176 J. B. JOHNSTON, 
Acipenser consists in the few collaterals given off by the fibres 
of the secondary vagus commissure while passing through the 
cerebellum. 
Comparison with the acustic centers of man. — To 
make this comparison it is necessary to consider the great changes 
which have taken place in the peripheral nerves concerned. Of 
these nerves the V is the most constant in Vertebrates, but it has 
become separated with its sensory nucleus from the central ap- 
paratus of the VIII nerve in the higher forms. The VIII and 
lateral line nerves, although more closely related to one another 
than to the V, present the most extreme differences between the 
lower and higher Vertebrates. In fishes the lateral line nerve is 
much larger than the VIII, while in Mammals the VIII has in- 
creased greatly in size and become differentiated into two portions, 
and the lateral line nerve has long ago wholly disappeared. In 
Acipenser both nerves end in the acusticum, cerebellum, and nucleus 
funiculi, and the VIII has in addition the Acusticuszellen as a part 
of its central nucleus. In higher forms, with the disappearance of 
the lateral line nerves there disappear also the greater part of the 
direct fibres to the cerebellum. The spinal VIII tract remains, but 
there are considerable modifications in the chief enters of the VIII. 
In Acipenser the cells of the acusticum and the Acusticuszellen send 
their neurites as arcuate fibres to the opposite side. These arcuate 
fibres are undoubtedly homologous with at least a part of the fibres 
of the corpus trapezoides, and probably correspond to those which 
arise in the tuberculum laterale and pass to the other side. If so, 
the tuberculum laterale is to be considered homologous with a part 
of the tuberculum acusticum in fishes. Its relatively smaller size 
in the higher Vertebrates, as compared with other structures in the 
medulla, would be explained by the separation of the V, and by 
the absence of the lateral line nerves. The greater development of 
the auditory organ in higher Vertebrates has led to modifications 
and development of the auditory centers of which little is fore- 
shadowed in the primitive acusticum of Acipenser. The only sug- 
gestion which I can make is that the Acusticuszellen correspond 
in position to the principal nucleus of the VIII in Mammals, on the 
floor of the IV ventricle, and they may constitute the fundament 
of that nucleus. 
Further considerations upon the relation of the acusticum to 
the cerebellum and upon the nature and origin of the PURKINJE 
cells will be found in the section on the cerebellum (page 186), 
