The Cranial Nerve Components of Petromyzon. 151 
so that each 100 of the scale represents one hundred sections 10 u 
in thickness. 
Some of the problems to be solved by this investigation may 
be mentioned before beginning the description. In most cases the 
interest in these questions is due largely to the expectation that the 
condition in eyelostomes will throw light upon primitive vertebrate 
strueture in addition to what we have received from the study of 
eranial nerve eomponents in other forms. 1) The question of the 
presence of general cutaneous components in the Xt, IXtk and VII“ 
nerves and their distribution. Especially the source of general eu- 
taneous fibers for the region in which the operculum arises in fishes. 
2) The distribution of the neuromast components to the organs of 
the ventral region. The relation of the neuromast rami to the pro- 
fundus and trigeminus, in view of the fact that in the embryo the 
placodes from which the ganglia of the supra-orbital and infra- 
orbital neuromast rami arise are closely related to the ganglia of 
tbe profundus and trigeminus. The question of the relation of the 
nervus lateralis to the dorsal spinal nerves. 3) The location of taste 
buds in the Ammocoetes and the distribution of the fascieulus com- 
munis components. The central relations of these eomponents were 
first deseribed in the author’s paper on the brain and of the peri- 
pheral distribution of this component we knew nothing except by 
inferenee. 4) The possible existencee of a sympathetie system in 
the head. 5) The position and relations of the VI!: nerve. With 
regard to all these questions except the last, facts of considerable 
interest and importance have been made out. 
The Trigeminus and Profundus. 
As previously described (20), the common sensory root of these 
two nerves forms the spinal V!* tract and sends a eonsiderable number 
of its fibers to the cerebellum. A smaller number of very coarse 
fibers belonging to the profundus part of the root run in the acusti- 
cum. These have not been distinguished in the present specimen 
but their probable significance will be noted below. The motor root 
arises chiefly from the cephalic end of the lateral motor column, 
but a small part of its fibers come from an isolated part of the 
ventral motor column situated opposite the VII—VII roots (20, p. 11). 
In the present series the sensory and motor roots leave the medulla 
oblongata between sections 360 and 336. The motor root arises by 
three rootlets which unite within the ceranial cavity (Fig. 3). At sec- 
11* 
