Sentence:  A medical journal, it seems to the writer, should be a medium for the propagation of useful knowledge, the instruction of the profession in the best and most scientific methods of fighting disease; and who is so well qualified for the work as the old <WSD>wheelhorses</WSD>, the men who have grown gray in the fight? 
Meaning of the word marked with <WSD> tags:  A person who labors heavily for a particular cause, without being concerned about recognition.