- Email: [email protected]
Computer-assisted Tomography of the Brain
1052
understand microbial diseases, their epidemiology, and their therapy, he is surely the person to interpret to the physicians and surgeons the implications of the laboratory findings for the management and control of infection. The integration of infectious-disease units into district general hospitals should enable the I.D. specialist to look after his own infective patients, whether originating in the hospital or in the community at large, and at the same time to advise on the management of the other cases of infection which, for whatever reason, need to remain in other units. However, a secure career structure and future must be demonstrated before it will be possible to recruit into this specialty the investigationally minded, enthusiastic young practitioners on whom the future of any major trained
to
specialty must rest.
Computer-assisted Tomography of the Brain
scanning is likely to change the whole investigative approach to the patient with suspected intracranial disease. This new technique, using EMI
scintillation detectors associated with computer processing of the output, allows small differences in the radiodensity of intracranial structures to be recorded. Horizontal slices of brain density are, displayed on a cathode-ray oscilloscope, from which a’ Polaroidprint can be taken; a paper print-out of the different numerical densities can also be obtained. The ventricles can be recognised (as areas of low density), and thus dilatation or displacement may be seen. Many focal pathological processes produce variations in density which enable lesions to be displayed directly. Research on this device started in the EMI laboratories, and the Department of Health helped with a clinical prototype. In the summer of 1973, production models were installed in a small number of British and North American centres, and reports of initial experience are now appearing.1-5 It is already apparent that supratentorial tumours can be reliably detected; tumour density varies, and with some types it may perhaps be characteristic; and the scan may also show whether the tumour is cystic or necrotic. An intravenous injection of an iodine compound may enhance the contrast between tumour and surrounding brain, and a doubtful abnormality may then be transformed into a strikingly positive picture. Metastatic tumours Infarcts can be may show multiple densities. confused with tumours, but enhancement (by iodine 1. Baker, H. L., Campbell, J. K., Houser, D. W., Reese, D. F., Sheedy, P. F., Holman, C. B. Mayo Clin. Proc. 1974, 49, 17. 2. New, P. F. J., Scott, W. R., Schnur, J. A., Davis, K. R., Taveras, J. M. Radiology, 1974, 110, 109. 3. Scott, W. R., New, P. F. J., Davis, K. R., Schnur, J. A. ibid. 1974, 112, 73. 4. Gawler, J., Du Boulay, G. H., Bull, J. W. D., Marshall, J. Lancet,
Aug. 24, 1974, p. 419. 5. Paxton, R., Ambrose, J. Br. J. Radiol. 1974, 47, 530.
will usually increase density only in tumours. Hxmatoma can be distinguished from infarction-a distinction not possible with any existing investigation-and this could be invaluable if strokes are to be more vigorously investigated in the future. EMI scanning promises to be particularly valuable in the assessment of acute head injuries with suspected intracranial haematoma; indeed, PAXicx and AMBROSE report that emergency angiography has been completely superseded in their unit.5 Posterior-fossa lesions are somewhat less easily detected, requiring special low views; even on these the dense petrous bones can make definition difficult, The finding of dilated ventricles, together with clinical suspicion, may indicate the need to focus attention on the posterior fossa and this will increase the likelihood of success. Cerebral atrophy can be inferred from ventricular dilatation and enlargement of the cerebral sulci. Brain oedema can be recognised - something no other technique can do-and much may be learnt now about the correlates of this important accompaniment of many acute intracranial The possibility of undertaking serial processes. investigations opens the way for observing the timecourse of developing or improving conditions, which could not be contemplated with more invasive investigations. Examples of such application would be changes in ventricular size after operations for hydrocephalus, the response of intracranial tumours to non-surgical treatment, and the detection of
injection)
