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It is shown that neither the oil damping nor the rest pauses in the current recommended use of the thrombelastograph eliminate the measurement of the viscous part of the complex modulus. The former, however, is advantageous in preventing unwanted oscillations but the latter is best eliminated. In addition to earlier published data, we now have some 30 thrombelastograms, about half from normal subjects and half from hospital patients with various diseases. Apart from severe haemophilia, for which the equation cannot be tested, all these are adequately described by means of a simple equation already discussed in earlier papers (slightly modified) and we conclude that the process of coagulation, which probably simply follows the polymerization of fibrin, may be described by the same kinetic equation.

The concept of “basic equations” is discussed. Maxwell proposed a basic equation for the relaxation of stress in a material depicted as an elastic and a viscous element in series; and Lord Kelvin proposed a similar equation for these elements in parallel. Although some materials obey these equations, their proposers were quite clear that further modifications would be needed for many real systems. The Maxwell equation for relaxation at constant strain and the Kelvin equation for the build-up of strain at constant stress give simple logarithmic relations between two dimensionless ratios, one rheological and the other temporal. Some years ago, an empirical equation was found to relate the complex modulus of coagulating milk and also blood, to the time following the first signs of coagulation. In the present paper, it is shown that, making the three most probable assumptions about the limits and form of the coagulation curve, this hitherto empirical equation is immediately derived. It shows a striking parallelism with the Maxwell and Kelvin equations. In the latest stages of normal blood coagulation, however, the equation does not hold. This “ceiling effect” is doubtless due to the rapidly increasing shortage of unattached possible junction-points on the long-chain molecules but its quantitative nature is still obscure.

Hartert’s thrombelastograph is used to help in the diagnosis of abnormal conditions of blood clotting. Its practical usefulness lies in its capacity to distinguish borderline cases. It is felt that its efficiency for this purpose would be enhanced if the equations for the rate of increase of the complex modulus during coagulation and of its decrease during subsequent softening (fibrinolysis and/or retraction) were known. The parameters of these equations could then be used for diagnostic and prognostic purposes and the equations should also throw some light on the mechanism of fibrin polymerization and of softening. Experiments have been done on bovine blood (also using a somewhat similar but larger torsiometer) and on human blood as well as on fibrinogen-thrombin. The polymerization process, especially in the case of fibrinogen-thrombin, appears to consist of two well-defined stages, an equation proposed in a previous paper holding in the earlier stage and a still simpler equation in the later stage; but the very last stage of coagulation does not follow any known equation. The effects of additions of substances known to induce or inhibit fibrinolysis and retraction in human blood were confirmed; but considerable differences were found, in some cases, in the behavior of bovine blood. Some marked effects on rate and extent of coagulation produced by these substances do not appear to have been previously recorded.

An apparatus is described which can be used on the farm and which measures the length of thread (l) which can be drawn from samples of bovine cervical mucus. The use of special forceps has, for the first time, made it possible to stretch the mucus without slipping at the grips. All the cows examined showed significantly higher values of lat and near the time of oestrus than at other times in the cycle (dioestrus).

There are no significant differences between the surface tensions of blood, plasma and serum. On glass, blood wets very slightly better than plasma which wets very slightly better than serum. The wettability of these materials on fibrin is in the same order as that on glass. No appreciable difference has been detected between the wetting of glass and fibrin surfaces by blood, plasma or serum. Our investigations demonstrate that no relation exists between the wettability of a surface with blood and the coagulation of the blood; since fibrin surfaces, which we found to be wettable, have previously been shown by one of us (A.L.C.) to act as anticoagulants both extracorporeally and in vivo. Our experiments with air emboli in the microcirculation of the hamster’s cheek pouch and of the mesentery of guinea pigs and of rats show that living vascular endothelium, contrary to the generally held contention, is wettable.

An equation is proposed to describe the rate of setting (i.e. rate of increase of rigidity) of both blood and milk gels under certain limited conditions. This equation is the third of a series whose first and second members represent an exponential and a power-relation respectively. The more obvious use of a logistic type of equation does not fit the experimental data. The molecular processes underlying the new equation are not yet understood and the treatment must be regarded as phenomenological, though not altogether empirical, since the equation fits the data best when independently determined time zeros are used, both for blood and for milk.