Fifty years ago the late Jim Linzell and I were writing our monograph, Salt Glands in Birds and Reptiles, for the Physiological Society's Monograph Series; it was published in May 1975. In this series I revisit some of the topics and people who followed up the discovery of salt glands in birds by Knut Schmidt-Nielsen.
One problem with writing a book 50 years ago was that the copy that arrives in the reader’s hands has a sense of finality. Last thoughts, changes of mind or new evidence can only be passed on by producing a new edition some tears later or covering the same ground in an invited review. That problem, although still present, is lessened by having a website covering a particular publication that can be amended accordingly.
One point we forgot to mention when we wrote Salt Glands in Birds and Reptiles was the possibility that secretion from other glands could perhaps be mistaken for secretion from the nasal salt glands of birds. Small drops or dampness seen within the nostril in response to cholinergic drugs or during periods of excitement could be interpreted as a sign of the salt gland being activated at a low rate or for a brief period. However there remains the possibility, particularly if the secretion that appeared was not analysed, that the small amounts could arise from other glands in the head, the secretions from which can find their way to the nostrils.
The Harderian gland is a relatively large gland that sits behind the eyeball. It is particularly associated with tetrapods which have a nictitating membrane. A number of functions of its secretion have been described or postulated including, of course, lubrication and protection, including immunoprotection, of the eye. In short, it is a second source of tears, alongside the lachrymal gland which in birds is very small. Just as when we cry, tears appear not only from the eye itself but also pass down the nasolacrimal duct to the nose; sniffling and lachrymosity go together.
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| We reproduced this drawing in Salt Glands in Birds and Reptiles I have added colouring to show the nasal salt gland and the Harderian Gland |
There is no indication that in birds with a nasal salt gland that the Harderian gland plays any part in removing excess salt. In geese, for example, we found no evidence for any increase in blood flow through the Harderian gland. By contrast, salt gland blood flow increased increased up to 26-fold when secretion was activated by a salt load. However, there is one intriguing report in the literature that the Harderian gland might be doing something in relation to salt and water balance in some birds.
In 1968 the late Maryanne Robinson Hughes and Frank E Ruch of the University of British Columbia noticed that in domestic ducks kept on sea water the feathers between the beak and the eye were wet and contained depressions which were filled with a clear fluid. Analysis of the fluid showed the sodium concentration to be similar to that of blood plasma. However the potassium concentration was much higher than that in plasma. Moreover, the potassium concentration increased with higher salinities up to full strength sea water, the main rising occurring when the salinity of the water exceeded that of plasma. No such effect was seen in gulls and the suggestion was that ducks living on sea water are at a precarious state of salt and water balance compared with gulls which can cope with the salinity of sea water much more readily. Although the source of the potassium-rich tears was not identified, the Harderian gland is the most likely since since the lachrymal glands are tiny, only one hundred of the weight of the Harderian glands.
Pertinent to the point of this article is that it was impossible to determine just how much potassium the Harderian glands were secreting in the ducks since tears could be passing down the nasolacrimal duct. However, they did find a band of smooth muscle around the opening of this duct which they suggested could act as a sphincter regulating the passage of tears from the eye by that route.
If only we had mentioned it at the time I would not have needed to write this article 50 years later.
Fänge R, Schmidt-Nielsen K, Robinson M. 1958. Control of secretion from the avian salt gland. American Journal of Physiology 195, 321-326.
Hughes MR, Ruch FE. 1969. Sodium and potassium in spontaneously produced salt-gland secretion and tears of ducks, Anas platyrhynchos, acclimated to fresh and saline waters. Canadian Journal of Zoology 47, 1133-1138.
Peaker M, Linzell JL. 1975. Salt Glands in Birds and Reptiles. Cambridge: Cambridge University Press.
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