EUOLOGY
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G
AN INTRODUCTION TO
GENERAL PHYSIOLOGY
AN "INTRODUCTION TO
GENERAL PHYSIOLOGY
WITH PRACTICAL EXERCISES
BY
W. M. BAYLISS, M.A., D.Sc., F.R.S.
PROFESSOR OF GENERAL PHYSIOLOGY IN UNIVERSITY COLLEGE, LONDON
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON, E.C.4
FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS
1919
All rights reserved
BIOLOGY
LIBRARY
G
PREFACE
To write a satisfactory text-book for those commencing the study
of a science is well known to be of greater difficulty than to write
one for advanced students. In fact, it would probably be true to
say that the former task is a severe test of the author's comprehen-
sion of the subject. I venture to hope that the thought given to
the present work may not be found to have been in vain. Any
readers who may find parts of it expressed insufficiently clearly are
requested to help the author with suggestions.
It was found to require much consideration to decide what
should be omitted. My firm conviction is, that a thorough under-
standing of the main fundamental conceptions is of more value than
a superficial acquaintance with a large number of facts and theories,
even if it leads, for the moment, to the omission of what many may
regard as essential. The student will probably be surprised to find
how many of the more detailed descriptions follow naturally from
a knowledge of a few general principles, when these are clearly
grasped. For this reason, more space has been given to certain
elementary facts of physics and chemistry than might be supposed
necessary ; but it has been my experience that, although a student
may have attended good courses in these sciences, he does not
readily apply the knowledge to physiological problems. Moreover,
there are some things essential to physiology, but often regarded
as outside the scope of an elementary course in the preliminary
sciences. I intentionally lay myself open to the charge of inserting
matter that is not, strictly speaking, physiological.
The rather " intensive " treatment sometimes adopted leads to
what some may regard as an excursion into advanced regions. But
this is unavoidable when the conception cannot be passed over
without risk of error.
In another way, the manner of presentation adopted may meet
with objection. Students do not easily remember the names of the
VI
PREFACE
discoverers of the facts they must learn ; and, when they do associate
names, they are frequently incorrect. Naturally so, since the dis-
coverers are, as a rule, mere names to those commencing the study
of the subject. For this reason, names, for the most part, have been
omitted, with the exception of those of a few outstanding men of
genius, such as Faraday, Claude Bernard, or Ludwig, which ought
to be familiar to all who claim to be educated. I would therefore
ask pardon of my colleagues who may find their discoveries referred
to without their names. At the same time, I have not hesitated to
refer to any personal or historical fact which might give added
interest to a dry description. Similarly, the only references given
are to my own larger " Principles," where those desirous of more
detail can find what they require, or, at any rate, indication of where
to find it. These references have the letter P prefixed to the page
indicated.
It is to be feared that the exposition may be regarded as too
dogmatic. This may be so, but it is of set purpose, and the
arguments have been carefully weighed before making a statement.
Where there is obviously insufficient knowledge, it is preferable to
state that this is the case, rather than to confuse the student by
conflicting views. In my early student days I was repeatedly pre-
sented with arguments on both sides, about as many on one side as
on the other, so that a blank remained, not only in the note-book,
but in my memory. If only a view is clearly grasped, it can easily
be corrected later, if necessary, since the point where it fails can
be seen and understood.
On the whole, it would have been preferable to leave details of
experiments to be shown to the student, or made by himself, to the
discretion of the individual teacher, in accordance with the resources
of his laboratory. But it has been represented to me that the value
of the book would be much increased if such details were included.
This has accordingly been done. As will be seen, however, the
experiments vary a good deal in their simplicity. Some of them
require apparatus that a particular laboratory may not possess, and
must perforce be omitted. On account of their importance, never-
theless, it would have been misleading to omit them on the ground
that the necessary means may not always exist. The value of
experiments, especially when made by the student himself, is very
great. They give a reality to description in words and a belief in
PREFACE vii
the truth of the statements made. There is of necessity much that
requires more difficult and lengthy work than is possible in the
time available for class work, since many of the most fundamental
facts could only be discovered by methods involving the greatest
accuracy in measurement.
The instructions as to experimental work are intended to assist
the teacher as much as the student. As already remarked, much
must be left to individual discretion. Probably instructive experi-
ments will occur to the teacher in addition to those given, and I shall
be very grateful for suggestions to be included in a future edition.
There are doubtless many shortcomings in this manual. Those
statements in the text which are capable of experimental illustra-
tion are marked with the letter E, and the page of the " Practical
Work " on which instructions are given.
It seemed scarcely possible to add summaries to the chapters,
as in my larger book, for the reason that all the matter contained
in so small a space is of nearly equal importance, and a summary
would have been almost as long as the chapter itself. It would also
tend to encourage what I wish most to avoid, namely, any kind of
merely learning by heart. The student may find it profitable to
make abstracts for himself.
Structural facts, whether anatomical, histological, or chemical,
are not given with more minuteness than necessary to understand
the mode of action of the organs they apply to, so far as it is known.
If further description is thought useful, such books as Quain's
" Anatomy," Schafer's " Essentials of Histology," and Plimmer's
" Practical Organic and Bio-Chemistry," may be consulted.
The best way in which the present book could be used would
be for the teacher to take it as a suggestion of what the author
regards as the fundamentally important things to be taught, and
to describe them to the student in his own words. The student
may use the book to remind him of what he has been taught, or
to obtain a different way of looking at the phenomena. In this
way, that most pernicious habit of learning a subject, already
alluded to, may be avoided, to some extent at least. The com-
mitting to memory a mass of statements without understanding
their relation to one another, or even what they mean, cannot be
too carefully guarded against. It is of no value whatever, either
as a means of education or for future use.
viii PREFACE
In justice to the student, it should be pointed out that the
requirements of no particular examination have been taken into
consideration here. It is to be hoped that the day is not far
distant when preparation for an examination, other than periodic
questions by the teacher himself, will be a thing of the past. It is,
no doubt, difficult to devise a method of replacing examinations as
a test for such purposes as medical qualifications, but it is surely
not impossible. Many of the existing examinations can be passed
without much real knowledge, while the 'preparation of students
for examinations, when these are conducted by others than the
teacher himsejf, has a most depressing effect on those who have
to do it. However important a new discovery may be, hesitation
is naturally felt in taking up the students' time with what they know
will not help them to pass their examination.
Some explanation is needed as to the diagrams given here. I
venture to think that, at all events in many cases, a picture which
attempts to represent what a part of a living organism actually looks
like is less instructive than one which frankly attempts no more
than to indicate what is essential to the working of the mechanism.
The real appearance can only be learned from actual specimens
and experiments. Such an excellent book as the " Practical
Biology" of Huxley and Martin shows that illustrations are not a
necessity.
W; M. BAYLISS.
UNIVERSITY COLLEGE, LONDON.
CONTENTS
PART I.— TEXT
CHAPTER I
LIFE AND ENERGY
PAGE
The Problem i
Protoplasm - 2
Brownian Movement and the Ultra- Microscope - 3
The Kinetic Theory 4
Amceboid Movement and Surface Tension 6
Heterogeneous Systems and Boundary- Surfaces - 8
Energy and its Laws 10
The Cell-Membrane 15
The Permeability of the Membrane 16
Osmosis - 17
Osmotic Pressure - 21
Electrolytic Dissociation - 23
Strength of Acids and Bases 26
Indicators 27
Electrical Resistance of Living Cells 28
Changes in Permeability during Life 29
The Colloidal State- 30
Precipitation by Electrolytes - 35
Electrical Adsorption and Staining 36
Hydrolytic Dissociation 38
The Nucleus of the Cell 39
Mitochondria 39
CHAPTER II
FOOD— DIGESTION AND RESPIRATION
Composition of Protoplasm - 41
Source of Carbon — The Sugars 41
Source of Nitrogen — The Amino-Acids 45
Optical Activity 48
The Green Plant 50
The Cycle of Nitrogen 56
Sulphur and Phosphorus 58
Salts
x CONTENTS
Accessory Factors -
The Supply of Energy
Digestion -*
Enzymes - -
Changes in Carbotfydrates - -
Fat - ...
Proteins - ...
The Large Intestine
Movements of the Alimentary Canal
Secretion - ...
Respiration -
The Lungs -
The Mechanism of Oxidation
CHAPTER III
WORK— THE MUSCLES
Length and Work -
Gradation of Contraction. " All-or-Nothing"
Refractory Period -
Staircase - -
Voluntary Contraction
Various Muscular Mechanisms
Posture Phenomena
Energy for other Purposes -
Maintenance and Regulation of Temperature
CHAPTER IV
STIMULATION— THE SENSES
Receptors in General
Pain-
Touch
Heat and Cold
Taste and Smell
Hearing
Sight
Position Receptors -
Proprio-Ceptors
CHAPTER V
ADJUSTMENT— THE NERVOUS SYSTEM
The Reflex -
The Neurone
Reflex Action
Inhibition
Reciprocal Innervation
Fatigue ...
The Cerebral Cortex. Conditioned Reflexes
The Nerve Impulse-
The Visceral Nervous System
CONTENT'S * xi
CHAPTER VI
TRANSPORT OF MATERIALS— THE VASCULAR^SYSTEM
General Arrangement — 132
The Blood - 135
Internal Secretions - - 136
The Kidneys • . - 137
Lymph 140
The Proteins of the Plasma 141
The Salts of Blood - 142
Viscosity 144
The Regulation of the Blood Supply 145
The Capillaries 147
The Regulation of the Heart Beat - 148
Origin and Transmission of the Heart Beat 150
CHAPTER VII
GROWTH AND REPRODUCTION
Fission 151
Conjugation 152
Sexual Reproduction 152
Heredity 156
Variation 157
Adaptation - 158
Struggle for Existence 159
PART II.— LABORATORY WORK
CHAPTER I
The Microscope - 163
Nature of Protoplasm 164
The Leucocytes of the Blood - 165
Movement of Protoplasm 166
Dark-ground Illumination - 166
Brown ian Movement 166
Surface Tension - 167
Adsorption - 168
Cell-membrane and Permeability - 168
Osmotic Pressure - 169
Blood Corpuscles 169
Plasmolysis 171
Turgor - 171
Contractile Vacuole - 171
Direct Measurements of Osmotic Pressure 171
Electrolytic Dissociation - 173
Indicators 175
xii CONTENTS
The Colloidal State
Colloidal Gold -
Emulsoids
Swelling
Surface Tension and Dispersion
Electrical Charge
Action of Electrolytes -
Mutual Precipitation
Staining and Electrical Adsorption
Rate of Reaction between Colloids
CHAPTER II
Chemical Composition of Organisms
The Polarimeter
Waste Products
Carbon Cycle
Water Culture -
Action of Green Plant on Carbon Dioxide
Chlorophyll
Formation of Starch
The Nitrogen Cycle
Bacteria
Formation of Nitrates in the Soil
Root Nodules -
Salts
Sources of Energy
Alimentary Canal of Frog and Rabbit
Enzymes and Digestion
Rates of Reactions
Hydrolysis by Enzymes
Enzymes Act at their Surfaces
Catalytic Action
Model
Various Digestive Enzymes
Amylase -
Invertase -
Pepsin
Trypsin
Absorption -
Histological Preparations
Voluntary and Involuntary Muscle
Contractions of the Frog's Stomach
Secretion
Pancreas
Living Newt's Stomach
Flow of Water -
Electrical Change
Respiration -
Tracheae of Insect
Haemoglobin as Oxygen Carrier
Absorption Spectrum of Haemoglobin -
Carriage of Carbon Dioxide
Stimulation of Respiration by Carbon Dioxide
CONTENTS xiii
I'AGE
Oxidation 202
Autoxidation 203
Peroxides 203
Peroxidase 203
Guaiacum Reaction 203
Reduction by Milk 203
CHAPTER III
Action of Jointed Bones 204
Contraction of Muscle 204
Tension in Muscle - 204
Spring 204
Nerve-Muscle Preparation - 204
Formation of Acid - 205
Effect of Length of Fibres - 206
Structure of Voluntary Muscle 206
Effect of Temperature 206
Production of Tetanus 206
Heart Muscle 207
Staircase - 207
"All-or-Nothing" 207
Refractory Period 208
CHAPTER IV
Spinal Frog 209
General Anatomy of the Nervous System - 209
Nerve 209
Structure 209
Electrical Change 210
Unexcited by Light or Sound - 210
Receptors of the Skin 210
Taste-Buds 211
Olfactory Cells 211
Heat and Cold Spots 211
Hearing - 212
The Eye - 212
Visual Purple - 212
Anatomy 213
Image on the Retina - 213
Structure of the Retina 214
Receptors for Position 214
Statocyst of Cyclas - 214
Semicircular Canals of Skate - 214
CHAPTER V
Anatomy of Central Nervous System of the Frog - 215
Sympathetic System of the Frog 215
Anatomy of the Central Nervous System of a Mammal 216
Spinal Neurones 216
Spinal Reflexes 216
Inhibition - 217
The Vagus in the Frog 217
xiv CONTENTS
CHAPTER VI
PAGE
Model of the Circulation - 218
The Circulation in the Frog's Web 221
The Heart of the Sheep - 222
Blood - 222
The Kidney - - 223
The Salts of the Blood 223
Function of Bicarbonates - 224
Vaso-Motor Effects 224
Action of Drugs on the Heart - 225
The Beat of the Heart 225
CHAPTER VII
Dividing Nuclei . 226
Development of the Frog - . .226
Ova and Spermatozoa ... 226
The Structure of a Flower - - 227
Process of Fertilisation in the Plant - - - 227
AN INTRODUCTION TO
GENERAL PHYSIOLOGY
PART I
CHAPTER I
LIFE AND ENERGY
The Problem
WE all know what living beings are, and it would be unprofit-
able to attempt to define life in such a way as to make an
inhabitant of another planet understand what we mean by it on
the earth. The most striking thing in the behaviour of living
things is their perpetual change — they are always doing things.
From that aspect, with which physiology has to deal, we may say
that they are extraordinarily complicated machines, in which the
laws of physics and chemistry are made use of in a way quite
different from that in which a machine made by an engineer uses
them. What we have to do, then, is to try to find the way in
which living machines work (p., p. viii).
It is natural that our own bodies should be, to ourselves, the
most interesting and important of living organisms ; but when we
come to investigate them, we find that there is nothing in them
which is not to be found in some form in what we call the lower
organisms. Certain arrangements, especially those connected with
the brain, are more complex, it is true. These would not be
understood, however, without a knowledge of the simpler arrange-
ments to begin with.
It must not be forgotten that physiology is not directly con-
cerned with the mind. Our thoughts and feelings, when investi-
gated with the view of finding out how they depend on one
another, is the province of another science, psychology. So that
when we speak of living machinery, it is not to be supposed that
a denial is made of the existence of anything else. When the
functions of the brain are discussed, it is the changes taking
place therein, as looked at from the outside, that we are dealing
with.
Plants, as well as animals, are alive, and we shall find that
there is very much in common between them. Strictly speaking,
GENERAL PHYSIOLOGY
" general " physiology should be confined to those properties which
belong to all living creatures. But this is not the meaning taken
here, because it would give us too limited an outlook. It is really
impossible to make a distinction between general and human
physiology. Perhaps the best way to explain the difference is
that it is the manner in which the subject is treated, rather than
the subject-matter itself. Thus, instead of taking an organ, such
as the liver, and talking about all the different things that it does,
we intend to discuss the processes in which it plays its part along
with other organs. This will become clearer as we go on, together
with the fact that whatever name we give to the particular form in
which the phenomena are presented, they are themselves always
the same.
Just as physics and chemistry deal with the laws of inanimate
nature, apart from and necessarily before their application to
practical industry, so there is a body of science dealing with living
nature, which is a necessary preparation for application to the
human body. But it would exist as a science quite independent
of any such application. The reader may be reminded, especially
if he is inclined to overrate the importance of what is obviously of
immediate practical use, that it is not possible to know beforehand
what and when pure scientific knowledge may become suddenly of
the greatest practical value. Electric waves in physics, and the
electrical phenomena of the heart in medical science, may be taken
as two of the numerous instances of the kind.
Protoplasm
Like every other machine, a living organism does its work
because it is made in a certain way. We must know its structure,
therefore. And, first of all, let us see what is the simplest structure
that we can call " living."
There are some minute creatures that consist only of a sub-
stance which has the appearance of a clear jelly, at first sight.
But when we look at it more closely under the microscope, we
see that it changes its shape, whereas a little lump of jelly would
not do so. This material, which is known as " protoplasm,"
behaves more like a drop of oil in water, except that it seems to
have the power of movement on its own account.
