the ventral 

 motor column of the spinal cord. This is particularly evident in case 

 of the hypoglossal nerve, whose nucleus and emerging fibres form a con- 

 tinuous line with the ventral roots of the cervical nerves, and their close 

 relation is shown by the tendency of the two to unite in the formation 

 of a plexus. The eye nerves are in the same series with the hypoglossal, 

 and always maintain a similar position near the median line and directly 

 beneath the floor of the ventricle, though they are separated longi- 

 tudinally at varying intervals. The character of the muclei of origin 

 is the same in all four nerves. 



There are no ganglia on these nerves such as are found in the spinal 

 region, though occasionally a ganglion and also at times a dorsal root is 

 associated with the more caudal roots of the hypoglossal. In such cases 

 the ganglion is to be regarded as a precervical one, the exact counterpart 

 of the spinal ganglia. This ganglion when present is the so-called 

 Froriep ganglion. 



