Part 38

The hormones represent a very ancient and primitive physiological mechanism. In organisms consisting of but one cell, in which there are very few differentiated organs, the messages from one part of the body to another are necessarily of a chemical or hormonic character. In higher multi-cellular animals the intercommunication between different parts of the body is maintained, for the most part, by a specially developed nervous system. Certain necessary messages are, however, still conveyed by chemical messengers. The development of the conception of hormones has been especially the work of the London physiologist E. H. Starling (1866-1927).

Internal secretions and especially hormones form part of the increasingly complex picture of the working of the animal body. They are not only of great physiological value, but have also entered the department of practical therapeutics. They are, moreover, of philosophical importance, since they yield us a conception of the body in which every part is dependent on every other part, and the whole is subject to a process of ‘integration’ or linkage into a unitary system. We have glanced at the mechanism of chemical integration. We have now to turn to the mechanism of nervous integration.

(b) Nervous Integration.

If the simple reactions of animal bodies are tested, it will be found that they clearly serve certain ends. Lightly touch the foot of a sleeping child and it will withdraw it. Tickle the ear of a cat and it will shake it. Exhibit savory food to a hungry man and at once his digestive process will get to work--his mouth will ‘water’. These instances might be multiplied a hundredfold. Such reflexes are admirably adapted to their ends. Many of them will continue in an animal in which the spinal cord is severed from the brain. Nevertheless, in the higher animals, and especially in man, they are controllable to a greater or less extent by the will. But to leave the question at that would give a false idea of the extremely complex integrative functions performed by the nervous system. Thus, the spinal cord, which, to the naked eye, is a longitudinal and little differentiated nervous mass, is, in fact, a collection of nerve-centers which have historically, both in the individual and in the race, been formed by the union of a series of separate segments. Each one of these segments is dependent on the action of the next segment in a fashion somewhat similar to that in which the actions of the cord itself are dependent on the brain. Each of the sections governs certain functions or movements of the body. There is thus a very complex process of integration which runs right through the nervous system.

The investigation of the bodily functions of a chemical and physical nature reveals that these activities are far more largely under nervous control and discipline than was at one time conceived to be possible. Thus, the main factor in the activity of any part is its blood-supply, but the blood-supply is largely determined by the state of contraction of the vessels of supply, which are in their turn under nervous control. So it is with the state of nutrition of the muscles, with the action of the sweat glands of the skin, with the mechanism of childbirth, and with a thousand bodily states with which both physician and biologist are concerned.

The investigation of nervous integration is especially associated with the name of Sir Charles Sherrington of Oxford. As the outcome of his work the picture formed of the nervous apparatus is that of a machine in which some parts work spontaneously, automatically, and with complete uniformity; others, though mainly automatic, are susceptible of various degrees of alteration and adjustment; others need intermittent or constant attention, and demand for their functioning fresh supplies of energy at longer or shorter intervals; while, finally, others have hardly yet taken a fixed form and are improvised as occasion demands. Thus the nervous system is a system of systems of every degree of independence.

These systems, each with a certain individuality of its own, date from every stage of Evolution, the more ancient being, as a rule, the more automatic and the less dependent on other systems. The most ancient, the chemical messenger or ‘hormonic’ system (pp. 306-8), we share with the lowest living things which consist of only one cell. Very recent are the factors in the nervous system that are specially developed in man as contrasted with the higher apes. Such are those associated with the delicate co-ordination of sensory impressions and motor impulses involved in such acts as speaking, reading, writing and the like. Each of these systems, high or low, ancient or recent, has its own place in the body. For many the exact position of the controlling center is demonstrable and some of the lower systems can function without the aid of any other systems save those which control their nutrition.

Among these nervous relations there is one which calls for special mention on account of its great clinical importance. The state of ‘shock’, the general nature of which is vaguely understood by everybody, has been given a more exact physiological meaning of late years, especially by the American surgeon G. W. Crile (1864-). It has been found possible to localize ‘shock’ experimentally. If a section of the spinal cord of an animal be cooled to a point just above freezing, the part of the body below the cooled level passes into a state of ‘shock’, that is to say, its reflexes no longer respond to irritation in the normal fashion. This shock effect is due to the removal of some influence exercised by the higher parts of the nervous system. In the experiment the shock effect is induced by an external agent, but there is an internal mechanism within the nervous system itself, which can cause it under appropriate conditions.

(c) Vitamins.