The organism called Amoeba is composed of such protoplasm,
but it also contains some other things floating in its body-sub-
stance (E., p. 164). The "nucleus" may be noticed particularly,
a spherical structure in one part or other of the protoplasmic mass.
There are also particles and drops of liquid of various sizes and
LIFE AND ENERGY 3
shapes. Neglecting these for the present, we will examine the
clearest part of the protoplasm. The very clearest part of all will
be noticed when a part of the protoplasm makes a protrusion,
called a " pseudopodium," because it serves the purpose of a leg,
and causes the animal to move along. In this part it will be
difficult or impossible to see any structure at all, a fact which
shows that the visible grains and so forth are not essential con-
stituents of living material.
The whole organism, protoplasm and nucleus, is known as a
" cell." The name does not seem very appropriate in this particular
case, but it was first applied to the constituent parts of vegetable
organisms, in which what corresponds to the whole amoeba is
enclosed in a box of material which is not itself living. Under
certain conditions, amoeba itself forms a coating around itself
and becomes quiescent.
The larger and more familiar plants and animals are composed
of a great number of cells, joined together in a community for
mutual help. Some of them do one thing, some do other things ;
whereas, in the unicellular beings, the one cell performs all the
functions of which the organism is capable. This fact, of course,
.makes it practically impossible to discover much from observa-
tions on these creatures. But we can find out many of the
fundamental and necessary properties belonging to all living
cells.
Apart from the fact that what we see in the moving protoplasm
of an amoeba conveys the unavoidable impression that it is liquid,
there are other facts which confirm this (p., p. 6). But before
we proceed to these facts, it should be pointed out that, so far as
can be made out, protoplasm sometimes sets into a jelly and ceases
to be liquid. This is a temporary state, and the liquid condition
returns again, as when an ordinary jelly is warmed (p., p. 19)4
In protoplasm, the change from one state to the other occurs*
without altering the temperature. This process will be better
understood after we have learned something about what is called
the " colloidal state."
Brownian Movement and the Ultra-Microscope
If we look at the smallest particles which we can see in proto-
plasm by the use of a fairly high magnification with the microscope,
we notice that they are in a perpetual kind of dancing movement.
This can also be seen with any small particles suspended in water.
The yellow resin, gamboge, used as a water colour, rubbed up in
water, shows it very well (E., p. 166). The name of the form of
4 INTRODUCTION TO GENERAL PHYSIOLOGY
movement we are considering has been given to it on account of
its first description by the botanist, Robert Brown. It will be
seen that the smaller the particle, the more lively its movement.
There are difficulties in seeing the very minute particles, because
they are so small. Now, when such particles are brightly lit- up
and looked at against a dark background, they are much more
visible. A ray of sunlight entering a dark room through a crack
shows up a myriad of bright particles when we look at it from the
side against a dark wall, whereas the surrounding air appears
quite empty. This is the principle of that method known as the
ultra-microscope or dark-ground illumination (p., pp. 79-82). The
rays from a bright lamp are sent sideways through the slide on
the stage of a microscope, so that they do not enter the objective,
and, if there were nothing on the slide, one would only be aware
of darkness. If, however, anything solid, able to reflect light,
were there, it would be lit up and send rays in all directions,
becoming a visible object. The particles of gamboge should be
examined first, and afterwards the living amoeba (E., p. 166). The
brighter the illumination, the smaller are the particles which can
be seen. If it is sufficiently intense, it will be found that even in
the clearest protoplasm there are particles to be seen.
The existence of these movements in the particles contained in
protoplasm shows that they must be free to move. In other words,
they are suspended in a liquid. In a solid mass, even with the
properties of a jelly, they would not be free to move. The
experiment can be made with gamboge in gelatin (E., p. 167).
The Kinetic Theory
The explanation of Brownian movement requires a few words
on the constitution of matter, as now generally accepted.
If we imagine a crystal of common salt to be divided up into
smaller and smaller fragments, we should find that at a certain
stage, which could not be arrived at merely by mechanical crushing,
although possible by dissolving in water, the separate fragments
are such that any further division changes their chemical properties,
and there are now two things present of different nature. The
particles which are the smallest possible without alteration of
chemical properties are " molecules ; " the two different substances,
arising from further splitting, are " atoms," and, as the reader is
doubtless aware, are sodium and chlorine. It was at one time
believed that atoms were incapable of further decomposition, but
the study of the radio-activity of certain " elements," and that of
the phenomena of the electric discharge, have taught us that atoms
LIFE AND ENERGY $
are themselves complex organisations, and sometimes divide up
into other smaller elements.
Molecules are not always composed of different kinds of atoms;
two or more of the same kind may be united together, as in the
case of those materials called chemical elements, in the free state,
such as the oxygen and nitrogen of the atmosphere, iron and
copper, and so on.
Now, suppose that we consider how these molecules are behaving
in a gas such as the atmosphere. It is clear from the fact that we
can, by pressure, make a particular volume into a smaller one, as,
for example, by pushing in the piston of a syringe with the nozzle
closed, that the molecules cannot have been in close contact
originally. They must have an actual size and, therefore, there
must also be free spaces between them. The molecules, indeed,
make up a very small part of the total volume of a gas. A rough
idea of how little it is could be obtained by taking a flask full of
the vapour of water and cooling it, so that the steam is condensed to
water. The total number of molecules must be the same in both
steam and water, or, more correctly, the number of atoms must be
the same in both, since we shall see later that some of the molecules
combine together when steam condenses to water.
Why, then, do we have to exercise pressure on a gas if we wish
to make its volume smaller? Why does it resist the process? It
is because the molecules are in a state of perpetual to-and-fro
movement, hitting against the vessel containing the gas with a
total pressure in proportion to the number of molecules that hit in
a given time. If we diminish the volume, we press more molecules
into the space than were previously there, so that we increase the
number of hits. Although these molecules hit against each other
occasionally, they are practically free from anything to hold them
together, so that, if a vessel containing a gas is connected to another
empty one, the gas divides itself equally between the two. This
movement of the molecules is due to their possession of that form
of energy which we call heat.
In a liquid, the constituent molecules are so close together as
to be within the distance at which they begin to attract one
another. Although this attraction does not begin to be appreciable
until the molecules are extremely near together, it reaches a very
high value at that position ; so that a very great force is required
to pull them further apart. The attractive force between molecules
shows itself as cohesion, and, in the case of a liquid, is known as the
internal pressure of that liquid, with which we shall meet again
presently. The molecules of a liquid cannot, then, move further
apart from each other, but they can rush about with a movement
like that of the molecules of a gas, so long as their distance from
6 INTRODUCTION TO GENERAL PHYSIOLOGY
the molecules among which they move does not increase. On the
other hand, the molecules of a solid are not free to move about ;
they can only vibrate backwards and forwards about the same
mean position.
The attractive force between molecules is doubtless due to
the structure of the atom as consisting of electrically charged
smaller constituents, "electrons," with negative charges, moving
in various kinds of orbits around a positively charged central
body.
In a liquid, then, the molecules are in constant movement,
hitting one another and rebounding. If a solid particle, large
enough to be hit by many molecules at the same time, be immersed
in water, the resultant force acting upon it will either be zero or
very small, because there will be about the same number of hits in
one direction as in the opposite one. The mass, again, of a large
particle would require to be hit in the same direction by a large
number of water molecules at the same time in order to move it.
But, if the particle is small, while still large enough to be visible
when adequately illuminated, it will be exposed to unequal
bombardment in opposite directions, and receive enough impulses
to send it moving until it is met by impacts sending it in another
direction (p., p. 86). This is Brownian movement, and we see that
it is a true representation of the molecular movements in the
liquid itself, so that, by looking at it, we get an idea of the way
molecules are in movement, as stated by the kinetic theory of
gases and liquids.
Amoeboid Movement and Surface Tension
We have noticed already how an amceba moves about by
means of a local protrusion of a part of its protoplasm and the
drawing up of the other part of the organism. The movements of
the protoplasm in a plant cell, such as one of those making up the
hairs on the stamens of Tradescantia, should also be examined, as
showing another form of protoplasmic movement (E., p. 166).
Now, while it would be rash to state that the process can be
completely explained in a simple way, there is no doubt that what
is known as " surface tension " plays a large part in it. Since
this property has important relations to numerous physiological
phenomena, we must give some attention to it.
First of all, let us convince ourselves by some experiments with
a soap bubble or film of soap solution that the film behaves as if
it were stretched (E., p. 167). The fact that a drop of oil suspended
in a liquid of its own specific gravity, so that it does not rise or fall,
takes a spherical shape also serves to show that the surface of the
LIFE AND ENERGY 7
drop is in a state of tension. The surface takes that form in which
its area is the smallest possible, that is a sphere (E., p. 168).
The surface of an amoeba is, then, in a state of tension, and if
the tension were the same everywhere, it would be a spherical
drop, like the oil. It does, indeed, become of this shape when
stimulated by an electric shock (E., p. 165). We will suppose
that it is, at a particular moment, spherical, and that at one part or
another something happens, either inside the organism or in the
water outside it, which makes the tension less at this spot. It will
be clear that the greater tension on the remaining part, which
tension, of course, produces a pressure throughout the liquid
protoplasm, will result in a pushing out of that part of the surface
where the tension is less. Thus a pseudopodium is formed.
Here we may note a further proof of the liquid nature of
protoplasm. If the drops of liquid, which are usually present in
an amoeba, sometimes containing organisms taken as food, be
observed, they will be seen to be spherical, whatever the shape of
the material inside them (p., Fig. 3, p. 2). The surface of the
liquid is free to take the form required by its surface tension. A
drop of fluid imprisoned in a jelly may be of any shape whatever.
But how is this surface tension to be explained ?
What, however, do we mean by "explanation " in science? We
have already " explained " Brownian movement by the kinetic
theory of gases and the formation of pseudopodia by surface
tension, and what we have actually done is to show that these
complex things are special cases of properties possessed by very
much larger groups of existences, not necessarily living. When we
now proceed to " explain " surface tension itself, what we do is to
show that it is a consequence of the properties possessed by liquids
as such. A further step might be to refer these properties back to
those of the molecule itself. It will be clear, nevertheless, that
scientific explanation must stop sooner or later. Even suppose
that everything has been explained in terms of the movement of
electric charges, this movement itself still remains a mystery. But
science does not pretend to be able to go beyond what can be
investigated by the powers we possess.
As physiologists, our task is to refer, as far as we can, all
phenomena of life to the laws of physics and chemistry. At present
we have to be content, in many cases, with a reference to more
general physiological laws, applying to a larger group of phenomena
than the particular ones under consideration, but themselves still
" unexplained."
And now we may proceed with that task.
S INTRODUCTION TO GENERAL PHYSIOLOGY
Heterogeneous Systems and the Phenomena
at their Boundaries
A glance at any living organism is sufficient to impress upon
us the fact that it is composed of a great variety of things that are
distinct from one another in space. In the amoeba, for example,
the nucleus and the particles scattered about in the protoplasm do
not mix with the rest of the cell substance. Moreover, we have
seen that even the clear part is full of tiny particles. The individual
cells, as well as the whole organism of a higher plant or animal,
are what the chemist would call " heterogeneous systems," as con-
trasted with such systems as solutions of salts in water. If we take
a sample from any part of a solution of common salt in water, we
find it to have the same composition. It is a "homogeneous
system." It could be made heterogeneous, however, by the addition
of a solution of silver nitrate. The precipitate of silver chloride
could be separated from the liquid. If we divided up a living cell
into parts, these parts would not have the same composition.
The various parts of a heterogeneous system — the parts that do
not mix with one another — are called phases. The name might
seem to imply that they have the same chemical composition, and
this is sometimes the case. Take, for example, ice floating on
water at the freezing point. They are separate phases, with the
same chemical' composition. But this is not necessarily the case.
Charcoal, suspended in water, forms one of the phases of this two-
phase system. There are certain laws which control the behaviour
of heterogeneous systems, some of which we may briefly consider
here. Others will be met with later.
Consider water in a basin. The molecules in the depth of the
water are exposed on all sides to the influence of molecules like
themselves, not only in chemical nature but in their state of
motion, etc. They are attracted equally in all directions. This
attraction, as we saw before, gives rise to the " internal pressure "
of the liquid. Those molecules at the surface, on the contrary,
are only exposed to the attraction of similar molecules on the
one side ; the other side is exposed to air, where the molecules
are very few in number, and not limited as regards their distance
from one another. There is, as a result, a continual force excited
on the water molecules at the surface, trying to pull them down
into the liquid. This could not happen, of course, without
diminishing the volume of the water, and even then there would
always be molecules at the surface. But the molecules are so
close together in a liquid that they cannot be made to get closer
except by enormous pressure, or by decreasing their kinetic
migrations by cooling them. The result of the pull inwards can
LIFE AND ENERGY g
only be that the surface takes the smallest area possible to it,
and resists any attempt to make it larger. In other words, it
behaves as if stretched.
It will also be clear that the molecules at the surfaces where
any unlike substances touch one another are similarly exposed to
forces different from those in the interior of the substances.
For the present, we are only concerned with that aspect of
dissimilar forces at the surface which results in surface tension,
a phenomenon which we can only detect when the molecules are
free to move, but unable to get away from the influence of their
neighbours ; that is, at the contact surface of liquids with gases
or other liquids. We can detect it indirectly at the contact of
liquids with solids, and there must also be related phenomena at
the contact of solids with each other and with gases. It will be
clear that there cannot be anything of the kind with gases them-
selves, because their molecules are completely free to wander
away into the interior of both, so that gases in contact always
mix up together. In other words, if gases form a part of any
heterogeneous system, they can only form one phase, however
many different chemical species this phase may be composed of.
The body-substance of an amceba, as is easily seen, does not
mix up with the water in which it lives. It forms a separate phase,
just as oil and water form distinct phases. But we know that
protoplasm consists largely of water, as can be seen when it dries
up and returns to life again when moistened, as sometimes happens.
Moreover, chemical analysis shows it to contain 80 per cent, or
more of water. We have seen that it is a liquid, so that it must
be a solution of various things in water, and it contains also other
things floating in it. If an amceba is killed by a strong electric
shock (E., p. 165), its protoplasm is dissolved up and disintegrated
by the water around it. Why, then, does it not mix with water
in normal conditions? It must be surrounded by some kind of
a layer that protects it. We have to find out how such a layer or
film, sometimes called the " plasma- or cell-membrane," is produced,
taking into account the fact that it is not a permanent rigid case,
like the cell wall of a plant or the shell of an egg. This is obvious
enough from observation of a pseudopodium. As it is formed
and increases in size, there is no mixing of its substance with the
water. Hence the membrane must be continually being pro-
duced at the contact between water and protoplasm. We shall
see later, moreover, that a dye, such as aniline blue or congo red,
is unable to pass through the cell-membrane, and that it is equally
unable to pass into a bit of protoplasm cut off from the main
mass, although, when either is killed in any way, the dye freely
enters.
to INTRdDUCTlON TO GENERAL PHYSIOLOGY
The difficulty will probably occur to the reader that solid
particles, such as the bacteria and algae used for food, enter an
amoeba, although solutions of dyes do not. The difference is due
to this very fact of the latter being solutions. They can do no
mechanical violence to the membrane, whereas a solid particle
breaks through. The hole in the membrane, however, is mended
as soon as the particle has passed. What happens is like the
dropping of a needle through a soap film. When the point touches
the film, it becomes covered with a continuous film, which prevents
an actual break. As the needle passes through, before the eye-end
has left the film, it also has a film over it, which is left behind as
the needle drops through.
It may be pointed out here that the substances resulting from
the digestion of the food of the amoeba are freely dissolved by
water. They would quickly be washed out if they could pass
through the membrane, and so be lost to the organism.
In order to understand how such a membrane could be formed,
we must direct our attention to the doctrine of energy, especially
in certain aspects. This is, in any case, a necessary preliminary to
further study.
Energy and its Laws
Living beings are always doing something, making changes
in their surroundings. This means work. When we have done
work, we feel that we have lost something that has enabled us to
do the work. Now, this is called " energy," and is actually
defined as the capacity of doing work. Moreover, it can be
accurately measured and shown to be exactly equal to the work
done. Since energy is not a thing to be seen, apart from tke
material bodies possessing it, we are rather apt to overlook its
importance in ordinary life. The chief use of the food we take is
to supply us with energy. If it were merely to make body-
substance, flesh or bones, we should need very little indeed. A
certain quantity of any particular food-stuff contains a definite
amount of energy, no more and no less, and will enable a certain
amount of work to be done, no more and no less.