postoperative complications. EMI scanning therefore provides as much information as any other single investigation currently available; and it does so without risk or discomfort, though the patient must lie still for four or five minutes, and young children and confused adults (those with recent head injury, stroke, or dementia) may require a sedative or even anxsthesia. Preexisting methods of investigation using contrast media entail considerable discomfort and some risk; moreover, they require the skills of a neuroradiologist, together with ready access to a neurosurgeon, because they sometimes create a need for urgent operation. For these reasons contrast neuroradiological procedures are largely restricted to inpatients in whom there is a reasonable expectation of finding a remediable intracranial condition. Seen against this background, the very ease with which the EMI scan can be performed may pose problems. GAWLER and his colleagues4 warn that scrupulous attention to detail in the performance and interpretation of the scan will be required if the method is not to acquire a reputation for inaccuracy, which they rightly say would be tragic for such a promising diagnostic method. This is what happened with isotope scanning of the brain, and the lesson of that experience should be heeded. With isotope scanning, as with computer-assisted tomography, there were hopes that the inconvenieace of contrast radiology might often
1053
be avoided and that some patients might be investigated without referral to a special centre. In the event, no satisfactory trials were carried out to establish the reliability of isotope scanning in an unselected patient population; only lately has a consecutive series investigated in a London teaching hospital been reported,and this showed isotope scanning to be falsely negative in nearly a third of patients subsequently proved to have a neurosurgical lesion. According to the National.Hospital team,4,7 three-quarters of isotope scans done at Queen Square are negative, and about a quarter cannot be justified at all; they cite evidence of similarly " wasteful and uninformed " use of plain skull X-rays8 and of the electroencephalogram.9 If this is what happens when patients are referred by neurologists, probably even less critical use would be made of such non-invasive investigations in general hospitals, where large numbers of patients present with minor neurological symptoms. Few such patients prove to have a remediable intracranial lesion, the exclusion of which is the aim of most investigations. Although isotope scanners are now available in many large general hospitals, it is doubtful whether the number of referrals to neurological centres has diminished, and most patients who are now scanned in other hospitals would previously not have been subjected to any tests at all. Even in the neurological centres few patients with obvious lesions are spared contrast studies, isotope scanning being largely used as a preliminary test. For a hospital to acquire a scintiscan or a gamma camera with the necessary supporting staff may therefore be a dubious enterprise in cost/ benefit terms, as far as investigating the brain is concerned. The EMI scanner presents a similar problem, but the stakes are higher; not only is the information it provides of more value, but the cost is much greater (over El 20,000)—and it can be used only for the investigation of the brain. ’Khe Department of Health showed wisdom when it contributed to the development of the EMI scanner, and one hopes that equal interest will be shown in evaluating its role in patient care. Because there is virtual central control over alternative means of investigation, it should be easier to make a valid assessment in Britain than in countries with a free market for medical care. In the free-market
situation, application of the scanner may prove to be somewhat different; many more machines are likely to be available, because the capital cost can readily be recovered by adjusting the charge to the patient and to the hours
a day that the scanner is used. Because cost/benefit equations are less easily solved in a National Health Service, it is all the more im-
