Tue Microscope. 123 
Human teeth are always more’or less exposed to pro- 
ducts of fermentation, and if that product is largely acidulous, 
all teeth should be more or less decayed where that product is 
most at rest about the teeth. But this is not so. Nor can we 
compare the teeth to individuals exposed to zymotic diseases 
where some succumb to the infection, while others do not; be- 
cause the teeth have no such functional activity as to make a 
comparison possible. 
Of one thing we can rest assured: ‘Great masses of mi- 
crococci, bacilli and leptothrix threads” are in the superficial 
layers of decayed dentine, extending somewhat into the deeper 
portion, but not into the deepest portion of the soft mass, there 
doing their work in the economy of nature, but that they are 
active in any other sense is yet to be proved. 
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STUDIES IN HISTOLOGY. 
LESSON ITI. 
Cc. H. STOWELL. 
SECTION CUTTING. 
i order to examine many of the tissues by transmitted light 
it is necessary to have them cut into thin sections. Very 
few of the tissues are in a suitable condition to be submitted to 
this method of procedure. 
Tissues that are of a soft and yielding‘nature may be exam- 
ined by snipping off a thin fragment with a pair of scissors 
with blades curved on the flat. Portions of these sections at 
the circumference will be found sufficiently thin for examina- 
tion, If larger sections be desired recourse may be had to a 
Valentin’s knife. This knife consists of two long parallel 
blades, which can be separated from each other by means of 
fine screws. The thickness of the section of course depends 
upon the degree of approximation of the blades. The sections 
obtained in this way are of limited dimensions and seldom of 
uniform thickness. It is a method of section cutting but little 
used and cannot be recommended. 
Many tissues prepared for study at the present time are 
cut with a freezing microtome, although embedding in the well- 
