The Tenor begins 'Ingemisco' with an arioso suggesting helpless groaning, but he takes heart with the beginning of the movement proper,('Qui Mariam absolvisti') - a rare case in Verdi of a compressed sonata design with two distinct themes in tonic and dominant respectively. The reason for this becomes clear when we reach the second of them ('Inter oves locum praesta'). The oboe and tenor throw the gentle second theme back and forth. It gives a remarkably direct evocation of a shepherd piping to his flock on a mountainside. If the oboe suggest the pipe, the shimmering violins rarified atmosphere, the dominant tonality serves to enhance the sense of height.

'Inter oves locum praesta' - realaudio

Clearly an orthodox reprise would ruin the effect. Instead there is a mere orchestral reminiscence beneath the tenor's closing lines, which in turn dovetails neatly into a cadence from the first theme.
A duet for the two women, a tenor 'aria'. We have nearly forgotten the bass by now..