  We use single-cluster Monte Carlo simulations to study the role of
topological defects in the three-dimensional classical Heisenberg model on
simple cubic lattices of size up to $80^3$. By applying reweighting techniques
to time series generated in the vicinity of the approximate infinite volume
transition point $K_c$, we obtain clear evidence that the temperature
derivative of the average defect density $d\langle n \rangle/dT$ behaves
qualitatively like the specific heat, i.e., both observables are finite in the
infinite volume limit. This is in contrast to results by Lau and Dasgupta [{\em
Phys. Rev.\/} {\bf B39} (1989) 7212] who extrapolated a divergent behavior of
$d\langle n \rangle/dT$ at $K_c$ from simulations on lattices of size up to
$16^3$. We obtain weak evidence that $d\langle n \rangle/dT$ scales with the
same critical exponent as the specific heat.As a byproduct of our simulations,
we obtain a very accurate estimate for the ratio $\alpha/\nu$ of the
specific-heat exponent with the correlation-length exponent from a finite-size
scaling analysis of the energy.
