The Theories on the Formation of the Aqueous Humour.

FOR many years there has been controversy, becoming very bitter at times, regarding the mechanism of formation of the aqueous humour. This fluid has at various times been considered a dialysate, a transudate, an exudate or a secretion, and very strong arguments have been brought forward for and against the various concepts by particular experimentalists partial to one or the other theory. Before going on to a study of the various theories in relation to the eye, it is important to define what is meant by the terms dialysate and ultrafiltrate, exudate, transudate, and secretion. As an introduction to their definitions I would like to refer very briefly to crystalloids and colloids, and to the permeability of membranes. Crystalloids and Colloids.-The division of substances into the two large groups of crystalloids and colloids is due to the work of Graham (1876) the pioneer in colloidal research. Crystalloids, he maintained, examples of which are sodium chloride, sugar and magnesium sulphate, are fast diffusers, whereas colloids, examples of which are albumen, caramel, silicic acid, gum-arabic and tannin, are slow diffusers. As a general rule when solutions of colloids are evaporated to dryness they leave a gluey or glassy residue in contrast to crystalloids that crystallise out in saturated solutions. There are, however, certain colloids capable of crystallisation such as vegetable proteins, haemoglobin, and egg and serum albumen, so that crystalloids in contrast to colloids might be simply defined as substances with a high rate of diffusion and crystallising out easily in saturated solution. The permeability of membranes.-A membrane may be completely impermeable. This is very rare but it occurs in the case of the membrane surrounding an egg. Most membranes are permeable to a small or large degree, depending on the fineness of the grain-i.e., the size of the holes through which the fluid passes. Filter paper permits the passage of water and all dissolved substances crystalloid or colloid. Gel membranes such as gelatin collodion or cellophane are of porous structure and they