The thymus gland and cot deaths
High cost of evolution
What causes cot deaths? We still don’t know. Over the centuries many ideas have been suggested. Since biblical times ‘overlying’ has been suspected (cf. 1 Kings 3:16–20). What happens is that the baby is in bed with his mother. In her sleep she rolls over and smothers the child.
Although this may have been an accident, in the eighteenth century it was often a method of infanticide.
In the early part of the twentieth century another cause was suggested: status thymo-lymphaticus. Let me explain the background.
It was often noticed at autopsy that a young infant who had died suddenly had a very big thymus gland. Sometimes the lymph nodes were enlarged too. The thymus is a fleshy organ situated inside the top of the chest overlying the windpipe (trachea) and great blood vessels as they leave the heart. Some babies with a large thymus during life develop croup (stridor) where breathing is obviously obstructed. In the past it was believed that the big thymus could press on the trachea and block it, causing the croup and even sudden death.
Apart from autopsy it is possible to see the size of the thymus on an X-ray of the chest. Sometimes in a healthy child the thymus may be so large as almost to fill the upper part of the chest.
It is only in more recent times that physiologists have understood the function of the thymus gland. We now know that it plays a dominant role in the development of immunologic competence in the fetus and young infant. It is the thymus which, in early fetal life, gives the body the ability to distinguish between ‘self’ and ‘non-self’. The thymus is relatively big in small infants. It continues to grow until puberty, but at a much slower rate than the rest of the body, and so looks as though it has ceased growing. In fact it disappears in adult life.
Function
Before the physiological value of the thymus gland was appreciated, it was regarded, following evolutionary theory, as a vestigial organ. A vestigial organ is a useless representative of organs which in allied kinds of animals are functional. It was believed that some organs of value in lower animals became vestigial in their descendants. Other organs considered to be vestigial were the appendix, the coccyx (thought to be a vestigial tail), the pineal gland, etc. In fact any part of the body which was fairly small and for which no obvious use could be found was considered vestigial. The higher the animal in the evolutionary tree the greater the number of such vestigial organs expected. Man, being the highest form of animal, was supposed to carry a large number of such organs.
Because the thymus was big in infants and later disappeared, and because no one knew what function it held, following evolutionary theory it was considered vestigial and useless. We now know the gland rapidly decreases in size when a child suffers from a severe debilitating illness such as gastroenteritis, which overwhelms the immunological system. Thus at autopsy in a child who has died from such a condition, the thymus will be small. But if death is very rapid (as in the cot death syndrome) the thymus does not have time to shrink and so will be big.
Doctors in the past fell into the error of believing that the large (unatrophied) thymus pressing on the windpipe was the cause of death in the cot death syndrome. Then, as now, cot death was much feared. If, therefore, the thymus was superfluous, unnecessary, just a vestigial organ—as evolutionary theory taught—it seemed unfortunate that such a useless organ could cause death. Surely it would be better if the big thymus were removed before it could do harm. In fact this is easily done. The thymus shrinks rapidly and permanently under deep X-ray radiation. So, in the early 1930s, preventative radiation of young infants was frequently performed in some centres.
Notice two errors in the above reasoning: (1) that the thymus was a vestigial organ and unnecessary, and (2) that a big thymus was the cause of sudden death in infants.
The results of preventative treatment were dramatic. The thymus shrunk rapidly, and the children so irradiated rarely died of cot death syndrome. (The reason for this is that cot death is very rare anyway.) But unfortunately, when the thymus is irradiated it is difficult to prevent radiation of the thyroid gland too. The two glands lie very close together, particularly in a small infant. So the result was that about 10 to 15 years later the irradiated children died of cancer of the thyroid gland, induced by the X-ray therapy. Belief in evolutionary vestigial organs had proved a very costly error. Wrong beliefs result in wrong practice.
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