Familia Romana CAP X
Spirare necesse est homini = Dative of interest
FR CAP XI
Puerum dormire necesse est. Why no Dative?
But would “Dormire necesse est puero” (Dative of interest) be correct?
FR CAP XI
Julius: Quia pes eius aeger est.
Nec modo pede sed etiam capite aeger est. (Ablative of respect)
Why no Ablative in Quia pes eius aeger est?
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It is necessary that the child should sleep. It does not say for whom it is necessary. Maybe for the parents who want to have some quiet time?
As you can see from L&S aeger, it may be used for the whole person (and then you would use the ablative to refer to the sick part) or for a part of the body (stomachus in Horace and also like in your example) or for the soul.
Yes, I now see the difference in meaning. Many thanks, bedwere.