190 J. B. Johnston 
unwarranted assumption in view of its central origin. As one of the 
results of the recent work upon the nervous system from the functional 
point of view, it should now be regarded as an established prineiple 
that the central relations of a nerve or nerve component are as im- 
portant for determining its homology as is its peripheral distribution. 
We can not say that because the nerve in question innervates an 
eye-musele which is apparently similar to that innervated by the 
_ abducens in other vertebrates, therefore the nerve is homologous with 
the abducens. It is exactly as just to say that because the nerve 
in question arises from the lateral motor nucleus in the brain, there- 
fore the muscle which it innervates must be a visceral muscle, de- 
rived from the lateral mesoderm. The only true course is to investi- 
sate both questions. The writer is satisfied that the course of the 
nerve is not yet well enough understood. How certain is our know- 
legde of the origin of the muscle may be judged from the following 
quotation from KoLTzorr [26, p. 338]: »Die von mir mit m. 3 in 
Fig. 19 bezeichnete Zellgruppe rechne ich zum dritten Somit, ob- 
gleich ich wiederhole, daß ich nur von der Möglichkeit ihres Ur- 
sprungs aus Zellen, welche früher dem dritten Somit angehört hatten, 
reden kann.« This is the cell group which develops into the m. 
obliquus posterior, and the italics are the author’s. 
The ventral spinal nerves from the 4!" to the 14% inclusive 
eontribute components to the epibranchial or so-called vagus trunk. 
These somatie motor components are to be compared, as NEAL [28] 
has shown, with the »hypoglossus« of other vertebrates. It is to 
be noticed, however, that there is formed here a sort of cervical 
plexus from which the supra-branchial body muscles, as well as 
the sub-branchial museles, are innervated. T'he fact that this plexus 
accompanies the vagus trunk is due merely to the great expansion 
of the branchial apparatus which has brought them into proximity. 
It is of interest to note that when the motor plexus separates from 
the vagus trunk behind the fourth branchial nerve, the somatie 
sensory components both general and special accompany the somatie 
motor nerve. 
Considering the eomplexity of the so-called vagus trunk in Pe- 
tromyzon and the differences which it presents from that of other 
vertebrates, we should bring our nomenclature into conformity with 
the facts and with recent usage. We may speak of an epibranchial 
trunk or plexus, in which we recognize two groups of components. 
The first is the somatie group ineluding somatie motor, general 