The reader is familiar, no doubt, with the two great laws at the
foundation of the doctrine of energy. They are usually known as
the First and Second Laws of Thermodynamics ; but it would be
better, especially from our physiological point of view, to speak of
them as laws of " Energetics," since they apply to all forms of
energy. They were first established by investigations of that form
of energy known as heat, hence their usual designation.
The words, " forms of energy," just used, imply that energy may
UFE AND ENERGY ii
be of various kinds. Let us consider for a moment what sorts of
properties objects may possess in virtue of which we can get them
to do work for us. A bullet is a very different thing lying on the
table from what it is just after it has left the rifle barrel. In the
latter case, it can do work because it is moving ; it is said to
possess " kinetic energy." A reservoir of water at a high level can
do work as it falls to a lower level, as through the turbine or over
the mill-wheel. This is due to gravity. If the water remains
dammed up, the energy is there, but not in use ; we say that
it is potential. The fire under a steam boiler makes the engine
do work — we have heat energy. The current of electricity in an
electro-motor enables it to drive machinery. The rays we receive
from the sun, some of which we call light ^ do an immense amount
of work. We may call this radiant energy. One of the most
important sources has not yet been mentioned. That is, .the
energy of chemical combination. Certain chemical substances,
when they combine together, give off energy in various forms.
These substances must originally have contained it in a potential
form. Consider the petrol of an internal combustion engine. It
gives off energy when it combines with the oxygen of the air.
The products, carbon dioxide and water, contain no energy that is
available for use. The greater part of the chemical energy that
we meet with is derived from combination with oxygen, which we
call combustion. The energy of our own bodies has the same
origin ; we burn up our food by means of oxygen obtained from
the air. The fact that one form of energy can be converted into
others is very obvious in this case. In the steam engine, the
combustion of the fuel comes out as kinetic energy. /.IT the engine
drives a dynamo, the energy of the combustion appears in part in
an electrical form. We obtain heat from our house fires, and light
from the burning of candles or from the electrical current. And
we can convert our electrical current back again into chemical
energy by the decomposition of water or by the use of the storage
battery.
T\\t first law of energetics is the expression of the fact found
to be true whenever it is tested, namely, that any form of energy
can be converted into any other form of energy, and that there is
no loss and no gain in the process. This is always found to be
true. If we measure accurately the amount of energy, supplied to
a motor by the current, and also that which is given out by it as
mechanical work, together with that appearing as heat in the
motor itself and the other parts of the arrangement, we find them
exactly equal. Similarly, if we compare the chemical energy of
the fuel burnt in a petrol motor with the mechanical work done
and the heat produced, we find them equal.
12 INTRODUCTION TO GENERAL PHYSIOLOGY
Now, one of the most significant and important results of
modern physiological investigation is that this first law has been
shown to apply to the human body itself. The amount of chemical
energy taken in the form of food can be measured, and that given
out in different forms can be converted to heat. When this is done,
the balance is found to be so close as to be practically perfect.
Although, as we have said, there is no loss in the conversion of
any one kind of energy to any other kind, so far as the final total
sum is concerned, there is a certain limitation in the case of heat.
With this circumstance the second law of energetics deals. This
law may be looked at in two ways. In the first place, it expresses
the fact that, while any other form of energy can be completely
converted into heat, heat itself, under the conditions in which we
live, can only be partially converted into other forms of energy.
The proportion is given by the well-known formula relating the
fall of temperature along which the work is done to the actual
height of the temperature above a particular point, at which heat
energy is absent, called the absolute zero of temperature. How do
we find out where this zero is ? Take a volume of a gas at o° C.
Owing to the heat energy present in it, the molecules are in a state
of movement, and sufficiently far apart that the volume taken up
by them is so small as to be a negligible fraction of the total
volume. Lower its temperature by one degree. The kinetic
energy of the molecules is reduced, so that the volume taken up
by the gas is diminished, if we keep the pressure from altering.
This diminution in volume is found to be 1/273 of its initial volume.
Hence, if the temperature is lowered by 273°, the volume will be
reduced to nothing, provided, of course, that nothing happens to
change the nature of the gas, and that we disregard the volume of
the molecules themselves. This temperature is the absolute zero
at which heat energy is absent altogether ; the kinetic energy of
the molecules has disappeared. In actual fact, of course, the
volume cannot decrease beyond that point at which the mole-
cules touch one another. The reason why heat has the peculiar
position as regards conversion to other forms of energy is, therefore,
because the temperature at which we work is so far above that at
which heat energy is absent. We can never completely get rid of
it ; whereas we can have a total absence of mechanical, electrical,
or chemical energy.
The other aspect of the second law is that otfree energy. WTe
have seen that the various forms of energy, with the exception of
heat, can be entirely converted into other forms ; and, so long as
they are not changed into heat, we may be said to be free to use
the whole of them. But, if ever we allow any of this free energy
to be " degraded," as is often said, to heat, we can only use a part
LIFE AND ENERGY 13
of it again to do work for us. Since free energy is continually
being converted into heat in all sorts of processes going on, it is
clear that the free energy of the universe is steadily decreasing.
This fact was pointed out by Lord Kelvin and called the " dissipa-
tion of energy," that is, of free energy. The energy that is lost in
this way has been given various names, " bound " energy, as
distinguished from that which is free, sometimes "entropy." The
last name is used when we wish to give a quantitative measure of
the fact, and we say that the entropy of a system is the ratio of the
bound energy to the absolute temperature. That energy may be
present, but in such a form that we cannot make use of it, may be
grasped by imagining that we have a hot ball of metal and a cold
one, so insulated from their surroundings that no heat can arrive
or escape. A certain amount of heat energy is present, and by an
appropriate mechanical device we can obtain useful work as the
heat passes from the hot to the cold body. But, as soon as the two
objects have reached the same temperature, one falling, the other
rising, no more work can be got, although the total amount of heat
present is unaltered, since none has entered or left.
In this connection there is an important fact to be remembered,
a fact for which no reason can be assigned, but which is one which
has never been found to be otherwise. In the present universe,
free energy always tends to become " bound," if it is possible for it
to do so. If one may so express it, it takes advantage of every
opportunity of losing its freedom. It is not impossible to imagine
a state of things, otherwise similar to that which we know, where
free energy would tend to increase ; but it is not so as matters are
now arranged.
There are three consequences of this second law which are of
special interest in regard to the phenomena of living organisms.
It has been pointed out that the so-called struggle for existence
is really one for the possession of free energy. There is unlimited
heat energy in the objects around us. What we demand is the
energy which is continually reaching us from the sun, and is con-
verted into the chemical energy of our food by the aid of the green
plant, as we shall find in the next chapter. The second point is
that, for the economical use of the energy we get from our food, it is
important that it should be converted into the other forms we
require, say that of muscular movement, without passing through the
stage of heat. We shall see later that appropriate means are taken
to ensure this. The third point is that it enables us to predict
many things that happen. If we find out that a process is associ-
ated with a decrease of free energy, we have every reason to reckon
upon its taking place, whenever it can. An instance of this will be
seen immediately.
I4 INTRODUCTION TO GENERAL PHYSIOLOGY
We may pause for a moment to point out that an explanation
of a phenon^enon as a consequence of the laws of energetics does
not tell us Iftout the mechanism by which it is effected. This
must be on the basis of the kinetic theory and the structure of
atoms and m6lecules.
The statement made at the beginning of this chapter about the
property of living beings to produce changes may now be made
somewhat more precise. It is in the process of change of one form
of energy into another that the phenomena especially characteristic
of life make their appearance. When this change ceases, or, as it
may be put, when equilibrium has taken place, we have a state of
death. Just as in commerce, money that is unemployed is of no
value.
The boundary surface of a liquid being in a state of tension,
it is clear that it may be made to do work. In a small way, the
experiment that we made with the soap film in a funnel shows
this ; the film rises and lifts up its own weight. In this case, the
tension in the small film is the same as that in the larger one, but
the area is much less, so that the energy is less. We could also
diminish the energy by reducing the tension without altering the
area. The fact reminds us that there are two factors making up
each kind of energy. One of these is always a sort of space or
mass, and is called the " capacity " factor. The other is what might
be called a strength or " intensity " factor. Some familiar instances
will make the conception clearer : —
Capacity Factor. Intensity Factor.
Soap film - - Area. Surface tension.
Water power - - Volume of water. Height above the earth.
Heat - - - Quantity. Temperature.
Electricity - - Current (ampere). Electromotive force or potential
(volt).
Chemical energy - Mass of material. Chemical potential.
Instruments for measuring these factors, with the exception of
that of chemical potential, which is measured in a more or less
indirect way, are in general use. This factor of chemical potential
is not so easy to grasp as the others. It has been loosely called
" chemical affinity," but it has clearly a real existence, as may be
seen by the consideration that equal quantities of different com-
bustible materials afford very different quantities of energy when
burned with oxygen. And again, the chemical potential of oxygen
and phosphorus is high enough for combustion to take place at
a rapid rate, whereas oxygen and sugar only combine very slowly
indeed, unless we raise the potential of the oxygen.
We may note in connection with chemical energy that there
can be no doubt that there is a change in the internal structure
LIFE AND ENERGY 15
and mechanism of an atom when it enters into chemical combina-
tion with another atom, and that it is in this way that energy is
given off when, for example, carbon combines with oxygen, and
that energy must be supplied when a chemical system of low
potential is to be raised to one of higher potential, as when carbon
dioxide is changed to sugar under the influence of the sun's rays.
There is one more point in connection with these two factors
of energy. The quantity of heat energy in bodies of the same
chemical composition at the same temperature is proportional to
their mass. Thus, a litre of water at 100° has twice the heat
energy of half a litre at the same temperature. So that if we
mix the two together we shall have three times as much heat
energy as we have in the half litre, but there is no change in the
temperature. The capacity factors, therefore, add together, while
the intensity factors do not.
The Cell Membrane
We have seen that if we reduce the tension at a boundary
surface, we reduce the free energy present. Now, nearly all
substances when added to water have this property, and the degree
to which the tension is reduced is in proportion to the amount of
the active material present, up to a certain value. Therefore,
suppose that there are things present in the protoplasm of an
amoeba that lower the surface tension of water, the more of these
that concentrate themselves at the contact surface of the organism
with the water, the greater is the decrease of free energy. The
second law of energetics tells us that this will happen. The name
"adsorption" has been given to the process. Such an accumula-
tion of a substance at the interface between two phases may go
so far as to exceed the limit of solubility of the substance, so
that it is deposited out of solution, and forms a more or less
coherent or rigid membrane. The fact can be well seen by blow-
ing a bubble with a solution of the vegetable product called
saponin, which is not very soluble in water, but has a powerful
effect in lowering surface energy (E ., p. 168). Certain substances
which we know to be present in protoplasm have properties like
that under discussion. We should expect, therefore, to find them
taking a chief part in the production of the cell membrane.
These are especially those which have a fatty nature, and also the
proteins, whose nature we shall learn in the next chapter. Fats
have a particularly marked effect in lowering the surface tension
of water. That at the contact surface between water and air is
notably depressed by merely stirring the water with the finger.
The cell membrane is then to be regarded as a part of the
1 6 INTRODUCTION TO GENERAL PHYSIOLOGY
protoplasm itself, and will vary in its composition, according to
the chemical processes going on in the cell. Further, we must
not forget that if the liquid outside the cell contains dissolved
substances, these will assist in the formation of the membrane.
This concerns especially the tissues of the higher organisms, which
are bathed by solutions of a complex composition.
This phenomenon of adsorption is met with in a great number
of cases, both in living organisms and in other heterogeneous
systems. A familiar instance is the use of charcoal for remov-
ing colouring matters from solutions of other things (E., p. 168).
The colouring matter is not destroyed by the charcoal, but
deposited on its surface, whence it can be removed by appropriate
means.
The Permeability of the Membrane
Having seen how th^ membrane is formed, we must next find
out what are its properties, especially in view of what has been
pointed out above as to the escape of matters from the cell. There
are some things that it allows to pass, others not. Its "perme-
ability " has to be investigated.
In the first place, k must allow water to pass through quite
freely, because, we can see cells swell up under some conditions.
What is the cause of this swelling is a rather difficult question,
which must be discussed presently. But does the membrane allow
anything which may be dissolved in the water to pass through?-
Unless the solute (that is, the substance in solution) is coloured, we'
cannot see directly whether it has gone in or not. But we can
test the behaviour to coloured substances, such as aniline dyes and
other pigments (E., p. 168). It is scarcely necessary to remark that
we must not make use of anything that injures the cell, because the
membrane would not then be in its normal state. Some aniline
dyes can be used ; aniline blue and congo red will be found not to
stain the cell protoplasm. We see, then, that there are some solutes
to which the cell membrane is impermeable^
In the cells of the root of the red beet, there is a pigment to
which their membrane is impermeable (E., p. 168). But we can
influence the membrane in such a way that it will allow the pigment
to escape. Killing by heat does this. Certain chemical agents also
do so. Moreover, some of these agents, if carefully applied, do not
permanently injure, so that we can get the membrane to recover.
This is important, because it shows the possibility of changes
during life, so that at one moment a cell membrane may allow a
substance to enter or escape, at another moment it may refuse
passage to it, according to the state of the cell itself. The cane
LIFE AND ENERGY 17
sugar which the cells contain can also be shown by chemical tests
not to be washed out by water, as long as the cell is normal.
At the interfaces between different phases inside the cell,
membranes must also be formed, and it is easy to see their
importance in keeping separate the various reactions going on
within a cell at the same time. The difficulty of finding out what
is happening in the space of a single cell is very great, and we do
not yet know much about it.
But, it may be said, supposing that the membrane is like a sieve,
with holes through which such small molecules as those of water can
pass, but which are too small for large molecules like the aniline
dyes, and there are many reasons for believing that such is their
structure (P., pp. 113, 114), how does it behave to molecules which,
although comparatively small, are larger than those of water, say
sodium chloride? The greater number of these substances of
physiological importance are colourless, so that some indirect way
of testing the permeability of the cell membrane to them must be
made use of. We have seen that the membrane is impermeable to
cane sugar, and we need to test it as regards glucose and sodium
chloride especially.
Osmosis
The most convenient way of doing so is by taking advantage
of the phenomena of '' osmosis " and their consequences. Here we
come upon a property of solutions that is of some difficulty to
explain and to understand. The reader may be reminded that
there are different ways of looking at it, but that given below is
probably the most intelligible to begin with.
Let us first make a few simple experiments to see what happens
to red blood corpuscles when placed in water and various other
solutions (E., p. 169). We take these bodies as convenient repre-
sentatives of the cells of the higher animals, especially so for the
present purpose since they are not attached together, and can be
examined in the uninjured state with ease. Having a thin film of
blood under the microscope, note the size of the corpuscles. Run
in a 10 per cent, solution of cane sugar. No change will be seen.
This being so, we may dilute the blood with such a solution at
once, a procedure which will render the observation of separate
corpuscles an easier matter. Next, try the effect of a 5 per cent,
solution. The corpuscles will swell up and may burst. This
occurs so rapidly if water itself be used, that it is difficult to see
what has happened. The only possible conclusion to be drawn is
that the corpuscles suck up water until they burst. Test, finally,
the effect of a stronger solution; the corpuscles will shrink. Similar
1 8 INTRODUCTION TO GENERAL PHYSIOLOGY
experiments can be made with the various other kinds of cells
making up the bodies of animals, but they require rather more
indirect methods. It has been found that the cells of warm-blooded
animals remain of a normal size in solutions of cane-sugar only
when it is about 10 per cent, the exact strength differing slightly
in the various species. The cells of the frog or fish require a
solution of less strength.
What is the explanation of this behaviour?
Suppose that we have a small hollow ball made of an elastic
material, which has minute pores in it large enough to allow the
molecules of water to pass through, but too small for those of cane
sugar to pass. This is filled with a 10 per cent, solution of sugar, and
immersed in water. It would swell up rapidly, and ultimately burst.
The fact which has to be explained is the rushing in of water
molecules at a greater rate than they escape, although the membrane
is completely permeable to them in both directions. It is somehow
due to the presence of cane sugar molecules on the inside of the
membrane and their absence on the outside, because this is the
only difference. But how? We call to mind the fact that molecules
have an actual size and that, in a cane sugar solution, a part of the
space is taken up by the solute and, therefore, there are fewer water
molecules than in an equal volume of water. Giving our attention
next to a particular area of the membrane, we realise that on the
outside the whole space is bombarded by water molecules, so that
wherever there is a pore, a water molecule can get through. On
the inside, a number of these pores will be hit by sugar molecules,
which cannot get through. As concerns those hit from the inside
and outside by water molecules, as many will pass in a given time
in both directions, since the space is merely a part of the general
mass of water. But where the sugar molecules hit, no water passes
outwards, while there is no hindrance to its passing inwards. The
amount that enters is, therefore, proportional to the number of sugar
molecules in a given volume of the solution.
