No organization has symbolized the essence of jazz any better or any longer than the Count Basie Orchestra, and it honors Basie's innovations that such a characterization can remain true more than a decade after his death. Lots of bands carry on in the name of their leaders, only to fall into precipitous decline as "ghost" bands, pallidly recapping hits of 40 years past; but the Basie band has actually expanded its horizons since its founder's passing. Thank Frank Foster. A tenor-sax star with Basie in the 50s and 60s, Foster also contributed a number of terrific compositions and arrangements to the band's expansive book; taking the helm in the late 80s, he quickly began to unearth long-buried charts that became suddenly fresh again, energizing both the band and its listeners. In addition Foster has supplemented the repertoire with new arrangements and has welcomed new works from the band's current members, thus echoing the spirit of Basie himself (as opposed to Duke Ellington, who maintained a stricter control over his band's material). And Foster has refined the classic balance between powerful horn textures and simple rhythms, one of Basie's great contributions to orchestral jazz. The Count may have courted royalty in his nickname, but the music always served to please and even ennoble the masses; Foster has managed to maintain at least a portion of Basie's elegant, slinky swing, which made his band so immensely popular among dancers in the first place. But under Foster's direction, the band has grown a new set of fangs, and it shows them off when it tears into the meatiest new and old arrangements. Friday through Sunday, Joe Segal's Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4846.