We have analyzed optical and ultraviolet spectra of nearby stars (r less-than-or-equal-to 200 pc) located within the area of Galactic coordinates 310-degrees less-than-or-equal-to l less-than-or-equal-to 330-degrees and 15-degrees less-than-or-equal-to b less-than-or-equal-to 25-degrees, in a study of the LISM toward the center of Loop I. The optical spectra show a multiple-component structure, with one prominent absorption plus one or two weaker features. The weaker components generally have Na I/Ca II < 1. One of them is identified with the nearby (d less-than-or-equal-to 5 pc) component P of Lallement, Vidal-Madjar & Ferlet. The most intense components generally have Na I/Ca II > 1, and show good velocity coincidence with the global flow derived by Crutcher. The analysis of the UV spectra reveals a sudden increase of the gas column density as a function of distance, from N(H I) congruent-to 10(18) cm-2 to congruent-to 10(20) cm-2, suggesting the presence of a wall of gas at 40 +/- 25 pc from the Sun. This result constrains the distance to the boundary of the Local Bubble-the soft X-ray emitting cavity around the Sun-in the direction toward the center of Loop I. At larger distances, the approximately constant N(H I), and the lack of detection of new interstellar components in the optical spectra, suggest the presence of another cavity of very low-density gas, identified with the Loop I Bubble. Along a few lines of sight, we have been able to estimate the physical parameters of the gas located at the boundary between the Local Bubble and Loop I Bubble by means of an analysis of optical (Na I) and UV (Mg I, Zn II) species, under the assumption of ionization equilibrium. We find evidence of warm gas (T congruent-to 6-7 x 10(3) K) in the direction of eta Cen and zeta Cen, and of colder gas in front of HD 129685 and probably HD 119921. The electron densities of the warm gas in front of eta Cen (congruent-to 0.03 cm-3) and zeta Cen (congruent-to 0.04 cm-3) are very similar to the space averaged densities derived by means of pulsar dispersion measures in the LISM. The fractional ionization of this gas is less than that predicted by McKee and Ostriker for the warm neutral phase of interstellar space.