If we immerse the ball in a solution of cane sugar of the same
strength as that inside it, the number of pores hit by sugar molecules
is the same on both sides, so that there is the same limited oppor-
tunity for water to pass inwards and outwards, and no change
takes place in the quantity of water within. If we place the ball
in a solution of half the strength of that inside it, what will happen?
Water will enter, because there are more pores free on the outside
than on the inside. But, as the water enters, the solution becomes
diluted. The ball will expand until its volume has become double
that which it first possessed ; since, then, the solution within will
have become of the same strength as the outer solution, supposing
that we had a large volume of solution outside, so that the water
LIFE AND ENERGY 19
lost by going into the ball made no perceptible difference in the
concentration of this solution. Now, imagine the ball placed in
a solution of twice the strength of that within it. The opposite
process will take place. Water will pass outwards until the strength
of the solution inside has risen to that of the solution outside.
The ball will shrink to half its size. We see, then, that such a
system behaves exactly like the living cell. But, it may be said,
the red blood corpuscles do not contain a solution of cane sugar,
True, but the above considerations require only that whatever
molecules there are in the solute should be unable to pass through
the membrane, no matter what may be the chemical nature of
these molecules. The effect is simply proportional to their number
in a given volume ; in other words, to the molecular concentration
of the solution.
The movements of water from one side of a membrane to the
other side, when caused by difference of molecular concentration,
are known as " osmosis'' A membrane which is permeable to the
solvent, but impermeable to any particular solute, is called "semi-
permeable" as regards that solute. The last name is not very
descriptive, but is used in the sense indicated. An "impermeable "
membrane would be one which does not permit either water or
solute to pass through, such as one made of glass would be.
We must next devote a little time to the conception of equi-
molecular solutions. It is obvious that for chemical operations it is
a great convenience to have solutions of which equal volumes
contain a known relative number of molecules. For example,
suppose that we want to precipitate a solution of sodium chloride
by one of silver nitrate. If the solutions are of equimolecular
strength, all that we have to do is to take equal volumes, without
the necessity of trial ; and if we find that a known volume of the
silver nitrate solution is just able to precipitate a particular volume
of the sodium chloride solution, we know that these volumes
contain an equal number of molecules of the reagents. In practice,
the most useful concentrations to take are those in which one litre
contains the molecular weight of the solute expressed in grams, or
solutions of simple relation to these. The molecular weight of a
substance expressed in grams is called a " mol," and hence solutions
containing one mol in the litre are " molar" Since the molecular
weight of cane sugar is 342, a solution containing 342 gm. in a litre
is a molar solution. A molar solution of sodium chloride contains
58.5 gm. in the litre, and so on.
The solution of cane sugar which we have been using contains
loo gm. in the litre, a.nd is, therefore, 100/342, or almost exactly
0.3 molar. If the red corpuscles behave as osmotic systems,
therefore, a solution of glucose of the same molecular concentration
20 INTRODUCTION TO GENERAL PHYSIOLOGY
(0.3 m.) as 10 per cent, cane sugar should preserve their normal
volume. Such a solution has a concentration of 5.4 per cent., since
the molecular weight of glucose is 180, and 180x0.3 = 54 gm. in
the litre. If we try the effect of such a solution we shall find it to
be equivalent to 10 per cent, cane sugar. There are many organic
substances which can equally replace cane sugar in the same
molecular concentration. We may say, then, that the molecular
concentration of the red blood corpuscles, so far as concerns those
substances to which their membrane is semi-permeable, is 0.3 molar.
But there are others, a solution of urea, for example, which
behave apparently just as water does. Is this because the
membrane is permeable to urea, as it is to water ? Let us consider
what would happen in such a case. For a moment, the number of
molecules in equal areas on both sides of the membrane is not the
same, but in a very short time urea molecules pass through the
membrane, and rapidly become equal in number on both sides, so
that there is no longer any difference, as far as urea molecules go,
and there is nothing to oppose the inflow of water caused by those
molecules which cannot pass through,
We see how we can utilise the changes in volume of cells to
find out whether or not their membranes are permeable to various
solutes, remembering, of course, that these solutes must not cause
injury to the cell membrane. It may also be pointed out that
there is no satisfactory explanation of this behaviour of cells to the
molecular concentration of solutions, and not to other properties,
other than that they have a membrane around them semi-permeable
as regards the particular solute in question.
As mentioned before, other cells may be used, and a method
with plant cells, known as that of " plasmolysis? has played a large
part in the investigation of the phenomena. In this method,
plant cells containing in a large vacuole inside the protoplasm
a coloured solution, "cell sap," are subjected to the action of
different solutions. The protoplasm forming a coating inside the
cell wall has a membrane of similar semi-permeable nature to that
of the blood corpuscles. If a solution of a higher molecular
concentration than that of the cell sap be applied, water will escape
through the protoplasm, and a space will be formed between it and
the cell wall, visible owing to the coloured fluid in the protoplasmic
bag. The experiment may be tried with the staminal hairs of
Tradescantia (E., p. 171). By testing various strengths of cane
sugar solutions, one will be found which is only just sufficient to
cause perceptible plasmolysis. The solution of equimolecular
concentration to that of the cell is, therefore, a little below this.
LIFE AND ENERGY 21
Osmotic Pressure
Hitherto, we have considered the effect of the entrance of water
in producing a swelling of cells. Suppose that they cannot swell, as
is the case with the cells of the higher plants, encased in a cellulose
box. What will happen ? We may imagine that the membrane of
our original ball is rigid and incapable of being stretched, and
that we have attached a vertical tube to it, so that the water which
enters in may find an outlet. If 10 per cent, cane sugar be inside
and water outside, we shall see that the solution rises rapidly in the
vertical tube, and finally runs over the top. A pressure is evidently
produced by the inflow of water, and this pressure must be greater
than that of the column of liquid of the height of the tube. This
would, indeed, be expected when we call to mind that the molecules
of water get in from the outside in virtue of their kinetic energy.
The amount of energy inside the ball is clearly greater than before
the extra molecules of water had entered. The increase of pressure,
which shows itself by raising the column of liquid in the tube, is
what is called the " osmotic pressure " of the solution. It is difficult
to make a simple experiment to show this fact, because, although
artificial membranes can be made which are semi-permeable as
regards cane sugar, it is not an easy matter. But there are some
organic substances whose molecules are large enough not to pass
through the pores of parchment paper, which are much larger than
those of the cell membrane. The experiment may be tried with
gum arabic, or with the protein of milk, called caseinogen (E., p. 171).
We must next get an idea of how great osmotic pressure is.
Returning to our membrane impermeable to cane sugar, let us try
a much more dilute solution, and instead of allowing it to raise
a column of itself in a tube, let it raise the heavier mercury, as can
easily be done by connecting the ball to a mercury gauge or
" manometer." Pressures of moderate degree are usually expressed
in millimeters of mercury, 760 mm. being the pressure of the
atmosphere. Taking a I per cent, solution, that is, 0.03 m., we
should find that the mercury rose to a height of 511 mm., and
if we took other concentrations, we should find that the pressure
was very nearly in proportion to the concentration, so that we may
say that the osmotic pressure of the red blood corpuscles and the
contents of other animal cells is about 5,110 mm. of mercury,
or 6.7 atmospheres. The osmotic pressure of cane sugar solutions
has been very accurately measured, and it has been found that the
volume taken up by the molecules and other connected phenomena
have to be taken account of. They naturally play a much larger
part when the solutions are concentrated.
The osmotic pressure of the cell contents is a high one, even
22 INTRODUCTION TO GENERAL PHYSIOLOGY
in the ordinary animal cells. In certain plant cells it is higher
still, and may amount to more than eleven atmospheres. It may
be asked, why do such cells escape being burst? In the plant
cell there is a rigid case around the protoplasm, so that the
osmotic pressure makes the cell very stiff (furgor\ thus preserving
the form and uprightness of even the fragile stalks of plants. If
the osmotic properties of the cell are destroyed (E., p. 171), the
rigidity of the structure disappears and the stalk collapses. In
the case of the animal cell, which is devoid of such a protection,
the liquid in which it lies has the same osmotic pressure as itself,
is " isotonic" so that the pressure on both sides of the membrane
is the same. When the outer fluid has a lower osmotic pressure
(hypotonic), the cells swell or burst. If the plant cell is surrounded
by a liquid of the same osmotic pressure as itself, its internal
pressure is compensated and the turgor disappears.
We may next take a further step. We have seen that the
osmotic pressure is proportional to the number of molecules in
a given volume and, as the student is well aware, so is the pressure
of a gas. What we have called a molar solution contains one
gram molecule in a litre. A gas at atmospheric pressure contains
one gram molecule in 22.4 litres. Therefore, if we want to have
one gram molecule in one litre of a gas, we must compress it, so
that 22.4 litres become one litre. This requires, by Boyle's law,
a pressure of 22.4 atmospheres. If we want only 0.3 gm. molecule
in a litre we require a pressure of only 22.4x0.3 = 6.7 atmospheres,
identical with the osmotic pressure of a solution of the same
molar strength. As mentioned above, however, when accurate
measurements of osmotic pressures of solutions are made, it is
found that, as would be expected with liquids, we have to make
allowance for the space occupied by the molecules in a more
important degree than in gases, although it has to be done in this
case also, as the reader is probably aware, from his study of the
Van der Waals' "equation of state" — a necessary modification of
the simple Boyle's law of simple, direct relation between pressure
and volume. We see, nevertheless, that the pressure of a gas
and the osmotic pressure of a solution are fundamentally the
same, and depend on the molecular concentration.
The dissolving of a substance in a solvent, as is well known,
raises the boiling point and lowers the freezing point of this
solvent. The effect is again found to be proportional to the
molecular concentration, and can therefore be used to measure
the latter. The boiling point is only of limited application in
physiology, since changes occur in the solutions with which we
have to deal when the temperature is raised much above that of
warm-blooded animals. The depression of the freezing point of a
LIFE AND ENERGY 23
watery solution (called A), on the other hand, is frequently made
use of (E., p. 172), since direct measurements of osmotic pressure
are difficult. The vapour pressure is also used for the same
purpose (P., p. 1 54). The fact that the vapour pressure of a solution
is lower than that of the solvent can be foreseen from consideration
of the energetics of the process. Imagine two vessels in an
enclosed space, one containing water, the other a sugar solution.
The pressure must be lower over the latter in order that the
osmotic energy of the whole system may be lowered by distillation
of water to dilute the solution. The reason why the vapour
pressure is less is of the same nature as that discussed in the
preceding pages. Air may be regarded as a semi-permeable
membrane to a non-volatile solute, since it is permeable to water
vapour, not to the solute, which has no vapour. A greater part of
the surface of the water is occupied by molecules escaping to the
air than in the case of the solution, where a part of it is occupied
by the molecules of the solute.
We have spent much time on the question of osmotic pressure,
because it is a difficult one ; but clear ideas upon it are of great
importance.
Electrolytic Dissociation
Pursuing our investigations on the osmotic pressure of the red
blood corpuscles, we shall find that we are led to another very
important characteristic of certain substances in solution in water.
When isotonic solutions of various materials were tested on plant
cells and blood corpuscles, it was found that some of them, although
of equal osmotic pressure, were lower in molecular concentration
than sugar. Thus, sodium chloride (E., p. 170), if taken in 0.3 molar
strength, was too strong and caused the cells to shrink. The
correct value was found to be 0.9 per cent, or 0.154 molar ; that is,
a little more than half its expected value. In other words, these
particular substances behaved as if they were split up, or " dis-
sociated," into a larger number of smaller molecules, each of them
acting as a separate molecule.
But what can these smaller parts be ? They cannot be ordinary
sodium and chlorine, because free sodium immediately reacts-
violently with water, forming caustic soda, and if there were free
chlorine in a solution of sodium chloride, it would easily be detected.
On further examination, it was noticed that all of these anomalous
substances were such as had been found to conduct electricity when
in solution ; they were salts, acids, or bases. Those that behaved
normally were organic compounds and non-conductors.
When an electrical current is passed through a solution of
24 INTRODUCTION TO GENERAL PHYSIOLOGY
sodium chloride by means of two carbon plates immersed in the
solution, the current enters by one plate and leaves by the other.
Faraday called the two plates "electrodes" that by which the current
enters being the "anode" that by which it leaves the "cathode"
Since the current flows from the anode to the cathode, the former
has the higher potential, or is electro-positive to it. Now we find
that chlorine is attracted to the positive pole, and is present around
it in solution. Sodium goes to the negative pole, and can be
collected if mercury is present to dissolve and remove it ; otherwise,
it reacts with water to form the hydroxide. But if the sodium
atoms are attracted to the negative pole, it must be because they
have an opposite, or positive, charge. Correspondingly, the chlorine
atoms must be electro-negative. Since there is every reason to
believe that atoms owe their chemical nature to their constitution
as electrons, or unit electrical charges, with a positive nucleus, it
is clear that if an electron is added or removed, making the atom
negative or positive to what it was before, the chemical properties
will be altered. A chlorine atom with an extra electron is not the
element chlorine, nor is sodium with an electron removed the
same thing as the metal sodium. It is only when the sodium and
chlorine " ions" as Faraday named them, on account of their
movements to the poles, lose their electrical charges by contact
with the opposite charges on the poles, that they are converted into
the ordinary elements. In Faraday's terminology, the sodium ion
is the cation, because it wanders to the cathode ; the chlorine ion is
anion, because it goes to the anode. If the current is allowed to pass
long enough, all the sodium chloride is decomposed by electrolysis,
and if we imagine that the last remaining molecule arrives as such
at either electrode and is not decomposed until it arrives there,
one of its constituent ions must be left free, and must pass through
the solution to the other pole. If this were so, it would exist
during its passage as an atom with a charge, that is, an ion. Hence
we must admit the possibility of the existence of free ions in the
solution, and it is natural to suppose that they are present as such
before the electrical current is sent through ; so that what this
current does is to carry those of opposite sign to the appropriate
electrode. This is the statement made by the theory of electrolytic
dissociation. The ions into which an electrolyte is dissociated in
water are the elements of which we are in search. To repeat, a
solution of sodium chloride or other electrolyte is already decom-
posed into its constituent ions, to a greater or less extent, before
any electrical current passes through it, and what the current does
is merely to attract the oppositely charged ions to the poles of
opposite charge to themselves and deprive them of their charges.
We see also that the passage of electricity from one pole to the
LIFE AND ENERGY 25
other is by means of the charges on the ions, each ion carrying a
definite quantity. There are various other reasons for regarding
this as the correct account of the phenomena (P., p. 173).
It has been agreed to denote the possession of a positive charge
by the addition of a dot to the chemical symbol of an ion, and a
negative charge by a dash. Thus the hydrogen ion is H', the
chlorine ion is CT. Ions may possess more than one charge,
according to their valency. Thus, sulphuric acid dissociates into
two H- ions, and one SO" ion, which must have two dashes to
satisfy the positive charges of the two hydrogen ions.
Since it is by the agency of water that the dissociation into
ions is effected, it is natural to expect that the more water there is
in proportion to the solute, the greater will be the degree of
dissociation. We have seen that 0.154 molar sodium chloride is
almost completely dissociated into its two ions, since it is equal in
osmotic pressure to a 0.3 molar solution of a substance which is not
dissociated (E., p. 170).
Acids and alkalies, as well as neutral salts, conduct electrical
currents excellently ; in fact, better than neutral salts. What are
the ions here? And why do they conduct better? In the case
of itcids, we find that hydrogen gas is given off at the cathode,
therefore the ion must be hydrogen with a positive charge. In the
case of hydrochloric acid, the other ion must be chlorine. All
acids are actually found to give hydrogen ions, while the anion
varies with the chemical composition of the acid. Alkalies deposit
the metallic or similar ion at the cathode. Sodium hydroxide is
decomposed into sodium and hydroxyl ions, but since the sodium
combines with water giving off hydrogen, it is this gas that actually
makes its appearance. At the anode, oxygen is given off, because
the OH when deprived of its charge cannot exist Two OH ions
unite, forming one molecule of water, and giving off oxygen.
It remains to mention briefly why some electrolytes, as Faraday
called those substances which conduct electricity when dissolved in
water, are better conductors than others. It has been found by
experiments, which cannot be described here, that different ions
move to their respective poles at different rates, and according to
their dimensions. H and OH ions move much faster than any
other ions. It is easy to see that the way in which electricity is
carried through a solution is by means of the charges carried by
moving ions, so that the more rapidly these ions move, the more
they carry across in a given time. Hence, substances which dis-
sociate with the production of rapidly moving ions are better con-
ductors than those producing slowly moving ions.
