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geminus group, the acustico-facialis group, and the vagus group. Each 
group contains both dorsal and lateral roots, and it is an interesting 
fact that the two kinds of roots are entirely distinct from each other. 
The dorsal roots join the brain along a tract of white matter, the 
ascending tract, while the lateral roots arise from groups of neuro- 
blasts that lie near the surface of the medulla, and some distance 
ventral to the union of the dorsal roots with the brain. 
The trigeminus group contains three ganglia. Of these the first, 
or ophthalmic ganglion, is homologous with the first Hauptganglion 
of Petromyzon. The distal end lies in front of the optic stalk, and 
gives off a nerve, which divides into three branches. The second, or 
maxillo-mandibular ganglion, is homologous with the second Haupt- 
ganglion of Petromyzon, and is the largest of the three. It lies over 
the posterior part of the first, and the greater part of the third 
ganglion, coming in close contact with both. The distal end is back 
of the eye, and gives off a nerve which divides into a maxillary and 
a mandibular branch. The third ganglion is the smallest of the three, 
and lies, for the greater part, between the brain and the second 
ganglion. The distal end is also back of the eye, and gives off a 
single unbranched nerve. The roots of the three ganglia join the 
brain near one another. The root of the first is entirely independent 
of the other two, and lies anterior to them. The roots of the second 
and third are in very close contact, the third being just ventral to 
the second. The lateral root, as has been said, arises entirely inde- 
pendently; it descends directly ventralwards, the anterior edge touching 
the posterior part of the maxillo-mandibular ganglion. 
The facial and auditory ganglia are united, but join the brain by 
two distinct roots. From the facial a nerve is given off, which extends 
downwards and forwards a short distance between the external epiblast 
and the maxillo-mandibular ganglion. The auditory ganglion extends 
backwards between the medulla and auditory vesicle, coming in close 
contact with the latter. The lateral root of the facial upon leaving 
the medulla first extends upwards, arching over and coming in close 
contact with the acustico-facialis ganglion, and passing between the 
dorsal roots. It then descends as a slender nerve, and joins the an- 
terior end of a series of epibranchial ganglia, which extend over three 
gill pouches, beginning with the first or hyomandibular pouch. It is 
possible the series extends even farther posteriorly. 
The ganglion of the vagus is small compared with the other 
cranial ganglia. It lies just back of the ear and is connected with 
the brain by a long, slender root, which extends upwards and forwards, 
