THE FUNCTION OF REISSNER’S FIBER 141 
partly calcified cartilage by particularly tough connective tissue 
with contained fin-rays. These several structures became 
greatly indurated during the prolonged paraffin embedding 
which was found to be necessary. Moreover, the various tis- 
sues contracted unequally during this process with the result 
that despite many precautions a very troublesome crumpling 
was often produced. 
This was most in evidence near the actual extremity of the 
tail and thus affected, principally, the region behind the incision 
so that, while it was usually easy to determine if the fiber had 
retracted backwards from the lesion it was sometimes extremely 
difficult to certainly recognize the contracted piece of fiber. 
Especially was this the case when a considerable infiltration of 
blood into the sinus terminalis had accompanied or followed the 
recoil of the fiber. 
In such sections, the filum terminale appears as a number of 
isolated pieces, often cut quite obliquely and a diagrammatic 
sagittal section through the sinus terminalis, such as that seen 
in text-figure 3, was but rarely obtained. 
Apart from this crumpling the tail usually becomes bent at 
the place where the incision was made, so that the lengths of 
filum terminale before and behind the incision rarely lay in the 
same plane, notwithstanding that weights were used during the 
process of embedding to keep the tissue as nearly flat as might 
be. In front of the experimental incision the crumpling was less 
noticeable, the vertebral axis being more rigid, and the muscular 
and other soft tissues liable to contraction having been, for the 
greater part, removed. Nevertheless, even here, a certain cur- 
vature almost invariably occurred. Further, the greater hard- 
ness of the cartilage in this region often caused the sections to 
cut very unevenly. This irregularity could be largely avoided, 
it was found, by cutting rather thick sections (not less than 30 1). 
The lumen of the central canal, however, in the hinder part of 
the spinal cord of the rays examined has a diameter which rarely 
exceeds 30 4 and in such sections, therefore, the whole of the 
central canal may be included within the thickness of a single 
section or a relatively thick layer of overlying tissue may seri- 
