118 JouRNAL OF COMPARATIVE NEUROLOGY. 
may be likened to so many telegraph poles; and the matted tan- 
gles extending laterally from these are the cross-bars for the 
wires. The wires to be supported, following out our compar- 
ison, are the horizontal axones of the granular neurones, which, 
as we have already noted, extend through the molecular layer 
in large numbers. 
5. Architecture and Physiology of the Cerebellum. 
It is now a well established fact that the principal func- 
tion of the cerebellum is to preside over the equilibration of 
the body. Moreover, a fairly direct connection is traceable 
between the morphology of this segment of the brain and the 
nature of the muscular activities which are characteristic of the 
animal. To the student of comparative neurology, therefore, 
the cerebellum holds problems of a special order, and is ever 
potential with no mean interest. 
We have seen that the cerebellum of Mustelus retains an 
external form of a far lower order than that found in the bird 
or mammal. The internal organization is not so inferior, how- 
ever, as we might, from this fact, be led to infer. The archi- 
tectural features of the lower and the higher types are so similar 
that the contrast is really one of degree and not one of kind. 
In both instances, the same sorts of neurones are present, 
grouped in a similar way, and related to each other physiologi- 
cally in essentially the same connections. This remarkable 
identity of features can only be interpreted to mean that the 
cerebellum became established in its organization quite early in 
the history of vertebrates. 
Undoubtedly, the key to the significance of cerebellar 
structure and physiology is to be looked for in the neurones of 
Purkinje. These striking cells, with their characteristic tree- 
like tops, apparently have all of the other structural elements 
present arranged contributory to them. Imbedded in the midst 
of the nervous matter, supported by the interlacing processes 
of the neuroglia there, a PURKINJE neurone sends its great 
branching dendrites outward into a veritable maze of possible 
physiological connections, for such the molecular layer really 
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