ube) with
hot water, and stand the capsule in a beaker of boiling water for a few
minutes to melt the gelatine.

7. Remove the solution of gelatine from the interior of the celloidin
case with a pipette.

8. Fill the sac with nutrient broth and place it, _glass tube downward_,
in a tube containing sufficient sterile nutrient broth to cover the sac
to the depth of 1 cm. Plug the tube and sterilise in the steamer in the
usual manner.

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.~--(_Anaesthetic, 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.

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