to discover not only the performance characteristics of the equipment but also the place which it should take alongside alternative methods. Du BOULAY 10,11 has already shown that computer analysis of neuroradiological data from large numbers of cases makes it possible to calculate in any future " case which is the next best test " 12 given the results of clinical and laboratory investigations already available. Similar comparative studies of investigative methods (for other bodily systems) have consistently resulted in test reduction, by making it possible to reach a diagnosis with less information than clinicians usually think necessary; some of these investigations have included calculations of the comparative costs of alternative diagnostic methods.13 Unless some such study is made of neuroradiological techniques, including the EMI scanner, resources are unlikely to be used in the best way. It is already obvious that the possible scope of application of the EMI scanner is so enormous that’ there is no hope of satisfying the potential demand. Rationally based decisions about where best to install these scanners, and how best to deploy machine time, require recognition of three quite separate roles for the method. One is as a diagnostic tool or therapeutic monitor for inpatients with established (or strongly suspected) intracranial disease in major neurological centres. This should result in replacement of alternative methods rather than duplication of investigation, once the necessary initial assessment by comparative studies is complete. An entirely separate use is as a scanning procedure for patients in whom the index of suspicion is low, which could include many outpatients in the major centres as well as most patients with cerebral symptoms in other hospitals. This could include any patient with epilepsy, dementia, or stroke. The Mayo Clinic1 reports that the EMI scanner has disclosed focal lesions in a considerable number of such patients who would not previously have had any special investigation. The third application is in clinical research; unless some time is specifically allocated for this activity a rare opportunity will be lost. For example, brain oedema and cerebral blood-flow could be studied serially and the effects of various measures recorded. There are therefore many potential users, from paediatricians to geriatricians, from accident surgeons to psychiatrists, and from pragmatic physicians to clinical scientists. Three of the common conditions for which it appears to be most reliable (craniocerebral trauma, cerebrovascular accidents, and cerebral atrophy) are mostly dealt with, in Britain at any rate, outside of specialised neurological centres; moreover, these are very The Mayo Clinic1 suggests that two common. standards of scanning may be needed-screening
portant
6.
Williams, J. O., Herzberg, L., Hicks, E. P., Williams, N. E., Croft, D. N. Lancet, 1972, ii, 642. 7. Claveria, L. E., Du Boulay, G., Zingesser, L. Proc. R. Soc. Med. 1973, 66, 823. 8. Bull, J. W. D., Zilkha, K. J. Br. med. J. 1968, iv, 569. 9. Matthews, W. B. Jl R. Coll. Physns Lond. 1973, 7, 207.
10. Du 11. Du
Boulay, G., Price, V. E. Br. J. Radiol. 1968, 41, 762. Boulay, G., McAlister, J. Proc. R. Soc. Med. 1970, 63, 926. 12. Du Boulay, G. H., Price, V. E. Br. J. Radiol. 1971, 44, 416. 13. Knill-Jones, R. P., Stern, R. B., Girmes, D. H., Maxwell, J. D., Thompson, R. P. H., Williams, R. Br. med. J. 1973, i, 530.
1054
using technicians (and perhaps extended working hours), and localising scans which would involve the radiologist. In these circumstances it is doubtful whether neuroradiologists, neurologists, and neurosurgeons, because of their specialised viewpoint, are necessarily the right people to advise on the strategic deployment of the EMI scanner in the community as a whole. Perhaps this is an opportunity for some of the community physicians to scans
show their worth.
SLOTH’S COMPENSATIONS AT a time when energy keeps
costing
more and do well to imitate temperatures keep falling, might the sloth, especially the slower of the two genera of sloths. First, get a really warm, wiry, and waterresistant coat, which needs a minimum of grooming and maintenance. Body heat can also be conserved by the remarkable retia mirabilia in limb roots. In these the limb artery gives off numerous branches which later re-unite, and the accompanying vein similarly gives off branches which lie among the arterial branches. Arterial blood passes outward with its chemical constituents to the limb, but leaves its heat behind in the returning venous blood.1 Our venae comitantes are comparable heat traps, but much less elaborate and less technically efficient, Thus we avoid the price to be paid for the strictest economy: when sloth limbs do get cold, they get very cold, numb, and inert; they do not shiver, and they take we
long to
rewarm.
