other ductless organs of which a number have been shown to have their
own ‘internal secretions’. Furthermore, it has been demonstrated that,
among organs which throw out their products through a duct, there are
those which also send an internal secretion into the blood-stream.
Among these are the essential organs of sex, the testicle and ovary.
The effect of castration on the general physique is well known. It may
be compared with the effect of ‘spaying’ or the removal of the ovary.
This operation leads to an assumption by the female, in more or less
modified degree, of the secondary sexual characters of the male.
Peculiarly interesting for their practical results have been certain
investigations made of late years upon the organ known as the
‘Pancreas’. The Pancreas has a duct which opens into the Intestine
just below the Stomach. It has long been known that the secretion of
the Pancreas is related to the amount and fate of sugar in the blood.
The association of disease of the Pancreas with the symptom known as
‘Diabetes’, in which sugar appears in the urine, was also familiar.
Later it became apparent that it was not the Pancreas as a whole that
was related to the process but only certain isolated and peculiarly
formed nests of cells. It is now possible to administer extracts of
these cell-nests with very favorable results on the course of certain
types of Diabetes. The extract is now in wide use under the name of
_Insulin_.
Among the ductless glands that have been best investigated are the
so-called ‘suprarenal bodies’, which lie above the kidneys. As with
the thyroid gland, the attention of physiologists was directed to
these bodies as a result of clinical observations. These observations
date back to the middle of the nineteenth century. In the last years
of that century it was observed that an extract of the suprarenal
bodies, injected into the circulation, caused a rise in blood-pressure,
an effect opposite to that following the extirpation of the glands.
The administration of extract from suprarenal bodies has found wide
clinical application. Unlike the extract of thyroid, the effect of this
extract is very temporary. It is easily oxidized and rapidly disappears
from the blood. It belongs to the group of substances which are
known as _hormones_. The active element in a suprarenal extract, the
‘suprarenal hormone’, has been recently prepared by a synthetic process.
The nature of hormones has only come clearly into view of late years.
The word ‘hormone’ is formed from a Greek word meaning ‘to excite’.
The internal secretions have, in general, functions of considerable
physiological complexity, and act, for the most part, slowly and
continuously. The hormones are, however, exceptions to this rule. They
act rapidly and in an excitatory manner. These substances appear to be
of relatively simple chemical structure. They are easily oxidizable,
so that they rapidly disappear from the body. They act, in fact, as
‘chemical messengers’, producing a state of ‘chemical correlation’ of
the different parts of the body which is comparable to the better-known
and more widely recognized ‘nervous correlation’.
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.
[Illustration: FIG. 131. DIAGRAM TO SHOW THE STRUCTURE OF THE EYE,
REPRESENTED IN SECTION. For description see pp. 314-15.]
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.
[Illustration:
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).
One of the most remarkable minds that has ever applied itself to
medical problems was that of the Quaker physician Thomas Young
(1773-1829). He was a man of immense learning, and is remembered
for having been the first to decipher Egyptian hieroglyphics. Young
explained the power of the eye to ‘accommodate’ for near vision. This
faculty of ‘accommodation’ was, he showed, due to changes in the
curvature of the crystalline lens (Fig. 132). In his memoir _On the
Mechanism of the Eye_ (1801), Young gave the first scientific account
of Astigmatism. His theory of color vision and his doctrine that light
is due to waves in the ether are still important. His ‘wave theory’
of light completely replaced the old view, the so-called ‘emission
theory,’ that light is due to something material which goes forth from
the luminous object. While we are referring to Young we may remind
the reader that his work on ‘Energy’ lies at the back of all modern
Physics, in the history of which he takes an extremely important place.
The operative treatment of the eye is of great antiquity. The most
important operative procedure is that for ‘cataract,’ a condition
caused by an opacity of the lens. ‘Couching’ for cataract, that is
depressing the opaque lens, was practised by Alexandrian surgeons
in the third century B.C. It is described by Celsus (p. 43) in the
first and mentioned by Galen (p. 50) in the second Christian century.
Contemporary with these authors are descriptions of the actual
extraction of the lens affected with cataract.
In Imperial Roman times there were surgeons who devoted themselves
exclusively to cataract operations. These were practised during the
Middle Ages by the Arabs and to a less extent by the Westerns. For the
most part the operations were performed by wandering quacks, who were,
however, often very skilful. In the sixteenth century operations on the
eye began to pass into the hands of recognized medical practitioners.
The advances in the knowledge of the anatomy and physiology of the eye
in the eighteenth century enabled the French surgeon Jacques Daviel
(1696-1762) to explain the real nature of cataract, which is usually
nothing but a senile change in the lens of the eye. His knowledge made
it possible for him greatly to improve the operation for extraction, so
that, over a large range of cases, he had only 11 per cent. of failures.