There are no current medical problems that are more discussed than those of nutrition. It has long been recognized that articles of diet may be classified according to their constitution into ‘proteins’, ‘carbohydrates’, and ‘fats’. If an animal is fed on a diet containing these in correct proportion, but in a perfectly pure state, it will become ill and ultimately die. The onset of illness and death will be the more rapid if it be a young animal. This fact, observed as long ago as 1880, was reinvestigated by F. Gowland Hopkins of Cambridge in 1906, from whence dates our real knowledge of a very important subject. He found that, in the case of rats, the addition of a very small quantity of milk to this chemically pure diet would induce normal growth. The milk must therefore contain some growth-promoting substance or substances other than protein, fat, or carbohydrate. The result of many similar experiments by a large number of observers has shown that almost all fresh food contains such growth-promoting substances. They have been named ‘vitamins’.

Several of these vitamins have been distinguished. None, however, has been isolated, and we depend for our knowledge of them on our investigation of their mode of action. One, known as Vitamin A, is produced in the growing green parts of plants, and is especially necessary for the promotion of growth. Vitamin A is abundant in cod-liver oil. It has been shown that the necessity for Vitamin A can to some extent be evaded if the animal is exposed to sunlight or ultra-violet rays. Moreover, it has been shown that the absence of Vitamin A or of some allied substance is associated with the disease of the bones known as ‘Rickets’ or ‘Rachitis’. The history of this disease (p. 181) is made intelligible by our knowledge of these facts. Rickets can be shown to be most prevalent under precisely those social conditions in which articles of diet containing Vitamin A are scarce and the amount of sunlight is inadequate.

Our knowledge of this topic is in the process of active extension. The question of the actual influence of sunlight and of the rays of various wave-length which go to make it up is still too uncertain for discussion here. There is a special aspect of this topic, however, to which we may refer. It has been demonstrated that stable-fed cows, fed not on fresh food but on oil-cake, yield milk of little antirachitic power. It has, however, been shown that this milk becomes antirachitic after exposure to ultra-violet light. Therefore, some antirachitic substance is produced in the milk, as in the body, by the action of ultra-violet light. Now recent research has shown that the antirachitic elements are associated with a chemical substance known as Cholestrol which is of the nature of a complex alcohol. Nevertheless chemically pure Cholestrol has no antirachitic power, though it, too, acquires it by exposure to ultra-violet light. By chemical means 99·9 per cent. of rayed and antirachitic Cholestrol has been recovered as pure Cholestrol without antirachitic power. Therefore the antirachitic power, that is the vitamin factor, resides in the remaining one-tenth per cent. of rayed Cholestrol. The further investigation of this fraction may be expected to yield results of great importance both theoretically and practically.

Another substance of the same order exists in the husks of rice. If animals such as fowls be fed on a diet of rice deprived of its husks, they develop a nervous affection. Now a somewhat similar nervous affection known as ‘Beri-beri’ is known in the East among natives who live on milled rice. The disease, whether in human beings or chickens, may be cured or avoided by giving the husks of the rice separately. The substance thus conveyed has been named Vitamin B. There is yet another disease, Scurvy (p. 170), which occurs in those who have been deprived of fresh food. Vitamin C, which cures this, is specially found in the juices of oranges and lemons. Our knowledge of ‘deficiency diseases’, of which Scurvy is one, is only just beginning. It may well be that they are of wider occurrence than has been supposed, and vitamins may be important curative and preventive agents.

§ 19. Knowledge of the Eye and its Disorders.

From an early date the treatment of ailments of the eye has stood somewhat apart from the rest of medical practice. Moreover, the knowledge of the structure and functions of the parts of the eye has not kept closely parallel with that of other departments of anatomy and physiology.

The eye is a roughly spherical organ, enclosed in a tough capsule, the Sclerotic coat (Fig. 131). The transparent front of this capsule, the Cornea, is the curved window through which we look upon our world. There is a watery space, the Anterior Chamber, behind the Cornea, at the back of which is situated the Lens, a horny transparent structure. In front of the Lens is a ring-shaped pigmented muscle which shuts out light from the Lens, except at the center, and gives the characteristic color to the eye. This circular colored muscle is the Iris, and the hole in its center is the Pupil. The pupil becomes smaller or larger with contraction or expansion of the Iris. This change is a reflex and unconscious act, depending on the amount of light and also on the degree to which the eye is adjusted to examine near objects.

The edge of the Lens of the eye is attached by the circular Suspensory Ligament to the circular Ciliary Muscle. The Ciliary Muscle, by contracting or relaxing, alters the form of the Lens (Fig. 132). This change in form of the Lens is part of the process of adjustment to near or distant vision. Behind the Lens is the large Posterior Chamber, containing a transparent gelatinous substance. At the back of the posterior chamber is the sensitive area or Retina, which is the essential organ of vision, and is backed by a pigmented coat, the Choroid. The Retina is continuous with the Optic Nerve, along which an artery enters the globe of the eye. At the point where this artery pierces the Retina there is the so-called Blind Spot.