Good insulation is a handicap in hot weather, when any exertion leads to overheating. The wise plan here is to live near your food, or indeed among it, as the sloth does in the hyacinth-smelling ymbahuba trees of Central America, where he eats the fruit and leaves. Thus feeding is easy, and can alternate with sleep taken sitting in the crotches of trees-a sleep so enviable that gunshots do not disturb it, and a plastic dish laid on top of a sloth’s sleeping head in the evening is still there in the morning.2 This indolent regimen must predispose to obesity ? But sloths are remarkably free from fat, and the usual subcutaneous adipose layer is generally lacking.33 Another helpful attitude is to be rather permissive about body temperature. Sloths keep their cool (rectal temperature 33 °C) and tolerate considerable swings (31-352°C) without obvious response, though they do maintain a rather higher temperature during pregnancy, reminiscent of the human rise in body temperature at ovulation which persists if conception occurs. Thus the undescended testis’presents no problem in the male sloth. His testes never descend; spermatogenesis proceeds within his relatively cool abdominal cavity; he does not need the scrotum, which is evidently required by the testes of animals maintaining higher central temperatures, as Crew first divined. 4,5 1. 2. 3. 4. 5.
Scholander, P. F., Krog, J. J. appl. Physiol. 1957, 10, 405. Tirler, H. A Sloth in the Family. London, 1966. Goffart, M. Function and Form in the Sloth. Oxford, 1971. Crew, F. A. E. J. Anat. 1922, 56, 98. Moore, C. R. Q. Rev. Biol. 1926, 1, 4.
Rash and ill-considered movements are foreign to the sloth’s nature, and his speed on the ground is measured in hours per mile rather than miles per hour. Surprisingly, he swims well, and can even do a competent overarm backstroke if placed in water on his back,3 since the supine position comes very naturally to him up in the trees. He has only red muscle, slower acting, and he is designed for economy in some routine movements. To look right behind us we must turn our trunk as well as our head, and more so as with the years cervical spondylosis tightens its grip. A sloth can This is possible by rotate his head through 360°. having two or three extra cervical vertebrae, and an extraordinarily long trachea, which has spare length enough to run normally down to the diaphragm and then double back.6 Presumably the extra dead space entails some extra respiratory work. But it also means that an overheated sloth need not pant as fast as a dog 3; he has more dead-space air to push in and out of his upper respiratory tract for cooling purposes, without upsetting alveolar ventilation. Sloths waste no more time in excretion than can be helped. A whole week may pass without defalcation (nowhere near the human record, but more than twice the upper limit found by Connell in normal inhabitants of Britain).7 Urine accumulates until it and the contents of the stomach may account for 30% of body-weight. Whole days may pass without urination. Some of the urea may return from the bladder urine through the bladder wall into the circulation, and certainly the sloth’s blood-urea is four to five times higher than the human level. The sloth may utilise this pool of urea effectively. His very large fourchambered stomach is populated by organisms which can probably convert the urea to ammonia and then synthesise aminoacids from it, or else allow the sloth’s liver to carry out the synthesis 8; much the same can be done by the organisms in the human colon, especially in urxmic patients.9 When energy is scarce, it pays to maintain a low profile. The sloth does this in every sense. His camouflage includes an inconspicuous rounded outline, and algx grow on his coat. His movements are slow and draw little attention, he is unaggressive,2 and he vocalises no more than is absolutely essentialfor instance, the female in heat makes just enough noise to entice the male. He keeps away from the jaguar by living on boughs too thin for it. He revives after an incredible twenty minutes of complete respiratory arrest, in part by avoiding struggling." Clearly the world has room for the slow as well as for the energy-rich. Everything hangs on how one goes about the business of living.
ANTIBIOTICS FOR DISEASE MANY elderly or immunologically debilitated patients survive modern surgical or medical treatment only to fall prey to pathogens such as Pseudomonas ceruginosa, Klebsiella aerogenes, and Serratia marcescens. These 6. 7. 8. 9. 10.
Britton, S. W. ibid. 1941, 16, 13, 190. Connell, A. M. Br. med. J. 1965, ii, 1095. Schmidt-Nielsen, B. Physiol. Rev. 1958, 38, 139. Wrong, O. M. Sci. Basis Med. A. Rev. 1971, p. 192. Irving, L., Scholander, P. F., Grinnell, S. W. J. cell. 1942, 20, 189.
comp.
