Laws of Cricket 1947 Code - Law 2
A Substitute shall be allowed to field or run between the wickets for any player who may during the match be incapacitated from illness or injury, but not for any other reason without the consent of the opposing Captain; no Substitute shall be allowed to bat or to bowl. Consent as to the person to act as substitute in the field shall be obtained from the opposing Captain, who may indicate positions in which the Substitute shall not field.
Notes on Law 2
1. A player may bat, bowl or field even though a substitute has acted for him previously.
2. An injured batsman may be "Out" should his runner infringe Laws [36], [40] or [41]. as Striker he remains himself subject to the Laws: should he be out of his ground for any purpose he may be "Out" under Laws [41] and [42] at the wicket-keeper's end, irrespective of the position of the other batsman or the substitute when the wicket is put down. When not the Striker the injured batsman is out of the game and stands where he does not interfere with the play.
Reproduction of the Laws of Cricket is by kind permission of Marylebone Cricket Club