A ray of light penetrating the eye from the center of the Cornea through the center of the Lens falls on or near a specially sensitive area, the Yellow Spot, and images formed there are more distinctly perceived than those formed elsewhere. When an object is examined closely, the observer makes the attempt to bring the image of it on to his Yellow Spot. Any injury to the Yellow Spot causes a great diminution in clearness of vision. Man and his allies, the zoological group known as the ‘Primates’, are the only mammals, except the cat tribe, that possess a Yellow Spot. There can be little doubt that the possession of this Yellow Spot has done much to raise the importance of vision among the senses in the Primates. It has thus been a very potent factor in the evolution and elevation of Man himself.

The eye is an optical instrument which, like other instruments, performs its functions with something less than perfection. Most purely optical errors of the eye can be remedied by spectacles. These aids to vision are of very great importance, since, by the time middle life is reached, few are fortunate enough to read in comfort without them. The introduction of spectacles, therefore, enormously extended the active intellectual life. Their social effects are incalculable.

The commoner optical errors may be classified under four heads.

First and commonest there is ‘old sight’. When a healthy eye adjusts to near vision, the Ciliary Muscle contracts towards its attachment at the junction of Conjunctiva and Sclerotic. This draws forward and relaxes the Suspensory Ligament. The elasticity of the Lens, no longer constrained by the Ligament, causes it to assume a more convex form. This more convex form is appropriate to the correct focussing of a near object on the Retina. At or about the age of forty-five the Lens usually begins to lose its elastic power, and thus has difficulty in adapting to near vision. The trouble is remedied by the use of convex glasses for reading or other near work.

A second common error is the so-called ‘far sight’. In this form--save in extreme cases--the eye is competent for distant objects, but those that are near are not clearly seen. The incapacity for near vision is due to a deformity--usually innate--of the eye. The eye is too short along the axis xy (Fig. 131). The resulting optical error can be remedied by the use of convex spectacle lenses.

FIG. 132. DIAGRAM TO SHOW THE NATURE OF ACCOMMODATION OF THE EYE TO NEAR VISION. The Ciliary muscle, by contracting, pulls forward the lateral attachment of the Suspensory Ligament to the Sclerotic. Thus the ligament is relaxed and in turn relaxes its pull on the Lens. The Lens thereon becomes more convex. As age advances the Lens loses this power and so the sight fails for near vision.

Thirdly, there is the so-called ‘near sight’. In this state near objects can be clearly seen, but vision fails with those that are more distant. Near sight is usually an acquired condition. The eye is too long along the axis xy (Fig 131). The resulting optical error can be remedied by the use of concave spectacle lenses.

Fourthly, there is ‘irregular sight,’ known as ‘astigmatism’. In extreme cases of this condition no perfectly clear image can be formed of any object, whatever its distance. It is in some measure both congenital and acquired, and is due to an irregular deformation of the optical apparatus of the eye. The remedy for astigmatism is a compensatory deformation of the spectacle lens, which may need, in other respects, to accord to the convex or concave form, according as the deformation of the eye is of the far-sighted or near-sighted type.

Historically optical errors of the eye were relieved by spectacles before the nature of the defects was understood. The first suggestion of the use of convex lenses as an aid to old sight was made by Roger Bacon (1214-94) in the thirteenth century. Spectacles with convex lenses for old or for far sight first came into use about 1300. By the fifteenth century they were widely known. It may well be that their adoption, by prolonging reading life, had an important effect upon that process of extension of knowledge that we dub the ‘Revival of Learning’. Concave lenses for the relief of near sight came in towards the end of the fifteenth century, but were not widely used till the eighteenth century. Astigmatic lenses were not contrived till well into the nineteenth century.

In 1874 S. Weir Mitchell (1830-1914), a very able American physician, showed that the eye strain resulting from astigmatism was associated with many nervous conditions. Weir Mitchell’s name is familiarly associated with a line of treatment of these conditions. Since his discovery it has been the practice to examine for optical error all sufferers with headache and other neurotic symptoms.

For long there was no means of estimating the degree of error, whether of old sight, far sight, or near sight, save by trial on the part of the patient himself. Spectacles were a common object of the hawker’s trays, and from them the sufferer selected the specimen that suited him best. The first essential improvement in this state of affairs was an elucidation of the mode of action of lenses. The paths of light rays in their passage through a lens were first correctly determined at the beginning of the seventeenth century by the astronomer Johannes Kepler (1571-1630). Knowledge of optics advanced during the seventeenth and eighteenth centuries, but the optical errors of the living eye were not accurately estimated until the time of the great Dutch ophthalmologist Frans Cornelis Donders (1818-89). The system of prescribing and fitting spectacles that is now in vogue dates from the publication of his work, The Anomalies of Refraction and Accommodation, in 1864. Hardly less important was the invention of the ophthalmoscope by Hermann von Helmholtz (p. 213). Very important also was the introduction of ‘test types’ for examining errors of vision by the Dutch ophthalmologist Hermann Snellen (1834-1904).