The interruption of the gas-flow prevents over-freezing and thereby lessens the amount of change in the cells. When sufficiently frozen the gas is then turned off; the knife, with bevel-edge down, is held in the right hand with its handle between the palm and the ball of the thumb with the back of the hand uppermost; and the edge of the knife set at an angle of 45° to the tracks, along which it is rapidly pushed back and forth, shaving the sections from the frozen tissue as the latter is pushed up above the level of the tracks by means of the micrometer screw turned with the left hand. The screw should have been adjusted at the proper height before freezing, so that no time is lost in getting the object-holder up to the height for cutting. By holding the elbow and fore-arm closely against the body and pushing from the elbow with wrist-joint fixed the shaving of sections can be accomplished very quickly, so that with one freezing several hundred sections can be obtained. The microtome-screw should not be turned by the left hand until the knife on its return has cleared the tissue. A little practice in co-ordinating movements is necessary for expert work. The sections should be allowed to collect upon the knife-blade until a large number have been cut; they are then swept off the blade by the finger into cold freshly-boiled physiologic salt-solution. Sections of tissue that have been previously fixed can be put into 60 per cent alcohol, where the sections will unroll and straighten out perfectly flat. When the method of freezing by alternately turning on and off the gas, as given above, is followed, and the amount of freezing in the interval noted, there is no danger of over-freezing, and the breaking and crumbling of sections, with the production of marked artefacts, is avoided. When over-freezing has occurred the block may be partly thawed out by the finger. But _over-freezing_, as well as _repeated freezing and thawing_, may cause so much damage to fresh tissue that a diagnosis cannot be obtained from the frozen sections. After thawing out in the physiologic salt-solution the sections of fresh tissue obtained by freezing are treated according to the methods given in the next section of this chapter. [Illustration: FIG. 55.—Knife for Bardeen freezing-microtome.]