The Scars of Evolution

by Elaine Morgan

  11

  Breathing

  ‘It is a strange fact that every particle

  of food and drink we swallow has to pass

  over the orifice of the trachea, with some

  risk of falling into the lungs.’

  Charles Darwin

  Darwin was not overstating the matter. It is indeed a strange fact, which looks like a deplorably clumsy piece of biological design – all the stranger because it is another of the many ways in which humans differ from all other land mammals.

  The basic mammalian design is one in which air enters through the nostrils and goes down through one tube into the lungs, while food and drink enter through the mouth and go down through another tube into the stomach. The tube for transporting air is the windpipe (trachea) and the tube for transporting food and drink is the gullet (oesophagus). In most mammals it is impossible for food or drink to ‘go down the wrong way’ into the lungs, because the upper opening of the windpipe is not situated in the mouth, but higher up, at the back of the nasal cavity. Everything the animal eats or drinks has to pass around or to one side of the upper section of the windpipe before reaching the gullet and descending into the stomach.

  It is a simple and foolproof system which keeps the openings of the two channels entirely separate. Anyone who has watched a thirsty horse drink (or especially a thirsty camel) will have noticed that the animal can go on drinking for a long time without having to pause to draw breath. Because the air and food channels are separate all the way, they can breathe and drink simultaneously.

  In all land mammals except man, the top of the windpipe is in contact with the palate, which is the partition separating the mouth cavity from the air passages. The front part of it is a bony partition, the roof of the mouth. Behind that in most animals the palate is extended backwards in a continuous sheet of tissue known as the soft palate. In primitive animals, such as some reptiles and a few present-day marsupials, the top of the windpipe (the larynx) passes through the soft palate and is permanently locked into position there. These animals are therefore incapable of breathing through their mouths, and they cannot utter any vocal sounds.

  In the majority of mammals nose-breathing is still the normal procedure, but the barrier against mouth-breathing is less absolute. The larynx is not permanently locked into position: it passes through a small aperture in the soft palate which is controlled by a sphincter – that is, a contractable ring of muscle. A dog, for example, may want to breathe through its mouth temporarily for the purpose of panting to cool down, or in order to bark. When that happens, the sphincter muscle is relaxed and the top of the windpipe slides down through the hole in the soft palate, just far enough to bring the larynx down into the mouth cavity. When a dog barks or howls it often stretches its neck, and that helps to disengage the larynx from the palate. When the panting or howling is over, the neck relaxes, the larynx goes back up through the hole, the sphincter tightens around it, and the dog is once again a nose-breather. Some version of this arrangement holds true for most mammals.

  In adult humans the larynx has lost all connection with the palate. It has slid right down into the throat, below the back of the tongue, so that, as Darwin pointed out, the opening of the windpipe is in front of the gullet opening and on the same level. Our soft palate is no longer a continuous sheet of tissue with a neat, closable hole in it to accommodate the larynx. There is a large gap in it. The uvula, the conical fleshy prolongation which can be seen dangling at the back of the throat, represents part of the upper rim of the hole. We can therefore breathe with equal facility either through our noses or through our mouths. But we cannot breathe and drink at the same time, and if anything makes us gasp while we are drinking, some of the liquid is inhaled into the lungs.

  This evolutionary change is known as ‘the descended larynx’, and much scholarly speculation has gone into trying to work out why it is an improvement on the normal arrangement. (There is always a tacit reluctance to believe that any anomalous features characteristic of our species can be other than an improvement.)

  Perhaps the most unexpected eulogy of the descended larynx came from an enthusiastic surgeon who called it an excellent arrangement, because human patients with injuries to the mouth cavity can be easily fed through a tube passing into the stomach via the nose. That is indeed one of the incidental benefits of laryngeal descent, but it can hardly explain why it came about.

  Like the spine of a quadruped, the normal mammalian blueprint for breathing has been perfected over more than two hundred million years and would not have been abandoned for any trivial reason. It might be as well to count up the advantages we have lost before searching for what we have gained.

  Nose-breathing is important to most mammals on five counts. Firstly, the sense of smell is of vital importance to them, and the airborne pheromones can only be detected if the air passes over the scent-detecting organs in the nasal passages. Secondly, nose-breathing protects the lungs by filtering out noxious substances. Particles of airborne dust are trapped by the hairs lining the nostrils. Finer particles are also arrested before reaching the lungs; they adhere to the mucus-lined passages of the nostrils and the nasal passages into which they lead, and are eliminated by one of a variety of routes – sneezing, nose-blowing, spitting, or being swallowed with the saliva.

  Thirdly, air which travels through the nasal passages is either warmed or cooled to near body temperature before it reaches the sensitive and vulnerable tissues of the lungs. Fourthly, if the air is arid, it is moistened by the secretions of the mucous membranes. Finally, some of the mucous secretions are mildly bactericidal. Air inhaled through the nostrils reaches the lungs filtered clean, warm, moist and to some extent sterilised.

  All these safeguards are discarded when we breathe through our mouths, as all of us do for part of the time – for instance, when we are talking, running, or doing any task involving physical exertion.