Ions have also the property of attaching molecules of water,
which increase their dimensions, and make them move more slowly.
26 INTRODUCTION TO GENERAL PHYSIOLOGY
The number of water molecules attached varies with the different
ions. This fact is of importance in connection with the permeability
of the cell membrane to them, since inorganic ions become larger
than would be expected.
In addition, however, to this cause of difference in conductivity
of solutions, there is another in the fact that different substances
are split up in very different degrees when dissolved in water. So
that, even if their ions move at the same rate, there are fewer of
them in the one case than in the other. It must always be kept
in mind that those molecules which are not split up into ions take
no part in the carriage of electrical currents.
It is not to be understood that all organic compounds are
similar to sugar in being non-conductors. Some of them are acids,
some are bases, and some are salts. But since they are, as a rule,
large and complex molecules as compared with inorganic com-
pounds, they are not such good conductors, although many of them
are better conductors than might have been supposed from the
dimensions of their molecules. Thus, solutions of congo red are
very good conductors, although it is a salt of an organic acid of
very large molecular dimensions with sodium.
Strength of Acids and Bases. — It is well known that some
acids are very much more powerful chemical reagents than others.
Thus, hydrochloric acid in dilute solution dissolves zinc with great
rapidity, whereas acetic acid in the same molecular concentration
has very little action upon it. Now, if we compare strong acids
with weak acids as regards their electrical conductivity, we find
that the former are much better conductors than the latter. This
might be due either to their being more dissociated, or to the rate
of migration of their ions being greater. We can decide this
question by diluting (E., p. 175). Suppose that we take hydro-
chloric acid and acetic acid, each in one-tenth molar concentration.
The former is a much better conductor than the latter. Next,
dilute each to ten times its volume. We find that the conductivity
of the hydrochloric acid is reduced almost exactly to one-tenth.
This means that practically no further dissociation has occurred ; or,
in other words, that it was at first almost completely dissociated.
On the other hand, the conductivity of the acetic acid is much
greater than one-tenth, hence it must have become more dissociated,
since the original ions would only account for a diminution to one-
tenth. By further dilution, we can make the conductivities of the
two acids approach one another nearer and nearer.
It is clear that these considerations suggest to us a method of
expressing the "acidity" of a solution in a numerical manner, a
fact of great convenience and importance. We have merely to
give the molecular concentration in ions ; and, since it is only the
LIFE AND ENERGY 27
hydrogen ion which is common to all acids, and is responsible for
their characteristic acidic properties, such as taste, and so on, we
always speak of the " hydrogen-ion concentration"' We shall find
later that physiological phenomena are extremely sensitive to the
precise value of this property of the medium in which they take
place, and that there are means taken to maintain it at its most
appropriate value.
Similar considerations may be applied to the case of alkaline
solutions, and their alkalinity may be expressed in terms of con-
centration of OH ions. But, since the product of the H and
OH ionic concentrations in all solutions is the same (P., p. 197),
it is best, for the sake of uniformity, to give the " reaction " of all
solutions in terms of H-ion concentration, from which the OH-ion
concentration can be easily calculated. The reaction of distilled
water being taken as the point of neutrality, those solutions whose
H-ion concentration is greater than this are acid, those below it
are alkaline (P., p. 184).
Indicators. — The question next arises as to how this hydrogen-
ion concentration is to be estimated. The most direct method is
by the use of the hydrogen electrode, in which a battery is fitted up
whose electrodes consist of hydrogen. The electro-motive force in
such a case is proportional to the concentration in H ions of the
solutions in contact with the electrodes, and can be measured in
the usual way (p., p. 190). But although this method is the most
accurate in cases where it can be used, in the physiological solutions
of most interest to us its application requires somewhat complicated
procedures if correct values are to be obtained. A case in point is
that of the blood. For this reason the more indirect methods are,
as a rule, more useful. Of these methods, the use of what are
known as " indicators " is the simplest. There are many coloured
chemical compounds which have a different colour, according to the
H-ion concentration of their solutions. There is some dispute as
to how this change of colour is related to the chemical changes in
the indicator, but this does not concern us here. In general, the
range is small, so that above a certain concentration there is no
further change in colour, nor is there below a certain concentration.
The particular concentration in hydrogen-ions, at which the more
or less sudden change in colour takes place, is not the same with
the different indicators (E., p. 175); so that it is posssible, by
taking an appropriate series, to obtain the H-ion concentration of
a given solution with some degree of accuracy (p., p. 189). In
most cases of interest to us, the H-ion concentration is not far
distant from that of distilled water, and in such cases the dye
known as " neutral red " is very useful, since it shows a series of
changes, from crimson through red and orange to yellow, in this
28 INTRODUCTION TO GENERAL PHYSIOLOGY
region. Moreover, unlike some other indicators, it is not particularly
sensitive to the presence of salts or proteins in the solution. Thus,
the H-ion concentration at which a change of tint occurs is
practically the same in their presence or absence, so long as they
are not in great excess.
That it is the concentration of hydrogen-ion that an indicator
really gives information about is well seen by taking a strong
solution of hydrochloric acid and diluting it with water. The dye
known as " crystal violet " will be found to show a series of definite
changes, although nothing has been done except to decrease the
concentration. That it is the hydrogen-ion, and not the anion,
may be seen by taking a different acid, say sulphuric, when we find
the same series of changes (E., p. 175).
The student may notice that this use of indicators differs some-
what from the usual one of determining the amount of total acid
present by " titrating " it with a standard solution of alkali, or
vice versa. In such cases, as is seen in practice, the degree of
dissociation of the acid does not play any part. Molar solutions
of hydrochloric and of acetic acids require the same amount of
caustic soda to neutralise them to an indicator. How is this to
be explained if the concentration of H ions is so much greater
in the former case ? We have only to remember that, as each
successive portion of H ions is combined with OH ions from the
alkali to make water, the remaining part of the acid, becoming less
and less concentrated, continues to become more and more
dissociated, until the whole of it, whatever the original degree
of dissociation, has passed through the ionised state, and the H ions
have been neutralised by OH ions.
The Electrical Resistance of Living Cells
Since the electrical current can only pass through solutions of
electrolytes by virtue of its carriage in charges on moving ions,
it is obvious that the conducting capacity of a solution depends
on the width of the channel between the electrodes, as well as on
its length. If, therefore, part of this channel is filled up with some
non-conductor, such as grains of sand, there must be an obstruction
to the passage of a current. Further, if the cell membrane is im-
permeable to the ions of a solution in which the cells are immersed,
these cells must behave simply as inert bodies, blocking the passage
of a current — in fact, as if they were grains of sand. This is found
by experiment to be the case, and has been used to determine the
number of blood corpuscles in a given volume of blood (E., p. i/'5).
If the cells are killed, the membrane becomes permeable, and the
conductivity rises, because the cells now admit of the ions of the
LIFE AND ENERGY 29
solution passing through them with very little resistance. This fact
may be regarded as further evidence of the presence of a semi-
permeable membrane on the surface of the cell. But, on the other
hand, it has been objected that the inorganic salts, shown by
chemical analysis to be present in the cell, might be combined
in a non-dissociable form with the organic constituents, or proteins,
of the cell. There are certain methods, which would require more
space to describe than can be allowed here, which show that there
are free electrolytes inside the cell (p., p. 123). But, apart from this,
an indirect proof can be given on the basis of the osmotic pressure
of the cell contents, a proof which is instructive in itself. We have
seen that the osmotic pressure is that of a 0.3 molar solution. The
smallest molecular weight met with amongst proteins is over 3,000 ;
haemoglobin has one of 12,000. Assuming that it is 3,000, a 0.3
molar solution must contain 90 percent, of the solute, an impossible
amount, since we know that only 20 per cent., at the most, of the
cell contents is solid matter. The cell membrane must be
impermeable to solutes of small molecular weight.
Changes in Permeability during Life
The consideration of the preceding paragraph leads us to a
brief statement of what evidence there is with regard to such
changes.
It has been pointed out above that the cell membrane cannot
always be semi-permeable as regards food materials — sugar, for
example — when the supply comes to it from the outside, as in the
higher animals. We have also seen reason to regard the cell
membrane itself as a local concentration of constituents of the cell
and of the surrounding medium. Its properties naturally depend
on the changes in the cell especially. Hence it is not surprising
to find that, in states of activity of the cell, the membrane becomes
permeable to substances to which it was previously impermeable.
There are not many cases in which, as yet, direct evidence of this
has been obtained (p., p. 124). Electrical stimulation of certain
contractile cells causes them to lose the pigment which is normally
kept within. Again, supposing that the natural electrical resistance
of the cells is mainly due to the impermeability of their membranes
to the ions of solutions in which they are immersed, it will be clear
that this resistance must decrease if the membrane becomes more
permeable. Such effects have been detected in muscle in contrac-
tion, in the process of fertilisation of egg-cells, and so on (p., p. 141).
We shall see later how this change of permeability to ions explains
the electrical phenomena which are frequently to be detected when
cells enter into activity.
30 INTRODUCTION TO GENERAL PHYSIOLOGY
The Colloidal State
We are now in a' position to understand more about the particles
which were revealed in protoplasm by our special optical methods.
Suppose that we imagine a small piece of gold immersed in
water, and by some means gradually divided up into smaller and
smaller fragments. Ultimately we shall arrive at the atoms,
beyond which we cannot proceed without altering the chemical
properties of the substance. But, before this state is reached, we
should find that the particles were small enough to be kept in
suspension by Brownian movement, and that the preparation would
show some new properties. It would appear clear but coloured,
and might be taken to be homogeneous unless a bright beam of
light were sent through it. When this is done the existence of fine
particles of gold is made manifest. Although such solutions of
gold, which are said to be " colloidal," could only be prepared with
great difficulty by simple mechanical disintegration, they can be
made easily by chemical decomposition of solutions of salts of gold.
The action of a reducing agent is to split up the salt, so that metallic
gold is obtained in a very finely-divided state (E., p. 176).
Similar solutions can be made by appropriate treatment of
various substances usually regarded as insoluble. The gamboge,
already used, is one of these. So is the suspension of carbon
particles known as " Indian Ink." Such are called " suspensoids,"
and consist of a solid phase suspended in a liquid phase. Since
the solid phase is completely surrounded by the liquid one, it is the
"internal phase," and may be compared to a number of islets
surrounded by the sea. But it is clear that the same constituents
might be arranged differently, similar to a number of small lakes
surrounded by land, such as might happen if the islands grew until
they touched one another. Here the solid phase would be external
and the water internal. The whole system would be solid, instead
of liquid.
Further, the constituents of a colloidal solution may be two
liquids which do not mix with one another. These systems are
" emulsions," or, when their internal phase is very finely divided,
" emulsoids." A good example is cream, where the internal phase
consists of oil globules, the external phase is a watery solution.
When made into butter, a redistribution of phases occurs by the
oil globules uniting together ; the fat becomes external, the watery
solution in droplets surrounded by it. The meaning of the terms
sometimes used will be plain. The internal phase is the "dis-
persed " one, the external phase is the " continuous " one. It is
very likely that changes in distribution of phases plays an important
part in the mechanics of the cell and of its membrane.
LIFE AND ENERGY 31
But an emulsoid system may also be formed by dispersion of
a solid in a watery phase, provided that this solid is one that
soaks up water by the process known as " imbibition." A well-
known case is that of gelatin (E., p. 177). Here the redistribution
of phases takes place merely on warming and cooling. A jelly
consists of droplets of a very dilute solution of gelatin encased in
chambers of the solid gelatin holding water in its substance by
imbibition. On warming, the more solid phase becomes internal,
particles surrounded by watery solution. Hence the system, as a
whole, becomes liquid. What the nature of imbibition is, is not
completely known. There is evidence that it is essentially an
adsorption of water by the surfaces of constituent elements of the
solid, owing to certain physical peculiarities of these surfaces ; but
the precise interpretation clearly depends on what these elements
are.
Whatever may be the nature of imbibition, a fact of importance
in the physiological behaviour of emulsoids is that the amount of
water present may vary in its distribution between the two phases.
The change is produced especially by electrolytes (E., p. 177), not
in virtue of their electrical charges, but owing to the effect they
have on the properties of water (p., pp. 96, 97). The importance
of being able to extract water from a system in which chemical
reactions are taking place will become more evident when we
study the actions of enzymes.
Remembering that a colloidal solution consists merely of a
substance very finely divided and dispersed in a liquid, we see at
once that the properties that distinguish it from those of a system
consisting of the same amount of material in a single lump immersed
in the liquid depend on the enormous extent of boundary surfaces
between liquid and solid phases, so that they may be regarded
as only differing in degree; but there is a very great difference
in degree. The properties are, therefore, those which manifest
themselves at such interfaces. These are especially those dependent
on surface tension, electrical charges, etc. We expect to find
adsorption phenomena in a marked degree, and we shall see,
presently, the way in which electrical charges play their part.
From this point of view we may note again that we cannot make
any hard and fast line of distinction between coarsely heterogeneous
and colloidal systems, except in degree. On the other side, it is
difficult to say at what stage of subdivision the properties of
surface cease and molecular properties begin. As will be seen
presently, some molecules are large enough to show the properties
of surface when single, but in most cases, and especially in the
suspensoid colloids, the particles consist of a large number of
molecules. It is generally agreed, however, to call those solutions
32 INTRODUCTION TO GENERAL PHYSIOLOGY
"colloidal," of which the dispersed phase is in large enough
particles, be these aggregates or single molecules, not to pass
through parchment paper, while at the same time small enough
to remain suspended permanently, or for a long time.
When such solutions were first described by Thomas Graham,
the colloidal state was thought to be a property of certain sub-
stances, such as gelatin or glue only ; hence the name (xoAA.?/, glue).
But we now know that any substance, by appropriate treatment,
can be brought into the state. In general, the treatment may be
described as reducing the material in question by some means or
other to a very fine state of subdivision. In the case of chemical
elements or simple compounds of small molecular dimensions, the
colloidal particles are aggregates of a large number of separate
molecules, but it is obvious that a single molecule, if large enough,
may exhibit colloidal properties. Such is the case with some dyes,
as congo red, and with the proteins, of great physiological im-
portance, whose nature we shall learn in the next chapter. We
must remember that the visibility of the particles depends on the
brilliancy of the illumination, and on the fact whether they differ
much in refractive power from the liquid in which they float.
There are, indeed, some substances which we know to be in
colloidal solution, because they do not pass a parchment paper
membrane, but which require a very powerful illumination to show
the presence of particles. Some only show a diffuse beam of light
when observed under the best conditions yet possible ; they have
not been actually resolved into separate particles. On the other
hand, if the illumination is sufficiently powerful, even simple
molecules may show a beam of scattered light ; in fact the blue
of the sky is such light scattered by the molecules of the gases
of the atmosphere.
We must now direct some attention to the properties which
belong to colloids in consequence of their enormous development
of surface. First of all; there are certain properties due to the
presence of surface tension, or rather of surface energy. Since the
larger the number of particles into which a given mass is divided,
the greater the total area of surface, there will always be a tendency
for these particles to aggregate together again into larger masses,
for by doing so there wfll be a diminution of free surface energy.
This tendency is opposed by the continual Brownian movement,
and we Tan also decrease it by diminishing the intensity factor
of surface energy, that is, the surface tension, by the addition
of some substance which lowers the surface tension at the interface.
Solutes in general do this, as we have seen, but there are some
which have a very marked effect of this kind. Such are the higher
alcohols, bile salts, fatty substances, saponin, etc. (E., p. 177). But
LIFE AND ENERGY 33
there is another phenomenon which takes part in the maintaining
of the particles in suspension, and is also of importance in other
ways. If we place a colloidal solution between electrodes,
connected to a battery so that there is a fairly high difference
of potential between them, we shall find, in nearly all cases, that
the colloidal particles are carried either to the positive or to
the negative pole, and deposited there. In the former case,
they must have a negative charge ; in the latter, a positive one
(E., p. 177).
We may ask, what is the effect of this charge on the surface
tension ? Remembering that charges of the same sign repel each
other, we may look upon the surface of each particle as made
up of areas charged with the same sign ; the parts of the surface
mutually repel one another, so that the surface tends to increase
its area. This is in opposition to the direction of the ordinary
surface tension, due to internal pressure, and the result is a
favourable one on the state of suspension of the colloid. The
mutual repulsion of the particles themselves also plays a part
in keeping them from aggregation and deposition.
