212 The Anatomy of a 12-mm. Pig 
20 mm. and becomes well defined in one of 24 mm. The hind brain 
possesses three well marked neuromeres followed posteriorly by a fourth 
shallow one. In a 9 mm. pig there are five which are distinct. 
Of the nerves shown in Plate I, the fifth is noteworthy as possessing 
no ciliary ganglion. Its ophthalmic division is not connected with either 
the third or fourth nerves, though it passes very near the latter. It 
finally divides into a shorter frontal and a longer nasal branch (Dixon). 
Ciliary ganglia have been drawn as if belonging with the fifth nerve by 
both His and Mall, but as shown by Dixon, 96, and admitted by Prof. 
His, this was an error. No ciliary ganglion could be found in this pig. 
The motor tract of the fifth nerve passing into the inferior maxillary 
division is drawn in Plate IV. 
The seventh nerve, Plate I, appears as a bundle of fibres free from 
cells passing out from the lower part of the lateral wall of the medulla. 
Its course within the brain proceeds from a point near the median line 
directly outward, transverse to the cerebral axis. Outside of the 
medulla a large ganglionic mass is placed over the motor bundle in 
equitant fashion. That part toward the fifth nerve is the geniculate 
ganglion from the inferior end of which passes a clump of fibres to fuse 
with the motor bundle already described, and form the main trunk 
of the facial nerve. The geniculate ganglion is in close contact with 
the acoustic ganglion and in a few sections is inseparable from it. 
From the geniculate ganglon there arises a slender fasciculus, the pars 
intermedia of Wrisberg. It proceeds backward under the entering 
root of the eighth nerve, to the wall of the medulla, Plate IV. After 
penetrating into the brain, the fibres lie just ventral to the oval bundle, 
and may be traced a short distance caudad, running parallel with the 
cerebral axis. Duval has followed them to the upper end of the 
glosso-pharyngeal nucleus (Quain). The slender pars intermedia, the 
large geniculate ganglion and the motor root are parts of a single 
nerve, secondarily united with the acoustic ganglion. Researches estab- 
lishing this opinion are cited by Thane, 95, Van Gehuchten, 97, and 
Wiedersheim, 02. The further course of the nerve is shown in Plate IL. 
It divides into prae-trematic and post-trematic branches, but the divi- 
sion is under the spiracle or auditory cleft and not over it as in fishes. 
The anterior branch of the seventh nerve passes into the mandibular 
arch as the chorda tympani; the remainder lies behind the auditory 
groove in the hyoid arch. 
The eighth nerve shows its upper vestibular and lower cochlear 
branches, but its ganglion is not yet subdivided. 
The trunk of the ninth nerve is cellular for some distance before it 