  The lungs of all mammals have some ability to dislodge – by coughing – any noxious particles which somehow penetrate the outer defences, but in most mammals this is a last-ditch emergency procedure; lungs did not originally evolve to cope with unprocessed air. Our pets breathe the same air as we do, but they do a lot less coughing. The ones most troubled with breathing disorders are breeds like Pekinese dogs, in which selective breeding has artificially reduced the length and efficiency of the nasal passages.

  The descent of the larynx entailed a whole sequence of modifications in the upper respiratory tract. The consequences of these changes are sometimes inconvenient, and occasionally lethal. One of them concerns what happened to the tongue.

  Swallowing is a uniquely complicated process in humans because of the need to prevent food and drink from entering the windpipe. The device which has evolved to ensure this is as follows. The back of the tongue, unlike that of most mammals, projects an appreciable distance beyond the back teeth. Every time we swallow food or a drink of water, or even our own saliva, the top of the windpipe moves upwards in the throat and buries its head (the larynx) underneath the back of the tongue. This movement forces a flap of cartilage (the epiglottis) to bend over and form a lid over the larynx which shelters it from the incoming liquid. When the liquid has passed safely down into the stomach, the windpipe drops back down to its normal site.

  If the swallower is an adult male, this manoeuvre is usually visible to the naked eye, due to the fact that in males around the age of twenty, a ridge of cartilage at the front of the larynx begins to be replaced by bone. In some men this bony projection, known as the Adam’s apple, is a prominent feature and can be seen bobbing up and down with every swallow.

  The backward projection of the tongue does not, of course, reach back far enough to block the airway – as long as we are conscious, perpendicular, and young. But with increasing age there is often a deterioration of muscle tone. Then, for a mouth-breather who falls asleep with the mouth open, problems can arise. The tongue may slump farther into the back of the throat by the mere f
orce of gravity, so that the air has to force its way through the diminished opening with the noisy rattle known as snoring. If the sleeper’s airway closes up entirely as it sometimes does, then the lack of oxygen in the blood stream alerts the body to its danger, and the mind emerges from its slumber at least far enough to ensure that the blockage is removed and free breathing is restored.

  However, if a similar blockage takes place during deeper unconsciousness, as for example in the case of concussion following an accident, it may be fatal. It is because of this uniquely human hazard that instructions for First Aid to accident victims begin: ‘First check the airway.’ Deaths caused in this way, like the occasional death of an alcoholic asphyxiated by his own vomit, are among the hazards incurred by having a descended larynx.

  Within the last decade publicity has been given to a condition which has been named ‘obstructive sleep apnoea’ (‘apnoea’ means ‘without breath’) or OSA. Normally a sleeper, when suffering a blockage of the airway, first tries to breathe in spite of the blockage, with spasmodic heavings of the rib-cage and diaphragm. This in itself can have deleterious effects, especially for sufferers from heart disease, by causing abnormal changes to air pressure in the lungs, increased blood pressure in the chest and decreased level of oxygen in the blood. However, it usually only lasts for about fifteen seconds before these symptoms cause a partial return to wakefulness, whereupon normal breathing is restored, and sleep is resumed. The cycle usually repeats itself many times, in severe cases several hundred times in a night.

  In North America there are now numerous clinics specialising in sleep disorders, and doctors have found a correlation between OSA and hypertension (high blood pressure). OSA is not a primary cause of hypertension but it may be a contributory or aggravating factor. According to one theory the correlation only means that both conditions result from some other cause such as, for example, being overweight. But a leading researcher, Christian Guilleminault, reports that only half of those who suffer from OSA are overweight, and hypertension still occurs in the lean OSA patients.

  The condition can cause psychological as well as physical ill-effects. Episodes of apnoea are normally commoner and more severe during the REM (rapid eye movement) phases of sleep when dreaming takes place; these are the periods of deepest unconsciousness. According to current thinking, they are also the phases we can least afford to be deprived of.

  When a patient’s REM sleep is disturbed by frequent episodes of OSA – even though he does not remember them in the morning – it may result in persistent sleepiness and fatigue during waking hours. Also there are often complaints of deterioration of memory and judgement, early morning confusion and headaches, and attacks of irrational anxiety and depression. The symptoms resemble in a milder form the effects induced in prisoners who are deliberately deprived of sleep while being interrogated. The recommended treatment varies from prescribing weight loss and the avoidance of alcohol late in the day to special appliances pumping air through the nose, or surgery to remove part of the soft palate and tighten the muscles in the throat.

  No baby is born with a descended larynx. For the first few months of life its respiratory passages are cast in the normal mammalian mould, with the larynx high up, in contact with the palate and in communication with the nasal passages. It is a habitual nose-breather. When a young baby suckles it can suck and breathe at the same time, and the milk flows steadily down into the stomach without the complicated swallowing procedures necessary in adult life.

  Some time between the third and sixth months after birth, the larynx loses contact with the palate and begins to descend. There is a transitional period during which the baby’s upper respiratory tract is not quite animal and not quite human, and its reflexes are in the process of learning to adapt to its new physiology.