Physiol.
understand microbial diseases, their epidemiology, and their therapy, he is surely the person to interpret to the physicians and surgeons the implications of the laboratory findings for the management and control of infection. The integration of infectious-disease units into district general hospitals should enable the I.D. specialist to look after his own infective patients, whether originating in the hospital or in the community at large, and at the same time to advise on the management of the other cases of infection which, for whatever reason, need to remain in other units. However, a secure career structure and future must be demonstrated before it will be possible to recruit into this specialty the investigationally minded, enthusiastic young practitioners on whom the future of any major trained
to
specialty must rest.
Computer-assisted Tomography of the Brain
scanning is likely to change the whole investigative approach to the patient with suspected intracranial disease. This new technique, using EMI
scintillation detectors associated with computer processing of the output, allows small differences in the radiodensity of intracranial structures to be recorded. Horizontal slices of brain density are, displayed on a cathode-ray oscilloscope, from which a’ Polaroidprint can be taken; a paper print-out of the different numerical densities can also be obtained. The ventricles can be recognised (as areas of low density), and thus dilatation or displacement may be seen. Many focal pathological processes produce variations in density which enable lesions to be displayed directly. Research on this device started in the EMI laboratories, and the Department of Health helped with a clinical prototype. In the summer of 1973, production models were installed in a small number of British and North American centres, and reports of initial experience are now appearing.1-5 It is already apparent that supratentorial tumours can be reliably detected; tumour density varies, and with some types it may perhaps be characteristic; and the scan may also show whether the tumour is cystic or necrotic. An intravenous injection of an iodine compound may enhance the contrast between tumour and surrounding brain, and a doubtful abnormality may then be transformed into a strikingly positive picture. Metastatic tumours Infarcts can be may show multiple densities. confused with tumours, but enhancement (by iodine 1. Baker, H. L., Campbell, J. K., Houser, D. W., Reese, D. F., Sheedy, P. F., Holman, C. B. Mayo Clin. Proc. 1974, 49, 17. 2. New, P. F. J., Scott, W. R., Schnur, J. A., Davis, K. R., Taveras, J. M. Radiology, 1974, 110, 109. 3. Scott, W. R., New, P. F. J., Davis, K. R., Schnur, J. A. ibid. 1974, 112, 73. 4. Gawler, J., Du Boulay, G. H., Bull, J. W. D., Marshall, J. Lancet,
Aug. 24, 1974, p. 419. 5. Paxton, R., Ambrose, J. Br. J. Radiol. 1974, 47, 530.
will usually increase density only in tumours. Hxmatoma can be distinguished from infarction-a distinction not possible with any existing investigation-and this could be invaluable if strokes are to be more vigorously investigated in the future. EMI scanning promises to be particularly valuable in the assessment of acute head injuries with suspected intracranial haematoma; indeed, PAXicx and AMBROSE report that emergency angiography has been completely superseded in their unit.5 Posterior-fossa lesions are somewhat less easily detected, requiring special low views; even on these the dense petrous bones can make definition difficult, The finding of dilated ventricles, together with clinical suspicion, may indicate the need to focus attention on the posterior fossa and this will increase the likelihood of success. Cerebral atrophy can be inferred from ventricular dilatation and enlargement of the cerebral sulci. Brain oedema can be recognised - something no other technique can do-and much may be learnt now about the correlates of this important accompaniment of many acute intracranial The possibility of undertaking serial processes. investigations opens the way for observing the timecourse of developing or improving conditions, which could not be contemplated with more invasive investigations. Examples of such application would be changes in ventricular size after operations for hydrocephalus, the response of intracranial tumours to non-surgical treatment, and the detection of
injection)
