What led to Hunter's far-reaching discovery was his investigation as to the cause of the growth of the antlers of the deer. Wishing to ascertain just what part the blood-supply on the opposite sides of the neck played in the process of development, or, perhaps more correctly, to see what effect cutting off the main blood-supply would have, Hunter had one of the deer of Richmond Park caught and tied, while he placed a ligature around one of the carotid arteries--one of the two principal arteries that supply the head with blood. He observed that shortly after this the antler (which was only half grown and consequently very vascular) on the side of the obliterated artery became cold to the touch--from the lack of warmth-giving blood. There was nothing unexpected in this, and Hunter thought nothing of it until a few days later, when he found, to his surprise, that the antler had become as warm as its fellow, and was apparently increasing in size. Puzzled as to how this could be, and suspecting that in some way his ligature around the artery had not been effective, he ordered the deer killed, and on examination was astonished to find that while his ligature had completely shut off the blood-supply from the source of that carotid artery, the smaller arteries had become enlarged so as to supply the antler with blood as well as ever, only by a different route. Hunter soon had a chance to make a practical application of the knowledge thus acquired. This was a case of popliteal aneurism, operations for which had heretofore proved pretty uniformly fatal. An aneurism, as is generally understood, is an enlargement of a certain part of an artery, this enlargement sometimes becoming of enormous size, full of palpitating blood, and likely to rupture with fatal results at any time. If by any means the blood can be allowed to remain quiet for even a few hours in this aneurism it will form a clot, contract, and finally be absorbed and disappear without any evil results. The problem of keeping the blood quiet, with the heart continually driving it through the vessel, is not a simple one, and in Hunter's time was considered so insurmountable that some surgeons advocated amputation of any member having an aneurism, while others cut down upon the tumor itself and attempted to tie off the artery above and below. The first of these operations maimed the patient for life, while the second was likely to prove fatal.