Most of what is known about the function of the lymphatic system has come from experimental work on animals. Although there were some notable experiments in the eighteenth, nineteenth and early twentieth centuries, the procedures were not easy until small, flexible, plastic cannulae became available in the mid-twentieth century. This made it possible to cannulate the major lymph trunks, such as the thoracic duct in rats, guinea pigs, and even mice, and to collect substantial volumes of lymph.

A major problem with this procedure was that it required relatively deep anesthesia as well as the postoperative restraint of the animals in specially designed cages. These factors induce major perturbations in the function of the blood vascular system and the levels of the adrenocortical hormones. Also, in the ensuing days, the loss to the animals of substantial amounts of fluid, protein, electrolytes and cells rapidly produce a state of metabolic chaos unless elaborate facilities are at hand to make good the deficits at physiological rates. Although such preparations can be a useful source of lymph cells, their value as physiological models is limited.

In the early 1960s Professor Bede Morris of the Australian National University initiated research into the lymphatic physiology of ruminant species, particularly sheep. The latter are docile animals that make no attempt to interfere with cannulae that have been inserted into them; they need no restraint and can be kept in ordinary pens to which they are accustomed. Also, the cannulation of a single peripheral lymph duct causes an insignificant metabolic deficit so that lymph from individual organs can be collected for periods of weeks from unanesthetized animals under physiological conditions. Because of the structure of the ovine placenta, it is even possible to cannulate lymphatic vessels of fetuses in utero. These types of preparations have yielded much useful and novel information, and will probably continue to do so. However, the risks of extrapolating data from a single species should not be overlooked.

Some useful data have been obtained from humans but ethical and technical difficulties have prevented systematic studies.

It may be worth noting here that while lymph always contains some lymphocytes, and may occasionally contain all other classes of blood leukocytes as well as some red cells, it never contains platelets. It does, however, contain fibrinogen and will clot unless anticoagulants are added to the containers into which it is collected.

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