9. To prepare the sac for use, empty it out of the broth tube into a sterile glass dish. 10. Grasp the tube near its junction with the sac in the jaws of sterile forceps, and with a teat pipette remove sufficient of the contained broth to leave a small space in the sac. Introduce the inoculum in the form of an emulsion by means of another pipette. 11. Still holding the tube in the forceps, draw it out and seal off near the sac in the blowpipe flame. 12. When cool wash the sac in sterile water, then transfer to a tube of nutrient broth and incubate over night to determine its impermeability to bacteria. 13. If the broth outside the sac remains sterile, insert the sac in the peritoneal cavity of the experimental animal. ~5. Intracranial.~--(_Anæsthetic, A. C. E._) [Illustration: FIG. 186.--Guarded trephine.] _Trephines and Surgical Engine._--The most useful instrument for intracranial operations upon animals is the small nasal trephine (Curtis) having a tooth cutting circle of 7 mm. The addition of an adjustable collar guard--secured by a screw--prevents accidental laceration of the dura mater or brain substance[13] (Fig. 186). This size is suitable for monkeys, dogs, cats and large rabbits. Other smaller sizes which will be found useful for guinea pigs and other small animals cut circles of 6 and 4 mm.; for very small animals--young guinea pigs and rats--a small dental drill or screw will make a sufficiently large hole to admit the syringe needle. The trephine can be set in ordinary metal handles and rotated by hand, but a surgical engine of some kind is much preferable on the score of rapidity and safety to the animal. The Guy's electrical Dental engine[14] (Fig. 187) which can be connected to a lamp socket or wall plug, and is operated by a foot switch, although inexpensive is eminently satisfactory. NOTE.--A fine dental drill attached to the dental engine renders the manufacture of aluminium handles needles (see page 71) quite an easy matter. (a) _Subdural._ 1. Anæsthetise the animal and secure it to the operating table, dorsum uppermost. 2. Shave a portion of the scalp immediately in front of the ears. [Illustration: FIG. 187.--Guy's electrical dental engine.] 3. Mark out with a sharp scalpel a crescentic flap of skin muscle, etc., convexity forward, commencing 0.5 cm. in front of the root of one ear and terminating at a similar spot in front of the other ear. Reflect the marked flap. 4. Make a corresponding incision through the periosteum and raise it with a blunt dissector. 5. With a small trephine (diameter 6 mm.) remove a circular piece of bone from the parietal segment. The centre of the trephine hole should be at the intersection of the median line and a line joining the posterior canthi (Fig. 188).