postoperative complications. EMI scanning therefore provides as much information as any other single investigation currently available; and it does so without risk or discomfort, though the patient must lie still for four or five minutes, and young children and confused adults (those with recent head injury, stroke, or dementia) may require a sedative or even anxsthesia. Preexisting methods of investigation using contrast media entail considerable discomfort and some risk; moreover, they require the skills of a neuroradiologist, together with ready access to a neurosurgeon, because they sometimes create a need for urgent operation. For these reasons contrast neuroradiological procedures are largely restricted to inpatients in whom there is a reasonable expectation of finding a remediable intracranial condition. Seen against this background, the very ease with which the EMI scan can be performed may pose problems. GAWLER and his colleagues4 warn that scrupulous attention to detail in the performance and interpretation of the scan will be required if the method is not to acquire a reputation for inaccuracy, which they rightly say would be tragic for such a promising diagnostic method. This is what happened with isotope scanning of the brain, and the lesson of that experience should be heeded. With isotope scanning, as with computer-assisted tomography, there were hopes that the inconvenieace of contrast radiology might often
1053
be avoided and that some patients might be investigated without referral to a special centre. In the event, no satisfactory trials were carried out to establish the reliability of isotope scanning in an unselected patient population; only lately has a consecutive series investigated in a London teaching hospital been reported,and this showed isotope scanning to be falsely negative in nearly a third of patients subsequently proved to have a neurosurgical lesion. According to the National.Hospital team,4,7 three-quarters of isotope scans done at Queen Square are negative, and about a quarter cannot be justified at all; they cite evidence of similarly " wasteful and uninformed " use of plain skull X-rays8 and of the electroencephalogram.9 If this is what happens when patients are referred by neurologists, probably even less critical use would be made of such non-invasive investigations in general hospitals, where large numbers of patients present with minor neurological symptoms. Few such patients prove to have a remediable intracranial lesion, the exclusion of which is the aim of most investigations. Although isotope scanners are now available in many large general hospitals, it is doubtful whether the number of referrals to neurological centres has diminished, and most patients who are now scanned in other hospitals would previously not have been subjected to any tests at all. Even in the neurological centres few patients with obvious lesions are spared contrast studies, isotope scanning being largely used as a preliminary test. For a hospital to acquire a scintiscan or a gamma camera with the necessary supporting staff may therefore be a dubious enterprise in cost/ benefit terms, as far as investigating the brain is concerned. The EMI scanner presents a similar problem, but the stakes are higher; not only is the information it provides of more value, but the cost is much greater (over El 20,000)—and it can be used only for the investigation of the brain. ’Khe Department of Health showed wisdom when it contributed to the development of the EMI scanner, and one hopes that equal interest will be shown in evaluating its role in patient care. Because there is virtual central control over alternative means of investigation, it should be easier to make a valid assessment in Britain than in countries with a free market for medical care. In the free-market
situation, application of the scanner may prove to be somewhat different; many more machines are likely to be available, because the capital cost can readily be recovered by adjusting the charge to the patient and to the hours
a day that the scanner is used. Because cost/benefit equations are less easily solved in a National Health Service, it is all the more im-
