The 200th birthday of La Scala - Teatro alla Scala, to give it its full name - on Aug. 3 is a day in the history of music that must not be passed over without all the proper grazie, merci, vielen Danken and heartfelt thanks of a world whose musical roots are still deeply embedded in Italy's great and most lasting gift to music, opera.

The theater in Milan whose name has become an international symbol of the operatic universe opened on Aug. 3, 1778, with Antonio Salierl's "Europa Riconosciuta" - "Europe Remembered." But it is not that now-forgotten opera, or even historic premieres by Rossini, Donizetti, Bellini, Verdi and Puccini that have played in the great theater that make it so vital a symbol for opera lovers.

Today it is not even our chief concern to speak of the eras of glory at La Scala when Arturo Toscanini, Victor De Sabata and, lately, Claudio Abbado brought nights of grandeur to the stage of the modest-looking opera house that stands at one end of Milan's galleria.

Rather, with a 200th birthday coming up - the kind of event made vivid for this country by the year 1976 - the central fact to remember in speaking of all that La Scala symbolizes is the singular focal power of Italian opera in the vast majority of the concert music today's world still loves most. Our symphonies, overtures, songs, solo piano literature and chamber music, as well as much of the standard orchestral repertoire, are direct descendants of Italian opera.

It is Italian opera, which dates back almost 400 years, that established our basic harmonic relationships - that hierarchy of chords that Leonard Bernstein used to call, before he knew their proper academic names, the "finishing chord," by which he meant the tonic; the "governing chord," which to him meant the dominant; and the "vice-governing" chord, which we know as the sub-dominant.

It is also Italian opera from which we received the basic song form A-B-A, which is not only the design followed in hundreds of opera arias and classical songs, but in 99 percent of our popular songs. From that form developed - not long after opera was invented through the regular meetings of Florentine thinkers who called themselves the camerata and a second, parallel group called the alterati - the principal elements of sonata form. It is through the manipulations of this basic form that composers' thoughts for several centuries have given us symphonies, concertos, string quartets and all their relatives.

When you hear a symphony by Mozart of Haydn, you are hearing music whose shape was determined by the Italian architects and builders of opera. When you listen to the introduction to a Mozart piano concerto, take the C Minor, K. 491 for example, you are giving your attention to music whose entire spirit was formed by the music of Monteverdi, Cavalli, Alessandro Scarlatti and their contemporaries.

Richard Wagner, the most lyrical of whose operas, "Tristan und Isolde," was heard here only a week ago, once insisted that his operas should be sung in the classical "bel canto" manner. Those who once heard the great Italian tenor, Edoardo Ferrari-Fontana, sing Tristan at the Metropolitan Opera in Italian, said they had never before heard the role make so rhapsodic an impact. Some hint of what this meant may be heard in recordings of the "Liebestod," sung by Maria Callas or Renata Tebaldi.

The operas of Weber, which exerted so powerful an influence on Wagner, were, like the early operas of Mikhail Glinka in Russia, directly patterned after Italian models, while opera in France traveled in a straight line north from Italy in the persons of Cavalli and Lully.

It is all of this and far more that should be recalled when Italy's - and one of the world's foremost - opera theaters marks an anniversary as bejeweled as a 200th. The salutatory fireworks on Aug. 3 should be seen not only in Milan and its environs; they should not even be confined to its sister houses in Venice, Naples and Rome. Those fireworks should be lighted in Paris and London, in Berlin and Moscow, in San Francisco, Buenos Aires, New Orleans, and on the homesites of every opera house and workshop around the world.

One of the happiest memories of the years immediately after World War II is that this country was helpful in rebuilding the theater which had been almost destroyed by bombs in August, 1943. Now, as it moves into its third century, La Scala looks very much as it always has. Six tiers rise from floor to ceiling, four of them of boxes. Lined in red, the house gleams with cream and gold. But the most important fact of all is that we simply cannot conceive of the world of music as it might be had not the Italians created opera, that form which itself led to the construction of La Scala.