Charles Ellicott Commentary John 19:36

"For these things came to pass, that the scripture might be fulfilled, A bone of him shall not be broken." — John 19:36 (ASV)

For these things were done (better, came to pass), that the scripture should be fulfilled.—The emphatic witness of the previous verse is not therefore to be confined to the one fact of the flowing of the blood and the water, but to the facts in which the fulfillment of Scripture was accomplished, and which establish the Messiahship of Jesus.

He saw—that which might have seemed an accidental occurrence—that they did not break the legs of Jesus; he saw—that which might have seemed a sort of instinct of the moment—that the Roman soldier pierced the side of Jesus; he saw in the water and blood which flowed from it visible proof that Jesus was the Son of man; but he saw, too, that these incidents were part of the divine destiny of the Messiah which the prophets had foretold, and that in them the Scripture was fulfilled. (Compare Note on John 13:18.)

A bone of him shall not be broken.—The reference is, as the margin gives it, to the Paschal Lamb, in which the Baptist had already seen a type of Christ (Compare Note on John 1:29), and which St. Paul afterwards more definitely identifies with Him (1 Corinthians 5:7). It is not equally apposite to refer to Psalms 34:20, as the thought there is of preservation in life, but the words of the Psalm are doubtless themselves a poetic adaptation of the words of Exodus.