On the other hand, where it is determined to open the sac, the pinching up of the sac must be managed with great care, to avoid injury of the bowel. There is generally a little fluid to be found at the fundus, which will protect the bowel. In one case in which Liston operated, he tells us, "there was no possibility of pinching up the sac, either with the fingers or forceps; it contained no fluid, and was impacted most firmly with bowel; very luckily the membrane was thin; and, observing a pelleton of fat underneath, I scratched very cautiously with the point of the knife in the unsupported hand, until a trifling puncture was made, sufficient to admit the blunt point of a narrow bistoury."[144] If the sac contains bowel and omentum, it is safer to open it over the omentum than over the bowel. When a small opening is made, an escape of the contained fluid takes place, and then the sac should be slit up as far as its neck by a probe-pointed bistoury, guided by the finger, introduced to protect the bowel, whenever the opening is sufficiently large. The forefinger must now be cautiously insinuated into the neck of the sac, the nail being directed to the bowel, the pulp to the crescentic margin of Gimbernat's ligament, and any constriction very cautiously divided. The bowel should then be drawn down a little, the constricted point carefully examined, and then returned or not, according to its condition. Two points require a brief separate notice:-- 1. In what direction is the crural arch to be divided? Not outwards certainly, on account of the vein, nor downwards, as the bone prevents that direction. Is it to be upwards or inwards? Not upwards, for such an incision would endanger the spermatic cord or round ligament, besides greatly weakening the abdominal wall by the division, partial or complete, of Poupart's ligament. Inwards then it must be; and little more need be said about it, were it not for the occasional existence of an abnormal course and distribution of the obturator artery. [Illustration: FIG. XXXII.[145]] The usual origin of this vessel is from the internal iliac, in which case (Fig. XXXII. N O) it never comes near the sac at all. In certain cases (1 in 3-1/2) it rises from the epigastric, and in a very few (1 in 72) from the external iliac. If rising from either of the two last, it most commonly passes downwards at the outer side of the hernia, in which case (Fig. XXXII. S O) no harm can possibly result; but in a few rare cases, perhaps 1 in every 60 of those operated on, the vessel winds round the hernia (Fig. XXXII. O), crossing at its inner side, and thus may be (and has actually been) divided by a rash incision. With due care, however, and by cutting a very little at a time, even this danger may be avoided.