  This period coincides with the peak incidence of Sudden Infant Death Syndrome (SIDS). Unexplained deaths in this age group used to be accounted for by saying that the baby had been ‘overlain’, and mothers were instructed that it was safer and healthier for babies to sleep alone. Now that babies do sleep alone, these fatalities are called ‘cot deaths’.

  Usually post mortems reveal nothing about the cause of death. Records show that the incidence of cot deaths is somewhat higher in winter. Correlation with recorded weather reports shows that the highest peaks occur not during but from two to five days after sudden spells of very cold weather, during which the baby’s resistance to virus infection may have been lowered. This is consistent with the possibility of an infection of the upper respiratory tract – in other words, the baby has caught a cold. But this does not explain why death should be caused – in this age group specifically – by an infection so mild that in the great majority of cases the parents have not noticed it, and the autopsy reveals nothing.

  The possible connection with the migration of the larynx was first perceived by Edward Crelin who discovered over twenty years ago, when about to operate on a baby, that there was no textbook on infant anatomy to which he could refer for guidance. Clinicians worked on the assumption that human babies were constructed like miniature adults. So Crelin set about compiling a book on the subject which is now used as a work of reference in hospitals all over the world. One of the major discoveries he made during his research for this project concerned the way babies breathe. It was a complete surprise to him. He had expected the baby’s anatomy to resemble a miniature version of a human adult’s anatomy, but when he came to the upper respiratory tract he found himself ‘dissecting the airways of a chimpanzee’.

  When the larynx first begins to descend, although physically it is becoming possible for the baby to breathe through its mouth, its reflexes are still those of a nose-breather. If the nasal passages are blocked because of an infection, it shows distress and may cry out. As in any other animal, vocalisation detaches the larynx from the palate and enables the baby to breathe, and it may drop off to sleep again with the mouth open.

  The risk arises if it is sleeping on its stomach, or on its back with the head low. The larynx may then slide back up to a position where the uvula can enter it and block it. After death, the muscles relax, the larynx disengages when the baby is picked up, and there is nothing to show the cause of death except perhaps some slight damage to the lung.

  Crelin believes that 90 per cent of cot deaths are caused in this way. His theory remains a theory, but it has been reinforced by clinical experience. For example, a Midwest paediatrician, the late Harvey Kravitz, advised mothers to raise the head end of the cot a little, and to keep the baby’s head up when out of the cot; in the next twelve years there was not a single cot death among the 1,800 babies he dealt with, whereas statistically about five would have been expected.

  A report from the Netherlands involved much larger numbers. In the early 1970s Dutch mothers were advised to place their babies face down to sleep, and by the 1980s at least 60 per cent of them were following this advice. During that time the incidence of cot deaths in the country went up from 0.46 per thousand births in 1969–71 to 1.13 in 1987. In 1987 well-baby clinics throughout the country began to advise against babies sleeping face down and a 40 per cent decrease in cot deaths was recorded between 1987 and 1988.

  These statistics, plus the fact that cot deaths are very rare before the age of three months and after the age of six months, are consistent with Crelin’s belief that they are connected with the transition from the infantile to the adult-human location of the larynx.

  Early colds and infections may cause trouble later on for children who do not succumb to cot deaths. If they are forced into mouth-breathing too early because of nasal obstruction, they sometimes acquire the habit of habitual mouth-breathing, known to doctors as HMB, and this leads to a kind of vicious circle.

  Because the nasal passages are being by-passed and not ventilated, secretions accumulate there which further obstruct the air flow; dried mucus forms a perfect breeding ground for micro-organisms which can result in inflammation and overgrowth of the ad
enoid tissue. Open-mouth breathing also leads to a higher evaporation rate of saliva, so that less of it needs to be swallowed. Normally, frequent swallowing helps to clear and ventilate the middle ear via the eustachian tube. A Netherlands team (Van Bon et al) in 1989 confirmed the hypothesis that HMB may be a cause of otitis media (earache) in young children. Early in this century, descriptions of slack-jawed, snotty-nosed slum children were sometimes taken as signs that they were hereditarily subnormal, rather than as a condemnation of the overcrowding in cold, damp conditions which spread the infections in the first place.

  Understanding of the evolution of the larynx grew slowly, and one reason was that anatomists study corpses and not living animals. Drawings of dissected animals by different anatomists showed the larynx in different positions, and led to heated controversy in the 1880s.

  It was finally realised that the different versions depended partly on how the animal died – if it died crying out, then the larynx would be found in the mouth cavity – and partly on how the body was manipulated during dissection. Once the sphincter is relaxed, simply stretching out the animal’s head and neck may cause the larynx to slide easily out of the nasal chambers and thus be found in a more human-like position.

  The matter was finally clarified by an examination of live and conscious animals – often by subordinates rather than the anatomists themselves. Dr R.L. Bowles in 1889 wrote of one of these unsung heroes: ‘My assistant Mr Stainer has introduced his hand into the pharynx [back of the throat] of many pigs of six months and upwards.’ Stainer found that in the living animals the windpipe passed up through the soft palate. It was held in place there by the epiglottis (a small flap attached to the top of the larynx) with the sphincter muscle of the opening in the soft palate ‘firmly clasping its root’.