NINE FIGURES
Freezing-drying and rehydrating of nerves for grafting purposes was proposed by Weiss and has since been used routinely in this laboratory. I n the early phase of the work freezing and drying were carried out in the manner outlined by Beiisley ( '33) and Hoerr ('36) f o r histological preservation. Later a number of modifications were introduced with the aim of simplifying the procedure and adapting it to the practical requirements of nerve grafting. While the method had led to successful results in its initial empirical form (Weiss and Taylor, '43; Weiss, '43) the results of different experimental series were by no means uniform, and it soon hecame obvious that further systematic progress was conditional on a standardization of the many variables involved.
The three phases of the processfreezing, drying, rehydrationvary within wide limits depending on the procedure followed. Since the condition of the processed tissue, and, consequently, its presumable adequacy for grafting purposes vary accordingly, a precise quantitative determination of the rate of freezing, drying and water uptake of tissues under various conditions was required. Therefore, the following study was undertaken.
I. FREEZING
The extent of formation and growth of ice crystals within tissues being frozen depends on the speed of the freezing process. Damage to tissue coiistituents will be the less, the smaller the crystals, that is, the shorter the period in which they can form. Evidence has been presented (Boell, '45) that not only the structural integrity of the tissue, but its biochemical integrity, as reflected in enzyme content, likewise is best preserved when freezing occurs most rapidly.