60 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



process grows the diploic spaces within it enlarge into air spaces. 

 Those round the antrum come to open into it, but the more 

 distal remain closed. These spaces occupy the whole of the 

 mastoid part of the temporal, but they also extend forwards in the 

 post-auditory process of the squamosal, and may spread backwards 

 to the occipital. Three varieties of mastoids are recognised : 



(1) Dense processes in which the air cells are minute or 

 absent ; 



(2) Containing numerous large spaces (pneumatic) ; 



(3) An intermediate type with large cells round the antrum, and 

 a few small ones near the surface. The third type is the commonest. 



The Floccular or Subarcuate Fossa.— At birth there is a fossa 

 situate on the posterior aspect of the petro-mastoid. It is filled 

 with a process of the dura mater, and over it, but not within it, 

 the flocculus is situated. The posterior semicircular canal sur- 

 rounds the fossa. This is the condition in most mammals 

 throughout life, but soon after birth the fossa becomes closed in 

 man, merely a remnant being seen above and internal to the 

 hiatus vestibuli in the bone of the adult. Its meaning is 

 unknown. 



The Acoustic Ganglia. — It has already been shown (Fig. 35) 

 that the mass of nerve cells which come in contact with the 

 otocyst arise from the neural crest of the hind brain in the same 

 manner as the ganglion of a posterior nerve root. The mass of 

 nerve cells is divided into three groups (A, B, C, see Fig. 46). 



One becomes the geniculate ganglion (A) ; in the formation of 

 the petro-mastoid it is included in the aqueductus Fallopii. Its 

 cells give rise to the great superficial petrosal nerve and chorda 

 tympani in the same manner as the ganglion of a posterior root 

 produces the sensory fibres of a spinal nerve (Dixon).- The pars 

 intermedia, in part at least, represents the afferent or ingrowing 

 root of the ganglion. 



The second or cochlear group (£) gives rise to : 



(1) The spiral ganglion situated in the lamina spiralis; 



(2) To the cells in the floor of the internal auditory meatus 

 which become connected with the saccule and posterior semi- 

 circular canal ; 



(3) Another part remains stranded on the restiform body as 

 the accessory and lateral root ganglia (Fig. 46). 