How is the presence of this electrical charge on the surface of
substances In contact with water to be accounted for? There is no
doubt that, in the majority of cases, it is due to electrolytic dissocia-
tion of the material at the surface of the particle itself. This takes
place in two somewhat different ways, according to the dimensions
of the molecule of the chemical compound concerned, giving rise,
on the one hand, to what have been called "'electrolytic colloids"
or, on the other hand, to " electrolytically dissociated colloids" As an
instance of the former, in which the particles consist of a large
number of small molecules aggregated together, let us take silicic
acid in the colloidal state. This substance is usually regarded as
being insoluble in water, but it is not absolutely so, as indeed no
substance is. When in solution, silicic acid, like all other acids,
dissociates into hydrogen-ions, which are freely soluble, and anions
of silicon oxide, which are practically insoluble. Consider now the
state of affairs at the surface of a particle of silicic acid in water.
The molecules of the surface layer are electrolytically dissociated.
The hydrogen- ions pass into the water, leaving behind on the
surface the insoluble silicic anions. These latter possess negative
charges, so that the particle, as a whole, will have a negative charge
consisting of the sum of the charges of the anions on its surface.
The particle becomes a kind of large composite ion, and may be
called a "colloidal ion" ; but it must be remembered that such ions
vary greatly in the number of molecules they contain, so that the
charge is not composed of a definite number of electrons, like that
of the true ion is. Similar considerations apply to particles of
34 INTRODUCTION TO GENERAL PHYSIOLOGY
basic substances, such as those of aluminium hydroxide. In these
cases the soluble ions which go away into the water are OH ions,
so that the particle is left with a positive charge.
Turning now to those substances which are present in solution
in single molecules, but are colloidal on account of one of the ions
being of large dimensions and insoluble, it is clear that these ions
will have charges of a definite number of electrons, according to
their valency. Otherwise, their behaviour is the same as that of
the previous kind. To distinguish them, however, they may be
called " electrolytically dissociated colloids" They are met with,
especially amongst complex organic electrolytes ; many of the
aniline dyes and the proteins are examples. The behaviour of the
latter is of special interest, and will be described in the next
chapter.
There is one point about the electrical state of such systems as
those just referred to that must not be left unmentioned. When
the diffusible ion goes into solution in the water surrounding the
particles it is endowed with kinetic energy, of course, as all the
other molecules of the liquid. In virtue of this, it naturally tends
to wander away into the solution. But this is prevented by the
powerful electrical attraction exerted by. the oppositely charged
solid particle. The soluble ion can only go so far as the balance
between its kinetic energy and the electrostatic attraction permits
it. A number of them form, thus, a sheath or layer at a very short
distance away from the particle. Such an arrangement is known
as the " Helmholtz double layer" and we shall have occasion to
return to it again later.
In certain cases where the dispersed phase shows on investiga-
tion that it has an electrical charge, it is not an easy matter to
explain it by electrolytic dissociation, although this may ultimately
turn out to be the case. Droplets of paraffin oil in water are
negatively charged. It has been suggested that this charge has
an origin similar to that of frictional electricity.
There is, again, a further cause of an electric charge on inert
particles in solutions of electrolytes. If particles of carbon are
suspended in water, surface tension is present at their contact
surfaces with the liquid. By the deposition of ions on this surface,
adsorption, in fact, the surface energy can be lowered. This may
be either in the mechanical way, or by imparting an electric charge.
It is a matter of experiment that if acid is present in the liquid
phase, hydrogen ions are deposited on the surface, giving it a
positive charge. If alkali is present, the surface becomes negative
by deposition of OH ions. There must be some reason why the
H and OH ions are deposited in preference to these of opposite
charge which are always present. It may be that the greater
LIFE AND ENERGY 35
velocity of the former ions is the cause, but the matter is not quite
cleared up.
Precipitation by Electrolytes. — The neutralisation of the
electric charge on colloidal particles will have the effect of throw-
ing them down from suspension, since the removal of the charge
acts both by increasing the surface tension and by abolishing the
mutual repulsion of the particles. The addition of an electrolyte
is an effective way of doing this. Suppose that we add to a
colloidal solution of arsenious sulphide, whose particles have a
negative charge, some sodium chloride in solution. There are now
present sodium ions, with a positive charge, and chlorine ions,
negatively charged. The sodium ions neutralise the negative
charge of the particles by being deposited on their surfaces, the
colloid is precipitated, carrying with it the ions required to
neutralise the charges on the particles. Since it needs several
univalent ions to neutralise the charge on each particle, it is clear
that many ions have to be met with by each particle before
sufficient opposite charge has been obtained. Bivalent or pluri-
valent ions afford two or more electrons at each encounter, so that
they are much more effective, as would be expected by the law of
chances (E., p. 178). When we have an electro-positive colloid, it
is the anions of the added electrolyte that are the active ones. We
see that a decrease of free energy occurs by such abolition of
charge, whereas if ions of the same sign as the surface were de-
posited on it, a gain of free energy would result.
Emulsoid colloids are, as a rule, much less sensitive than sus-
pensoids to the action of electrolytes. But it is only a matter of
degree (P., p. 92). If we call to mind that the two phases of which
the former consist differ only in the amount of water contained, it
will be understood that the forces at the interface of contact, whose
magnitude depends on the difference in nature of the two phases,
must be less than when the two phases are altogether different in
chemical composition.
This is an appropriate place to remind the student that the
various physical properties to which, for the time, our attention is
being directed, depend on the chemical nature of the substances
concerned. While we discuss the properties which belong to
certain constituents of the cell on account of their being in the
colloidal state, we must not forget that they also react chemically
with other constituents and with substances coming from the
outside. Substances in the colloidal state, however, do not so
readily enter into chemical reaction with other substances, since it
is only the surface of the matter of which they are composed that
comes into relation with other reagents. On the other hand, the
physical properties of the surface can be brought into play very
36 INTRODUCTION TO GENERAL PHYSIOLOGY
rapidly, a point of some importance in connection with the inter-
pretation of certain physiological phenomena, such as that of
muscular contraction (E., p. 179).
When electrolytes are added to colloidal solutions, if more is
added than necessary to neutralise the charges, the particles may
have conferred upon them a charge of the opposite sign to their
original one and be re-suspended. It is somewhat difficult to give
a satisfactory explanation of this fact. The probable reason is
that the excess ions are adsorbed, owing to their effect on the
mechanical surface tension. If an ion is adsorbed owing to an
effect of this kind, independent of the sign of its charge, the surface
must obtain a charge of the sign of that of the ion in question
(E., p. 179).
The addition of a colloid of opposite electrical sign to another
colloid has the effect of precipitating both (E., p. 179). Excess of
either causes re-suspension, owing to the excess charge of one sign
or the other.
The precipitate in this last case is evidently composed of both
colloids, although not in chemical combination. It is a representa-
tive of a large class of substances, sometimes called "adsorption
compounds." The components of these are present in no relation
to chemical combining proportions, but to certain physical properties,
which, it may be pointed out, although less simple to determine in
any particular case, follow laws as definite as the purely chemical
ones. Much confusion of thought would be avoided if the expression
" chemical combination," or even " combination," were strictly
confined to those cases where the chemical properties of the atoms
or molecules are changed — where the internal structure and energy
of the atom is altered. The name " adsorption compound " is not
to be recommended ; a better name is "colloidal complex." Such
complexes may be formed between colloids and crystalloids, as
when charcoal takes up iodine, as well as between two or more
colloids (P., p. 64). There are other cases, such as those of mixed
crystals and that of the water of crystallisation, where physical,
rather than chemical forces appear to be concerned.
From the general properties of contact surfaces, as outlined
above, we see that complexes between colloids and substances that
lower surface energy are very apt to occur. They often cause
difficulty in the separation and purification of the compounds
present in cells and secretions, as will be more obvious later. Many
errors in interpretation have been made on this account.
Electrical Adsorption and Histological Staining
The dyes used for the purpose of making evident various con-
LIFE AND ENERGY 37
stituents in cells are nearly always neutral salts, but in the one set
the coloured ion is a complex organic acid combined with an
inorganic cation, usually sodium ; in the other set, the cation is the
coloured one, and is combined with an inorganic acid, usually
hydrochloric or sulphuric. The former set is often called that of
the " acidic " dyes, the latter, " basic " dyes, but such names are
clearly misleading, in that they suggest that the dyes themselves
have the properties of acids or bases. It was supposed at one time
that chemical combination occurred between particular constituents
of the cell and dyes of a definite chemical composition, so that the
staining of some particular structure indicated that it had some
particular chemical composition. Although this seems to be the
case in some rare instances, further investigation has shown that
a great variety of physical conditions also play a part, and that a
conclusion of the kind referred to cannot be drawn without other
evidence. Some points that are instructive may be mentioned
here. The ordinary form of adsorption must play a part, but there
are also those phenomena in which electrical forces come into
action, and sometimes in a rather complex fashion. Most of the
surfaces in cells have negative charges, and in order to see how
they behave to various dyes, some experiments with filter paper
should be made ; since this has a negative charge in water, the
conditions in general can be readily controlled (E., p. 179). The
results obtained apply, naturally, with the appropriate change of
sign, also to surfaces having a positive charge.
Let us take pure white paper and stain some pieces of it in
crystal violet, a "basic" dye, and others in congo red, an "acidic"
dye. The former will rapidly become deeply stained, the latter
very faintly. The explanation is, no doubt, that in the first
case the ion which stains the paper is the electro-positive one, and
is attracted ; in the latter case, the coloured ion is electro-negative,
and is repelled by the paper. What Staiffing occurs in the case of
congo red is the mechanical adsorption due to direct effect on the
surface tension. That this is so is shown by the curious fact that
the coloured matter deposited in the case of <k basic " dyes is the
free base, whereas in the other case it is the neutral salt itself.
This is the reason why, in our previous experiment with charcoal
(E., p. 1 68), we used acidified alcohol to remove the dye from
the surface. Next, add a neutral salt, say sodium chloride, to both
the stains, and repeat the above experiments with filter paper. It
will be found that congo red stains very deeply, while crystal violet
stains less deeply than in the pure state. Why is this? The
negative charge on the paper -is neutralised, or changed to a positive
one, by adsorption of sodium ions from the solution, so that the
attractive and repulsive powers of the paper towards the two
38 INTRODUCTION TO GENERAL PHYSIOLOGY
»
opposite signs of coloured ions are reversed. This is the explanation
of the effect of electrolytes in certain histological staining reactions.
The phenomena in general are spoken of as " electrical adsorption"
a term which also includes the adsorption of the precipitating ion
in the ordinary process of precipitation of colloidal solutions by
electrolytes.
Hydrolytic Dissociation
There is a form of dissociation to be met with in salts of weak
acids or weak bases which is not of an electrical nature, and, to a
certain extent, antagonistic to electrolytic dissociation. This is
known as " hydrolysis," or, better, " hyclrolytic dissociation," because
it is brought about by interaction with the hydrogen and OH ions
of water.
We have already seen that the distinction between weak and
strong acids or bases is that the former are only slightly dis-
sociated electrolytically, unless very strongly diluted. If we take
a salt of such a weak base, say ammonia, with a strong acid, such
as hydrochloric acid, or of a strong base, such as sodium hydroxide,
with a weak acid, such as acetic acid, or of a weak base with a weak
acid, such a salt as ammonium acetate, and dissolve in water, we
find that the solution is not neutral in reaction. There is evidence
of the presence of free acid and free base. The reaction will
be either acid or alkaline, according to which is the stronger.
Ammonium chloride is acid, sodium acetate is alkaline. Further
details of the process will be found in the larger works (p.,
p. 196).
Although the fact has sometimes to be reckoned with, it is not
to be supposed that it is usually of any great magnitude, unless
both the acid and the base are extremely weak ; and in such cases
the question naturally arises as to whether we are justified in
speaking of their being in combination at all. It is usually not
more than I to 2 per cent, and is sometimes absent when it might
have been expected to be present.
The nature of the phenomenon may be realised somewhat in
the following way. If we take a. solution of sodium acetate and
suppose that electrolytic dissociation occurs in the usual way, it
would be almost completely dissociated into sodium ions and
acetic anions. Now, although the former can exist in high con-
centration in water, the latter cannot, since acetic acid is but little
dissociated. The acetic anion accordingly combines with hydrogen
ions from the water, forming undissociated acetic acid ; more
hydrogen ions are set free from the water until the normal
proportion of dissociated and undissociated acetic acid is present.
LIFE AND ENERGY 39
The result is that an excess of OH ions remains, giving an alkaline
reaction to the solution.
The adsorption of the free base of " basic " dyes is connected
with the hydrolytic dissociation of these salts, since the base is
a weak one. Being insoluble in water, it forms a colloidal solution
therein, and, owing to its giving off OH ions, becomes electro-
positive and powerfully attracted by a negative surface. Most of
the " acidic " dyes are salts of fairly strong acids (sulphonic acids),
and are very little, if at all, hydrolysed in solution.
The Nucleus of the Cell
The presence of a special component in the more highly
developed cells has been mentioned. Most of the facts concerning
the nature of protoplasm, given in the preceding pages, apply also
to the nucleus, but it has functions peculiar to itself. As yet very
little can be said about how these are performed. We know that
if a cell, such as an amceba, is divided so that one part only retains
the nucleus, this part will continue to live, while the other part
will, sooner or later, die and disintegrate.
When nucleated cells multiply by subdivision, the nucleus
usually undergoes a complicated process of activity, to which
further attention will be given in the last chapter. It has been
supposed, also, to be concerned with the formation of certain
structures which appear in the cell, but it has to be confessed that
we are still very much in the dark as to its mode of operation.
Mitochondria
The same statement of uncertainty must be made with respect
to those bodies or granules in the cell protoplasm, to which the
above name has been given. They have various shapes, and have
been seen in living cells, where they appear to undergo changes in
the course of the activity of the cell. They have a special attraction
or affinity of some kind for a particular group of dyes, known as
the derivatives of di-ethyl-safranin.
The subject matter of the preceding chapter is undoubtedly a
difficult one, but a comprehension of it is necessary before we can
proceed further with profit. The student is recommended to refer
back to it from time to time as he meets with phenomena, which
require a knowlege of the particular facts referred to here if they
are to be understood.
CHAPTER II
FOOD-DIGESTION AND RESPIRATION
THE first question that occurs to us in this connection is — Why
do living beings require to take food, that is, some material
from the outer world which supplies something that they are in
need of?
When such a creature is actually engaged in making greater
the amount of substance in its body, is growing, as we say,
it is quite clear that this extra substance must be obtained from
outside, and what is taken in this way must contain the correct
chemical constituents that are wanted to make up the new body
tissues.
Further, even in the adult, when growth has ceased, there is a
certain loss of material, due to wear and tear in the process of
activity, as well as the growth of some parts, such as hair, which
continues to take place. As a motor car uses up tyres, piston
rings, bearings, etc., so the cell machinery requires replacement
of parts worn out. This is sometimes called maintenance^ and
has practically the same requirements as growth. But not
altogether, since there is evidence that some parts, once con-
structed, never require replacement, somewhat like the fly-wheel
of a petrol motor, which lasts as long as the engine itself, apart
from accidents.
The amount of food needed by the adult for the purpose of
replacing wear and tear is very small. It might be expected, for
example, that the structure of muscle would be worn away to some
perceptible degree by vigorous exercise. It is a rather remarkable
fact that it has been found impossible to obtain evidence of any
loss of the actual muscular structure itself, except after such severe
work as to be abnormal. There is more evidence of wear and tear
in some peculiar forms of muscular work, as we shall see later. It
might, perhaps, be said that in ordinary muscular work the products
of the wear and tear are used up again to repair the machine.
This may be so, but, as far as the necessity of supply from the
outside is concerned, the result is the same.
40
FOOD— DIGESTION AND RESPIRATION 41
Nevertheless, as every one knows, a fairly large amount of
food is necessary to the adult, especially if he is doing hard work.
The last remark indicates the purpose of this food. In fact, by
far the greater part of the food taken, both by the adult and the
growing organism, is for the purpose suggested, not for growth
or maintenance. To do things, to cause changes, requires energy ;
and, when energy has been used, it must be replaced if more
work is to be done. This is the chief function of food.
We will discuss the two uses of food in turn, taking first that
for growth and maintenance.
Since the object of this is to make new substance or to replace
what has been lost, it is clearly necessary to know what is the
chemical composition of protoplasm, and of the various structures
made by it. We may, indeed, to begin with, take the general
composition of the organism as a whole. We find that it is
composed of organic and inorganic substances (E., p. 181). The
latter are not present in very large amount, but are of some
variety, and of great importance. Organic compounds, as the
student will not need to be reminded, are the compounds of carbon.
