This injunction was necessary in view of the fact that pagan peoples often ate raw flesh at their sacrificial meals. As to the prohibition against boiling the paschal lamb, see on v. 8.
His head with his legs.
Ancient Jewish expositors correctly understood this to mean that the lamb was to be roasted whole, including both head and thighs (v. 46).
The purtenance.
The viscera were to be roasted along with the rest of the lamb, the former first being cleansed. The preparation of the lamb typified the fact that the body of Jesus was not to be broken (see John 19:33, 36).