根据以下材料，回答题
The dog has always been considered man' s best friend.Always noted for being particularly faithfulin watching over children, he also has his place by the fireside, in the cow pasture, on the sheep range(放牧区), and beside the hunter in forest.He is easy to train, works hard, and often performs astonishingfeats.And in the frozen polar regions he was once the principal motive power, before being latelydisplaced by the plane and helicopter.
Because he howls or whines in the presence of impending death, the dog was once thought to havesupernatural powers and believed to be capable of seeing gods and ghosts invisible to men.Actually, thebasis for these beliefs lies in the hound' s sensibility to people' s feelings and his superior hearing ability andsense of smell, which enable him to detect signs hidden from human observation.His record of savinglives is outstanding, for he often gives warning of fire and other dangers not noticed by his master.
The dog' s major contribution, however, has been to medical research.Both his diet and hisstructure are comparable to those of the human being, and so he has been the subject of countlessdemonstrations and experiments.Open-heart surgery has been made possible largely because of the dog.But his sacrifice has repaid his own species as well by safeguarding it from rabies(狂犬病), distemper,and other diseases.
The dog has always been noted for________.