Of these there are an enormous number known, and a very large
number are produced by living beings. The other chemical
elements making up these latter carbon compounds are nitrogen,
hydrogen, oxygen, and to a less extent, sulphur and phosphorus.
Iron and magnesium are found in two very special compounds, as
we shall see presently.
The four elements, oxygen, hydrogen, sulphur, and phosphorus,
are obtained in the course of taking as food those substances which
are necessary as sources of carbon and nitrogen, since animals
cannot build up their structure from elementary carbon, and
nitrogen. Plants cannot utilise the former, but a few exceptional
micro-organisms can take nitrogen from the atmosphere and form
compounds useful for food to higher organisms.
Source of Carbon— the Sugars
While there is a large variety of organic compounds which
serve the animal for this purpose, it has been found that none
simpler than the sugar called glucose is of use. Green plants, on
the other hand, are able, by making use of the sun's energy, to
produce glucose for themselves from the carbon dioxide of the
atmosphere. Our study of energetics has shown us that, since
carbon dioxide cannot be further burned up, it must be converted
into a compound that can be so oxidised, if it is to serve as a
source of energy. It must have energy supplied to it for this
42 INTRODUCTION TO GENERAL PHYSIOLOGY
purpose, and the source of this supply is the sun, whose rays are
absorbed by the green pigment of those plants which possess it.
Other plants, fungi, require sugar or similar substance to be
supplied ready made, although a few of them are satisfied with
somewhat simpler carbon compounds, so long as these have a
higher chemical potential energy than carbon dioxide has.
Before we proceed further it will be useful to remind ourselves
of the reasons why carbon forms such an enormous variety of
different compounds, and is, therefore, particularly fitted to be the
basis of the chemical changes taking place in living organisms.
In the first place, owing to its possession of four valencies, it is
able to form a great variety of derivatives of any one compound,
one valency combining with a group of one kind, another with a
different one, and so on. Secondly, the power that carbon atoms
have of combining with one another, gives the possibility of great
complexity and size of compounds. Thirdly, carbon is able to
combine with elements of opposite characters, owing to its position
in the middle of the periodic table. Thus, it can combine with
hydrogen or oxygen, with nitrogen or chlorine. It can, therefore,
be alternately oxidised and reduced, thus acting as a carrier of
energy. The reduced compounds give off energy when oxidised
or burned, while the oxidised compounds require the addition of
energy in order to reduce them. Fourthly, it alters its character
according to the groups with which it is combined. Thus, while
NO2— C=H2 is usually "negative," that is, has special affinity for
elements like hydrogen, CH3 is positive, like hydrogen, and has
affinities similar to those which hydrogen has. Fifthly, carbon com-
pounds react slowly, or are comparatively stable. Reactions which
proceed of themselves with explosive rate are incompatible with
vital phenomena. H2SO3 (sulphurous acid) is much more reactive
than HCH3SO3 (methyl-sulphonic acid). On the other hand,
this same property enables large molecules of high potential
energy to be built up, which remain stable when left alone, but
decompose with great violence when the powerful shock of a
detonator acts upon them.
Returning now to glucose, we note that there is a class of
compounds containing carbon atoms and water molecules in an
equal number. For this reason they are called carbohydrates. The
actual numbers of the carbon atoms vary from one to six, or more.
The most important ones, from our present point of view, are those
of six atoms. They are the sugars called hexoses. The five carbon
sugars, or pentoses, are of frequent occurrence in plants, and form
an important, although not large, constituent of certain com-
pounds in the nucleus of the animal or plant cell. It appears,
FOOD— DIGESTION AND RESPIRATION 43
however, that the pentoses are formed indirectly from the
hexoses.
The simplest " carbohydrate " is naturally that with one carbon
atom ; what is this ? If we try to represent in a formula a com-
pound of C and H.,O, we find that it must be the following :
H
I,
H— C=O, which is known as formaldehyde. Looked at from
another point of view, it is an aldehyde group (CHO) combined
with H. It is the starting point of a number of hexoses, including
glucose, which have the properties of aldehydes, and are hence
called " aldoses" Adding further carbons and waters so as to form
a chain, they must go between the carbon and one of the hydrogens
of formaldehyde, and consist of a series of carbons united to H on
the one side, and to OH on the other. Thus : —
HCO
!
HCOH
HO
HCOH
:OH
HCOH
HCOH
H
Note that there is now an aldehyde group at one end, and an
alcohol group at the other end. Aldehydes have powerful reducing
properties, taking up oxygen to become acids, so that the CHO
group becomes COOH. This latter group is known as " carboxyl,"
and confers acidic nature on the compounds in which it is
present.
Looking at the general formula given above, we see that one
or more of the Hs or OHs may be changed from one side to the
other of the central line. Thus a number of different sugars are
possible, many of which are known, although of the aldoses only
glucose, galactose (in milk sugar), and mannose (a rare hexose)
are of use to the organism. The capacity of dealing with the
others is absent. Also, to avoid error, it should be pointed out that
there is reason to believe that the simple chain formula, as given,
is that produced by the action of reagents, and that the usual state
of these sugars is in that of a closed ring by union of the aldehyde
carbon to the oxygen of the fourth carbon below, the hydrogen
44 INTRODUCTION TO GENERAL PHYSIOLOGY
of this hydroxyl then becoming transferred to the aldehyde group.
Thus :—
H C^ OH
H C OFT
I >0
H C OH
I
H C
H C OH
H C OH
H
Changes of this kind are common in organic chemistry. If we
designate the neighbouring carbon to the aldehyde by the prefix a,
that one united to the aldehyde in our formula will be y, and the
compound is a y lactone, being of the nature of an internal
anhydride.
Next, if we transfer the two hydrogens from the a-carbon to the
aldehyde group, we have another kind of sugar which has an
alcohol group at both ends and a CO next to it at one end. Thus : —
H
HCOH
CO
HCOH
HCOH
HCOH
HCOH
H
CO is the characteristic group of the ketones, and the sugar in
question is called a " ketose." It is known as " fructose," and
exists, combined with glucose, in cane sugar.
The student should never forget that the representation of
chemical compounds in the way that we have done is a conventional
diagram of the facts shown by the properties of these compounds
with regard to the particular constituent atoms which are united to
each other. For one thing, we are compelled to write them on a
plane surface, whereas, of course, they are solids, with three
dimensions in space. The " bonds," again, are not hooks, or
similar rigid attachments, but forces, probably of electrical nature.
Further, there is every reason to believe that an element in a
particular kind of combination is not the same thing as it is in
another kind of combination, or when free, although the change from
FOOD— DIGESTION AND RESPIRATION 45
one state to the other may be brought about more or less readily.
The energy obtained when a compound of carbon with hydro-
gen is oxidised to C(X and H.,O must arise from changes in
the internal structure of the atoms, as already pointed out.
We have just seen that O=C— H and O=C— OH have quite
different properties, although both contain O— C— . The carbon
is united to H in the one case, to OH in the other. The energy
content of the former is greater than that of the latter.
Source of Nitrogen — the Amino-Acids
We may now pass on to the consideration of the simplest
compound of nitrogen that will serve for animal nutrition, leaving
the case of the plant to be dealt with later. The animal organism
cannot utilise any compounds of nitrogen simpler than those
known as amino-acids. What is the chemical nature of these
substances ?
The carboxyl group, COOH, is a characteristic of acids.
Although its place can be taken by sulphur or phosphorus
derivatives of similar nature, the most important organic acids
concerned with the functions of living cells are the carboxylic acids.
The free bond of the carboxyl group must, of course, be united
with some other molecular group, and the most obvious to begin
with is hydrogen ; thus we get formic acid, H — COOH. Many
series of compounds of ascending degree of complexity and size
are formed by the successive addition of CH2, one of the free bonds
of the carbon being used to join on to the original compound,
while the other serves to attach further groups. Adding CH., then
to formic acid, we get acetic acid, H— CH2— COOH. Continuing
the process, we have the numerous straight chain fatty acids :
H— (CH,X— COOH.
One of the commonest and most important compounds of
nitrogen is ammonia (NH3), so that it is not surprising to find that
its derivatives form the basis of the source of nitrogen for living
matter. It is joined on to an acid, such as acetic acid, in the form
NH2 — , taking the place of one of the hydrogen atoms combined
with a carbon other than that of the carboxyl :—
H
H— C— COOH
I
NH,
which is amino-acetic acid, or glycine.
46 INTRODUCTION TO GENERAL PHYSIOLOGY
At this point we may note that the only set of amino-acids of
use to protoplasm is that in which the NH2 is attached in the
a-position as regards the carboxyl. Thus amino-butyric acid
might be —
CH3— CH2-CH(NH2)— COOH or CH3CH(NH2)CH2COOH
but it is only the first that is of value. The nitrogenous constituents
of the cell structures are all of this a-series.
Practically all organic acids — and some of them have very
complex structure — can form amino-derivatives. In some com-
pounds we find that another hydrogen atom has been lost from
ammonia, and we have the group NH=, the bivalent imino-
group.
When we try to feed an animal on amino-acids only, as source
of nitrogen, we find that it can be done if we take a sufficient variety,
but that one kind alone is insufficient. There are some particular
ones that are necessary, because the animal cannot make them out
of the appropriate fatty acid and ammonia as it is able to do in
the cases of others. The complex one known as tryptophane, in
which a ring containing nitrogen is united with amino-propionic
acid, or alanine, is one of these. If we set about making an electric
motor, we discover that certain parts of it must be made of
substances with definite properties, different from those of other
parts. While the wire must be a conductor, the segments of the
commutator must be separated from one another by a material
which is an insulator, and any conductor would be useless for this
purpose. It is possible also that some special chemical groups may
be required for the manufacture of substances of importance as
regards their action on protoplasmic processes, not as actual
components of the machinery. For the lubrication of the bearings
of our motor some oil is wanted, and water would not suffice.
With regard to the general properties of amino-acids, it is to be
noted that, while the carboxyl group confers those of an acid, the
NH2 group is basic. Thus, these acids are what are called
"amphoteric," being both acids and bases. But it must be
remembered that the acidic and basic properties are potential only.
Amino acids in which the two characters are almost balanced, as
when there is one acidic and one basic group, are unable to combine
with neutral salts, nor even with weak acids or bases. The
probable explanation of this behaviour is that such acids exist,
even in solution, in a closed ring form. Thus glycine : —
H.,C— COOH
"I
H2N
POOD— DIGESTION AND RESPIRATION 47
becomes, by formation of an internal salt : —
:oo
Before they can unite with acids or bases, they must be converted
into the hydrolysed form. This can be done by strong acids and
bases only, not by weak ones, nor by neutral salts.
There are some amino-acids which possess two carboxyls and
one NH2, while others have two basic groups to one acidic group.
The former, of course, are much stronger acids than those in which
the two functions are nearly balanced, while the latter are strong
bases. Both of these classes are good conductors of electricity,
whereas the mono-amino-mono-carboxylic acids are scarcely con-
ductors at all, being electrolytically dissociated only to a minute
degree.
Although a sufficient variety of amino-acids, as said, suffice as
nitrogen supply to an animal, it is not in this separated form that
we take them in our food. In fact, if we wanted them so, we should
have to make them from the materials which we actually use, and
with great difficulty and expense. These materials are the "proteins"
of which there are a great variety, differing in the particular amino-
acids they contain and in the number of these combined together.
This number is always a large one, although there may be several
molecules of one kind of acid. Familiar examples are white of egg
and the lean of meat.
The way in which amino-acids are combined together is by the
union of the amino-group of one acid with the carboxyl group of
another, water being eliminated in the way that is so common in
organic chemistry. The head of one molecule joins on to the tail
of another, as it were : —
HNH
I
C— C— (OH H)NH
II II I
H., O C— C— OH
II II
H2 O
which represents the production of what is called a "dipeptide,"
namely, glycyl-glycine. The union of — OC — and — NH — to
— OC — NH — is known as the " peptide linkage." By continuing
the process, more and more acids can be united, forming "poly-
peptides," and ultimately proteins.
Considering the large dimensions of their molecules, we naturally
expect proteins to behave as colloids. They have, in fact, the
48 INTRODUCTION TO GENERAL PHYSIOLOGY
properties of the emulsoids. One of these, as we saw, is that of
taking up more or less water, according to surrounding conditions :
an important fact. They have also the properties due to the
possession of surface, as well as those more definitely due to their
chemical composition. Of these we may note that, owing to
terminal free NH2 and COOH groups, as well as those of some
amino-acids attached as side branches, they act either as acids or
bases towards strong bases and acids respectively.
Optical Activity
We have seen that, when there are alternatives in chemical
compounds of the same general structure, such as the a- and other
series of amino-acids, the living organism has been evolved in such a
way as to be able to make use of one kind only. This applies to the
proteins and to the carbohydrates. Of the eight possible forms of
the aldo-hexoses, only three are utilised, namely, glucose, galactose,
and mannose.
There is, moreover, in addition to this exclusiveness, a further
one to which we must give a little attention.
If we write the formula of methane thus :—
H
H— C— H
H
we see that the carbon atom is symmetrical on all sides. On the
other hand, writing alanine thus : —
CH,
H,N— C— H
COOH
the central carbon atom is obviously differently weighted on all
sides. By representing such an arrangement in space (p., p. 282),
it can be seen that by interchanging positions of two of the groups
a compound is obtained which is different in space arrangement,
and cannot by any turning about be changed into the first one. It
is, in fact, the image of it as seen in a mirror. All compounds
which contain asymmetrical carbon atoms, that is, attached to four
different groups, show the same characteristic, which is, indeed,
a geometrical necessity. There are then two " isomers " of each
of these compounds. How can we distinguish them ? It is by
FOOD— DIGESTION AND RESPIRATION 49
their behaviour to polarised light. A beam of ordinary light
consists of a number of ether elements vibrating in all possible
directions at right angles to the direction of the beam. If looked
at endwise, these vibrations fill up, as it were, the whole cross
section of the beam, in all directions across it. There are, however,
certain crystals which, owing to their structure, only allow vibra-
tions of one particular direction to pass through, the others being
blocked out or absorbed. The same thing can also be done by
reflection from glass at a particular angle, in which case all the
vibrations except those of one direction pass through ; those of
the particular direction are reflected. The beam is then said
to be " polarised," because it has properties different in one
direction from those in another direction. Suppose such a beam
to be sent through a compound which contains asymmetric carbon
atoms. Owing to their being different in one direction from
that in another, such atoms will turn the plane in which the
polarised light is vibrating through a certain angle. They are
said to rotate the plane of polarised light, and to be "optically
active." Now, according to the side of the carbon atom which is
the more heavily weighted, the plane of the polarised light
will be turned either to the right or to the left. Hence the two
kinds of " optical isomers," as we may call them now, can be
distinguished. That particular form of glucose, which is the only
one utilised by the organism, rotates to the right ; that form of
alanine used rotates to the left. The means used for the detection
and measurement of the degree of this rotation of polarised light
is the instrument called the " polarimeter " (E., p. 181). The prin-
ciple of it is this : light sent through the instrument is first
polarised by a prism of Iceland spar cut in a particular way. At
the eye end there is another similar prism which can be rotated,
and the angle of rotation measured. If the plane of vibration
of the light passed by the polarising prism is the same as that
passed by the second prism (" analyser "), the light reaches the eye.
If not, there is darkness. When a solution of an optically active
substance is placed between the prisms, the analysing prism
requires rotation in order to correspond with the plane of vibration
of the light which has been rotated by passing through the
solution. In the actual instrument there is a device which
increases its sensibility, so that very small differences of rotation
can be measured accurately.
It appears that living organisms must have first made their
appearance under the influence of some asymmetrical forces, so
that they developed a bias towards one set of optical isomers.
Once established, this would tend to become more and more
exaggerated. The question is a difficult one, but it must not be
50 INTRODUCTION TO GENERAL PHYSIOLOGY
supposed that the production of optically active compounds is
confined to the living organism, as sometimes suggested. All
compounds with asymmetrical carbon atoms must be optically
active. The point is that in the laboratory the two oppositely
rotating isomers are nearly always formed in equal amount, so
that the actual rotation is zero. The living organism produces
one only, because the formation takes place by means of asymmetric
agents, which are themselves already optically active, since they
consist of one isomer only.
The Green Plant
Since animals cannot do with less complex sources of carbon
and nitrogen than glucose and amino-acids, we have next to
inquire where the supply comes from. They are only found in
nature in the bodies of animals and plants. These bodies, or
materials extracted from them, are taken as food by other animals.
After being used they are rejected in simpler forms, deprived of
energy, the carbon in great part as carbon dioxide (E., p. 182), the
nitrogen combined with part of the carbon mostly as urea (E., p. 182),
but sometimes in other more complex forms. None of these will
serve again as food, until they have been built up by the supply of
energy to more complex forms.
