  Let $G$ be a simple connected graph. We use $n(G)$, $p(G)$, and $\eta(G)$ to
denote the number of negative eigenvalues, positive eigenvalues, and zero
eigenvalues of the adjacency matrix $A(G)$ of $G$, respectively. In this paper,
we prove that $2n(G)\geq d(G) + 1$ when $d(G)$ is odd, and $n(G) \geq \lceil
\frac{g}{2}\rceil - 1$ for a graph containing cycles, where $d(G)$ and $g$ are
the diameter and girth of the graph $G$, respectively. Furthermore, we
characterize the extremal graphs for the cases of $2n(G) = d(G) + 1$, $n(G) =
\lceil \frac{g}{2}\rceil$, and $n(G) = \lceil \frac{g}{2}\rceil - 1$.