to discover not only the performance characteristics of the equipment but also the place which it should take alongside alternative methods. Du BOULAY 10,11 has already shown that computer analysis of neuroradiological data from large numbers of cases makes it possible to calculate in any future " case which is the next best test " 12 given the results of clinical and laboratory investigations already available. Similar comparative studies of investigative methods (for other bodily systems) have consistently resulted in test reduction, by making it possible to reach a diagnosis with less information than clinicians usually think necessary; some of these investigations have included calculations of the comparative costs of alternative diagnostic methods.13 Unless some such study is made of neuroradiological techniques, including the EMI scanner, resources are unlikely to be used in the best way. It is already obvious that the possible scope of application of the EMI scanner is so enormous that’ there is no hope of satisfying the potential demand. Rationally based decisions about where best to install these scanners, and how best to deploy machine time, require recognition of three quite separate roles for the method. One is as a diagnostic tool or therapeutic monitor for inpatients with established (or strongly suspected) intracranial disease in major neurological centres. This should result in replacement of alternative methods rather than duplication of investigation, once the necessary initial assessment by comparative studies is complete. An entirely separate use is as a scanning procedure for patients in whom the index of suspicion is low, which could include many outpatients in the major centres as well as most patients with cerebral symptoms in other hospitals. This could include any patient with epilepsy, dementia, or stroke. The Mayo Clinic1 reports that the EMI scanner has disclosed focal lesions in a considerable number of such patients who would not previously have had any special investigation. The third application is in clinical research; unless some time is specifically allocated for this activity a rare opportunity will be lost. For example, brain oedema and cerebral blood-flow could be studied serially and the effects of various measures recorded. There are therefore many potential users, from paediatricians to geriatricians, from accident surgeons to psychiatrists, and from pragmatic physicians to clinical scientists. Three of the common conditions for which it appears to be most reliable (craniocerebral trauma, cerebrovascular accidents, and cerebral atrophy) are mostly dealt with, in Britain at any rate, outside of specialised neurological centres; moreover, these are very The Mayo Clinic1 suggests that two common. standards of scanning may be needed-screening
portant
6.
Williams, J. O., Herzberg, L., Hicks, E. P., Williams, N. E., Croft, D. N. Lancet, 1972, ii, 642. 7. Claveria, L. E., Du Boulay, G., Zingesser, L. Proc. R. Soc. Med. 1973, 66, 823. 8. Bull, J. W. D., Zilkha, K. J. Br. med. J. 1968, iv, 569. 9. Matthews, W. B. Jl R. Coll. Physns Lond. 1973, 7, 207.
10. Du 11. Du
Boulay, G., Price, V. E. Br. J. Radiol. 1968, 41, 762. Boulay, G., McAlister, J. Proc. R. Soc. Med. 1970, 63, 926. 12. Du Boulay, G. H., Price, V. E. Br. J. Radiol. 1971, 44, 416. 13. Knill-Jones, R. P., Stern, R. B., Girmes, D. H., Maxwell, J. D., Thompson, R. P. H., Williams, R. Br. med. J. 1973, i, 530.
1054
using technicians (and perhaps extended working hours), and localising scans which would involve the radiologist. In these circumstances it is doubtful whether neuroradiologists, neurologists, and neurosurgeons, because of their specialised viewpoint, are necessarily the right people to advise on the strategic deployment of the EMI scanner in the community as a whole. Perhaps this is an opportunity for some of the community physicians to scans
show their worth.
SLOTH’S COMPENSATIONS AT a time when energy keeps
costing
more and do well to imitate temperatures keep falling, might the sloth, especially the slower of the two genera of sloths. First, get a really warm, wiry, and waterresistant coat, which needs a minimum of grooming and maintenance. Body heat can also be conserved by the remarkable retia mirabilia in limb roots. In these the limb artery gives off numerous branches which later re-unite, and the accompanying vein similarly gives off branches which lie among the arterial branches. Arterial blood passes outward with its chemical constituents to the limb, but leaves its heat behind in the returning venous blood.1 Our venae comitantes are comparable heat traps, but much less elaborate and less technically efficient, Thus we avoid the price to be paid for the strictest economy: when sloth limbs do get cold, they get very cold, numb, and inert; they do not shiver, and they take we
long to
rewarm.
