If an employee has a permanent disability or has sustained a permanent injury in service in the Army or Navy of the United States or in another employment other than that in which he receives a subsequent permanent injury by accident, such as specified in Section 42-9-30 or the second paragraph of Section 42-9-10 he shall be entitled to compensation only for the degree of disability which would have resulted from the later accident if the earlier disability or injury had not existed, except that such employee may receive further benefits as provided by Sections 42-7-310, 42-9-400 and 42-9-410 Title if his subsequent injury qualifies for additional benefits provided therein.