THE NERVOUS SYSTEM. 



499 



The VII, IX and X are, as already mentioned, branchial (splanchnic) 

 nerves and the central processes of their ganglia all have a common destina- 

 tion; they grow into the lateral surface of the medulla oblongata, enter the 

 marginal layer of the alar plate, and there bend caudally, forming a comrion 

 descending bundle of fibers in the marginal layer, the tractm solita.-ius 

 (Figs. 436 and 470; see also pp. 462, 465). 



The acoustic ganglionic mass is elongated at an early stage, and is in c on- 

 r.ection with an ectodermal thickening (placode) which gives rise to the acoi stic 



Roof plate 



If"- Alar plate 



Sulcus limitans 



Basal plate 



Floor plate 



FIG. 437. Transverse section through the acoustic region of the rhombic brain of a 10.2 mm. human 

 embryo. VI, Abducens and its nucleus; VII G. g., geniculate ganglion; 77/7 G. c., cochlear 

 ganglion of acoustic nerve; VIIIG.v., vestibular ganglion of VIII nerve. His. 



receptors (p. 591). From the upper part of the mass a bundle of peripheral 

 processes forms a branch which subsequently innervates the ampullae of the 

 superior and lateral semicircular canals and the utricle, while from the lower 

 part a branch develops to the ampulla of the posterior canal and to the saccule. 

 The nerve and ganglion (ganglion of Scar pa) is thus at first vestibular and at 

 this stage the cochlear part of the ear vesicle is not indicated as a separate out- 

 growth. As the lower border of the vesicle grows out into the cochlea, the 

 lower border of the ganglion becomes thickened and develops into the cochlear 

 ganglion (the ganglion spirale). It will be recalled that the vestibular part of 