Good insulation is a handicap in hot weather, when any exertion leads to overheating. The wise plan here is to live near your food, or indeed among it, as the sloth does in the hyacinth-smelling ymbahuba trees of Central America, where he eats the fruit and leaves. Thus feeding is easy, and can alternate with sleep taken sitting in the crotches of trees-a sleep so enviable that gunshots do not disturb it, and a plastic dish laid on top of a sloth’s sleeping head in the evening is still there in the morning.2 This indolent regimen must predispose to obesity ? But sloths are remarkably free from fat, and the usual subcutaneous adipose layer is generally lacking.33 Another helpful attitude is to be rather permissive about body temperature. Sloths keep their cool (rectal temperature 33 °C) and tolerate considerable swings (31-352°C) without obvious response, though they do maintain a rather higher temperature during pregnancy, reminiscent of the human rise in body temperature at ovulation which persists if conception occurs. Thus the undescended testis’presents no problem in the male sloth. His testes never descend; spermatogenesis proceeds within his relatively cool abdominal cavity; he does not need the scrotum, which is evidently required by the testes of animals maintaining higher central temperatures, as Crew first divined. 4,5 1. 2. 3. 4. 5.
Scholander, P. F., Krog, J. J. appl. Physiol. 1957, 10, 405. Tirler, H. A Sloth in the Family. London, 1966. Goffart, M. Function and Form in the Sloth. Oxford, 1971. Crew, F. A. E. J. Anat. 1922, 56, 98. Moore, C. R. Q. Rev. Biol. 1926, 1, 4.
Rash and ill-considered movements are foreign to the sloth’s nature, and his speed on the ground is measured in hours per mile rather than miles per hour. Surprisingly, he swims well, and can even do a competent overarm backstroke if placed in water on his back,3 since the supine position comes very naturally to him up in the trees. He has only red muscle, slower acting, and he is designed for economy in some routine movements. To look right behind us we must turn our trunk as well as our head, and more so as with the years cervical spondylosis tightens its grip. A sloth can This is possible by rotate his head through 360°. having two or three extra cervical vertebrae, and an extraordinarily long trachea, which has spare length enough to run normally down to the diaphragm and then double back.6 Presumably the extra dead space entails some extra respiratory work. But it also means that an overheated sloth need not pant as fast as a dog 3; he has more dead-space air to push in and out of his upper respiratory tract for cooling purposes, without upsetting alveolar ventilation. Sloths waste no more time in excretion than can be helped. A whole week may pass without defalcation (nowhere near the human record, but more than twice the upper limit found by Connell in normal inhabitants of Britain).7 Urine accumulates until it and the contents of the stomach may account for 30% of body-weight. Whole days may pass without urination. Some of the urea may return from the bladder urine through the bladder wall into the circulation, and certainly the sloth’s blood-urea is four to five times higher than the human level. The sloth may utilise this pool of urea effectively. His very large fourchambered stomach is populated by organisms which can probably convert the urea to ammonia and then synthesise aminoacids from it, or else allow the sloth’s liver to carry out the synthesis 8; much the same can be done by the organisms in the human colon, especially in urxmic patients.9 When energy is scarce, it pays to maintain a low profile. The sloth does this in every sense. His camouflage includes an inconspicuous rounded outline, and algx grow on his coat. His movements are slow and draw little attention, he is unaggressive,2 and he vocalises no more than is absolutely essentialfor instance, the female in heat makes just enough noise to entice the male. He keeps away from the jaguar by living on boughs too thin for it. He revives after an incredible twenty minutes of complete respiratory arrest, in part by avoiding struggling." Clearly the world has room for the slow as well as for the energy-rich. Everything hangs on how one goes about the business of living.
ANTIBIOTICS FOR DISEASE MANY elderly or immunologically debilitated patients survive modern surgical or medical treatment only to fall prey to pathogens such as Pseudomonas ceruginosa, Klebsiella aerogenes, and Serratia marcescens. These 6. 7. 8. 9. 10.
Britton, S. W. ibid. 1941, 16, 13, 190. Connell, A. M. Br. med. J. 1965, ii, 1095. Schmidt-Nielsen, B. Physiol. Rev. 1958, 38, 139. Wrong, O. M. Sci. Basis Med. A. Rev. 1971, p. 192. Irving, L., Scholander, P. F., Grinnell, S. W. J. cell. 1942, 20, 189.
comp.
Physiol.