THE FUNCTION OF REISSNER’S FIBER 133 
related to the swollen and retracted portion of the fiber. In- 
stead, in the region in which there has been a dislocation of the 
fiber, minute spherules of some highly refracting substance are 
found plentifully, close to or in contact with the free surface of 
the ependymal cells. That these are the contracted remains of 
such connecting fibrillae, which were, indeed, component fila- 
ments of Reissner’s fiber is therefore extremely probable. The 
withdrawal of the fiber would inevitably snap such connecting 
filaments in the region affected and these broken protoplasmic 
strands would naturally shrink backwards towards the surface 
of the parent cells. 
The view that Reissner’s fiber is a thread of modified proto- 
plasm, formed by the complete coalescence of numerous delicate 
filaments (or hypertrophied cilia) is indicated by its origin and 
is confirmed by its staining reactions. Moreover, it is an inter- 
pretation which renders comprehensible its singular elastic recoil 
notwithstanding its apparent structureless condition. Such spiral 
retraction is met with only, so far as I. am aware, in the little 
differentiated protoplasm of the Protozoa, among which gr ou 
the fusion of cilia is also no uncommon feature. 
II. THE SCOPE OF THE PRESENT INVESTIGATION 
From what has been stated above it will be seen that, while 
there has been a great variety in the suggestions made as to the 
nature of Reissner’s fiber, there have been put forward but three 
theories as to its function. 
The disproof of Sargent’s statements as to the nature of the 
fiber disposed, at the same time, of his ‘optic reflex theory.’ 
Ayer’s suggestion was based, as I have shown, almost entirely 
upon an erroneous idea of the nature and normal condition of — 
the fiber and its relation to the ventricles; in any case his view is 
not one which could easily be tested experimentally. 
There remained Dendy’s theory which might readily be put 
to the test of experiment if a way could be devised of breaking 
the fiber without damage to the central nervous system. 
Such an operation became possible with my discovery (’10, 
p. 527) of the actual condition of the hinder end of the filum 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 27, NO. 2 