A word of explanation is needed as to urea. The whole of the
nitrogen contained in the protein food is not needed for repair
purposes, and urea is the way in which the waste ammonia groups
are got rid of by combination with carbon dioxide. Urea is
obtained from ammonium carbonate by removal of water, and can
easily be reconverted by hydrolysis. Thus :—
0-NH4 .NH9
C0< = C0< " + 2H00
XO— NH4 XNH2
Thus urea is the diamide of carbonic acid.
There is, then, a continuous using up of available carbon by
animals, and the same is true for plants, with the exception of
certain special structures in the green plants. It is only by the
aid of these that the life of both animals and plants on the earth
is preserved from final extinction.
In the oxidation of food, not only are useful carbon compounds
used up, but the oxygen of the atmosphere also. We have now to
learn something about the wonderful mechanism by which they
are both restored in the course of the same reaction. This is
probably the most interesting mechanism that exists, as well as
being that on which the continued existence of life on the earth
depends.
FOOD—DIGESTION AND RESPIRATION 51
We have, first of all, to convince ourselves that a green plant
is able to make use, in some way, of the carbon dioxide in the
air as a source of carbon to build up the complex compounds of
its own structure. These compounds afterwards serve as carbon
food for animals.
If we grow a plant from the seed in such a way that it can get
no carbon except from the atmosphere, we find, nevertheless, that
its bulk increases far more than would be possible by the use
only of the material originally present in the seed (E., p. 183). It
must have obtained its carbon from the atmosphere.
Another experiment which should be made is the following
(E., p. 183): Fill a vessel, under which a green plant is growing,
with expired air from the lungs. Take a sample of the air at
once and determine the percentage of oxygen and of carbon
dioxide in it by gas analysis The oxygen will be low, the carbon
dioxide high, as compared wit?h atmospheric air. Expose to
sunlight for a day or two. Determine the composition of the
gas again. The oxygen will have increased, the carbon dioxide
decreased. Therefore, oxygen has been produced from carbon
dioxide. If the experiment be done in the dark, this will not
happen. In fact, if the gas analysis is very accurate, the opposite
will be seen to have occurred, namely, an increase of carbon
dioxide and a decrease of oxygen, just as in animals. Moreover,
if a colourless plant, such as a mushroom or other fungus, be used
instead of the green plant, there will be increase of carbon dioxide
and decrease of oxygen even in the light.
Two things, therefore, are required — light and the pigment
that gives the green colour to plants. It is clear that the energy
used in the process has come from light, and that the means by
which it is utilised is the green pigment, called '• chlorophyll" or
" leaf-green." This substance has several remarkable properties,
but that which concerns us most is its relation to light-energy.
The enormous quantity of radiant energy that we receive from
the sun is transmitted in the form of transverse vibrations in the
ether of space. The rate of these vibrations is of a wide range.
The wave-length depends on the rate, when the velocity of propa-
gation is the same, being the distance one wave has travelled
before the next one follows it. The lowest rates, or longest wave-
lengths, are only perceived by us as heat when they strike on the
skin. Those of a certain medium wave-length are perceived by
the eye as light, differing in colour according to their wave-length.
The most rapid vibrations, or shortest wave-length, the ultra-violet,
are not perceived directly at all, but are capable of causing chemical
actions of various kinds to occur. Those rays, which we call
" light " can also produce chemical changes when they fall upon
52 INTRODUCTION TO GENERAL PHYSIOLOGY
an appropriate system. It will be perfectly obvious that none of
these rays can produce any change in bodies on which they fall
unless they are absorbed. To produce change requires energy,
and if the energy of the light is as great after passing through a
body as before it impinges upon it, no energy has been given
up to that body, and no effect produced in it, All rays from the
sun can be converted into heat when absorbed, and their energy
measured in this way. But, as we have seen, if we want the most
efficient conversion of their energy into other forms, such as
chemical energy, it must take place without previously passing
through the stage of heat. This is ensured by the aid of certain
coloured substances which absorb the energy of light, and enable
it to effect chemical changes directly. These substances are
sometimes called "optical sensitizers," because they make it
possible for a chemical system to absorb light of a wave-length
which it would otherwise be unable to do. A familiar instance
is the dye with which " red-sensitive " photographic plates are
stained.
If we take a coloured solution, say one of a green tint, and look
through it at a white surface, we realise that the light which reaches
the eye must be that which has not been absorbed. A green
solution absorbs the light of both ends of the spectrum, leaving the
green part in the middle. If we next examine a solution of
chlorophyll with a spectroscope (E., p. 186), we find that there is a
particular region in the red in which the light is greatly absorbed,
showing a dark band in dilutions such that very little absorption is
shown elsewhere. A spectroscope is an instrument which sorts out
the mixed wave-lengths of white light in series, according to their
wave-length. This it does by means of a prism, or other device,
which deflects the rays from their straight course in proportion to
their wave-length on account of the fact that in passing through
the prism the red rays are turned aside less than the violet rays.
They are deflected less because the rate of propagation of light
waves in a dense medium like glass is lower than in air, and that
of rays of short wave-length is affected more than that of the
longer ones. So that when a wave-front strikes obliquely, more
effect is produced on the shorter waves.
The reason why a substance absorbs rays of a particular wave-
length is because the rate of vibration of certain of its molecular
constituents coincides with that of the light absorbed. The energy
of the light is thus transferred to the absorbing substance by what
is known as "resonance" This may be understood by taking a
pendulum at rest and giving a series of very slight blows. The
first of these will produce a very small movement of the pendulum,
which will swing back beyond its resting position and then return
PQOD-DIGEST10N AND RESPIRATIOX 53
in the direction in which it was driven at first. If the second blow
arrives just at the time in which this last movement starts it will
increase it, and a repetition of the blows at the correct moments
will finally result in a vigorous vibration of the pendulum. We
have converted the energy of the blows into a movement of a large
mass. Unless the blows are timed to the natural rate of the
pendulum, some of them will push it in the wrong direction and
undo the work of the rest. The energy of blows so delivered,
instead of setting the system into its natural rate of vibration, will
be wasted as heat. Similarly, the light energy taken up by
resonance is converted into molecular movement of the natural
rate, and may so increase this movement that chemical change
occurs, and is thus used in chemical work without becoming heat.
What, then, is the reason why chlorophyll has this especially
great absorption in that part of the spectrum which we see as red ?
It has been found by measurement that the energy of the rays in
the solar spectrum is greatest about the yellow. This is due to the
fact that these measurements were made on a high sun in a clear
atmosphere. Since the atmospere absorbs rays of short wave-
length more than it does those of the longer wave-length, it is
possible that the position of the chlorophyll band may be in that
of maximum energy for the greater part of the day and the greater
part of the weather, especially in the higher latitudes.
So far we have seen what provision is made for absorbing
radiant energy, and we next inquire as to the chemical changes
which it causes to take place. We saw that the final result is that
carbon dioxide is decomposed and that oxygen is given off. Let
us take the carbon dioxide part of the problem first. It is easy to
show that starch is the final product (E., p. 186). Now starch is an
insoluble carbohydrate formed by the union of a large number of
glucose molecules by removal of the elements of water from two
neighbouring ones, in a similar way to the union of two amino-
acids described above. The advantage of its being insoluble will
be seen later, when we come to learn about enzymes. The forma-
tion of starch from glucose is not a result of the light, so that what
we have to account for is the production of glucose or a similar
hexose. From the composition of glucose we see that hydrogen
has to be introduced into the molecule of carbon dioxide to start
with. This, of course, comes from the water present. The final
net result is : —
6CO., + 6H2O = CoH^O,. + 6Oo
and we see that there is a large increase in chemical potential
energy, which has come from the sun. But this process must
consist of several stages. What is known about these? We
54 INTRODUCTION TO GENERAL PHYSIOLOGY
have seen that the simplest compound of the same percentage
composition as glucose is formaldehyde, CH2O, and it is natural
to imagine that this might be the first result of what is sometimes
called " photo-assimilation," the taking up of carbon by the agency
of light. Although there is much probability that this is the case,
from the general properties of formaldehyde and other evidence, it
has not yet been found possible to obtain really satisfactory proof
of its production in the green leaf. This may be due to the fact
that it is a powerful chemical agent, and injurious to living
protoplasm if present in any but minute quantities. For this
reason it would be rapidly converted into sugar. In the laboratory
it can easily be caused to unite, six molecules at a time, to form a
hexose. A further ground for the belief that formaldehyde is
formed by the action of light in the chlorophyll system is that
certain artificial systems, under the action of light, can produce it
from carbon dioxide and water. One of these is colloidal ferric
hydroxide under the action of ultra-violet light. This fact is of
further interest, because it suggests a possibility with regard to the
mechanism in the plant. To understand this we must consider
the form in which chlorophyll is present in the cell. It is insoluble
in water, and is therefore present in particles or colloidal solution,
and is not distributed generally throughout the cell substance, but
located in special structures usually, but not always, of spherical
shape (E., p. 1 86). These " chloroplasls " contain other things in
addition to chlorophyll, and of some of these it is interesting to
find that iron is a component. Although iron is very common
in cells, and has functions connected with oxidation, its presence
in the chloroplast is significant, and suggests that the function of
the chlorophyll itself may be to absorb light energy, bringing it
into intimate relationship with the chemical system, and that, after
absorption, the iron may come into play and cause the production
of formaldehyde, as in the ferric hydroxide above.
At the same time, chlorophyll itself has so peculiar a chemical
structure that it is difficult to believe that this does not, in some
way, play a part. The suggestion has been made that carbon
dioxide and water are taken into combination with the pigment,
then reduced to formaldehyde, which is given off.
. exjstsjhat chlorophylljtakes opjc.arbo«~4ioxide. By itself, even in
(the presence of light ancFof carbon dioxide, it does not produce
formaldehyde or sugar. It appears that the other parts of the
chloroplast are necessary.
Although the chemical nature of chlorophyll has not, as yet,
thrown much light on the photo -chemical reaction with which we
are concerned, this structure is in itself an interesting one. In
connection with the possible function of iron, it is remarkable that,
FOOD— DIGESTION AND RESPIRATION 55
although this element is not a part of the molecule of chlorophyll,
yet if a plant be grown from the seed in absence of iron, no green
pigment is developed until iron is supplied. Chlorophyll, or rather
the important green part of the molecule is, briefly, a number of
pyrrol derivatives, four to be exact, united by magnesium. Thus
the magnesium is in organic combination. On the other hand, the
red colouring matter of the blood, haemoglobin, is a similar pyrrol
derivative in which iron takes the place of magnesium. The
properties of haemoglobin are as remarkable in another way as
those of chlorophyll are, as we shall see presently. The structure
of pyrrol is that of a ring of four carbon atoms andjcme_ nitrogen
atom, each united to hydrogen, and is produced from proteins by
destructive distillation. In those derivatives which form chlorophyll
and haemoglobin, two of the hydrogens are replaced by methyl and
one by ethyl. The magnesium may be removed by the action of
acids, without destruction of the green colour, a fact which makes
its presence somewhat puzzling, if the function of chlorophyll is
merely that of absorbing light of a particular wave-length.
On the whole, it must be admitted that we know little about
the mechanism. The system is a very complex one, and photo-
chemical reactions, even of a simple kind, are still obscure in many
respects.
The way in which oxygen is produced is still more difficult to
explain. We can only point out how it might happen, on the
basis of certain facts which are known. It is not an uncommon
action of radiant energy to bring about the formation of peroxides,
raising the chemical potential of oxygen. Peroxides are oxides
containing more oxygen than the simple oxides. Thus, water being
H.,O, the peroxide of hydrogen is H.2O2. This may be represented
either as H— O— O— H, or if the rise in' potential of oxygen implies
H-0
its becoming quadrivalent, as ', In either case, the extra
H-,0.
oxygen atom is readily available for oxidising other substances, or
being set free from two molecules together, given off in the form of
gaseous molecular oxygen. If, in any way, peroxides wer$ pro-
duced in the leaf under the action of light, oxygen could be
obtained from them under appropriate conditions. There is
evidence that peroxides are formed in chlorophyll systems by light,
although it is not certain that they do not arise from destructive
oxidation of the pigment itself. Such organic peroxides give rise
to the production of hydrogen peroxide by interaction with water.
Further, there is an enzyme, called catalase, present in all green1
leaves, which decomposes hydrogen peroxide with the evolution
of gaseous oxygen. This is about as far as we can go at present.
56 INTRODUCTION TO GENERAL PHYSIOLOGY
We see, however, how dependent we are on the sun for our
continued existence, and more definitely how the "struggle for
existence " is one for the possession of the free energy of the sun's
rays.
The assimilation of carbon dioxide and the production of
oxygen is the great function of the enormous area of green leaves
that is to be seen on the earth. Since it is necessary for them to
receive as much light as possible, we see why they are in the form
of thin sheets, and why they spread themselves out in such a way
as to receive the maximum amount of light. Sometimes in the
tropics the sun's light is destructively brilliant, and the chloroplasts
take such positions as not to be subject to its full intensity.
The Cycle of Nitrogen
As already stated, waste nitrogen leaves the animal body
chiefly in the form of urea, a small amount as more complex
compounds. When death takes place, the proteins of the tissues
are broken up by the agency of those minute vegetable organisms
called "bacteria." In the case of plants, although their protoplasm
probably gives off some simple substance, such as urea, in the
course of its chemical changes this is very small, and the nitrogen
of the plant structures finally passes to the soil in the same way as
that of the animal body does.
What, then, are bacteria ? They are microscopic plant organ-
isms, devoid of chlorophyll, and exist in great variety of forms
and properties (E., p. 186). They are present all around us, to
some extent blown about in the air, but chiefly on the surfaces of
all kinds of materials and in the waters and soil. One of the most
striking things about them is that, although the number of distinct
shapes assumed is not great, the chemical activities they perform
are of an enormous variety. The chief forms are : small spheres
(micrococci), sometimes attached together in chains, bacteria (short
rods with rounded ends), bacilli (longer rods with flat ends), and
twisted rods of various lengths and closeness of coils (spirilla and
vibrigs). Some of these in certain stages move about by the
agency of threads which are contractile (flagella). Others are
frequently found in a resting state as spores, surrounded by a layer
of material which makes them very resistant to the action of heat.
The name " bacteria " is commonly used as a general name for the
whole group. The name " micro-organisms " includes also yeasts,
moulds, and small animal organisms, protozoa, of which amceba is
one. Protozoa are abundant in the soil and in stagnant water.
Those bacteria concerned in the destruction of animal and plant
remains are the cause of what is known as "putrefaction" the final
POOD— DIGESTION AND RESPIRATION 57
result of which, in the case of nitrogen compounds, is ammonia or
closely related compounds. Many of these micro-organisms are
responsible for certain diseases, owing to the production by them
of poisonous substances, " toxins" which have powerful actions on
physiological processes, differing according to the particular organ-
ism producing them. In order to avoid putrefactipn, those bacteria
already present must be killed and access of others prevented.
This is the process known as " sterilisation" familiar in the domestic
operation of bottling fruit. To avoid the access to wounds of those
organisms causing disease was the object of the " antiseptic method "
introduced by Lister. In his time the hospitals were swarming
with noxious organisms to such an extent that it was necessary to
dress wounds with chemical substances destructive of bacteria.
With the steady progress of general destruction, the use of anti-
septic chemicals has become less necessary, and the sterilisation of
the hands and instruments usually sufficient. But it must not be
forgotten that the principle on which Lister worked was the
exclusion of infection by any method whatever. The modern
" aseptic " method is merely one form of his treatment, made
possible by the previous antiseptic methods. It is clearly a difficult
matter to find an antiseptic chemical which kills bacteria without
injury to the delicate new tissues growing in a wound, although
some progress has been made in this direction. The need of it
has been made evident by the wounds of the late war, which
naturally became infected with all kinds of organisms.
There are then in the soil micro-organisms which convert the
remains of animals and plants into ammonia compounds. Urea
and other nitrogenous excreta are also converted into ammonia by
the same agency. Now, green plants and some fungi can make use
of ammonia as a source of nitrogen, but it is rather remarkable
that green plants do better with nitrates. Indeed they are said to
suffer from nitrogen starvation when ammonia is their only supply
of nitrogen. It would have been expected that this would more
readily yield the NH2 groups required for the production of amino-
acids and proteins.
A part of the ammonia is probably made use of by the plant,
but the greater part is oxidised in the soil to nitrates by certain
bacteria present therein. The first stage is the production of
nitrites by a particular group of organisms. Another group then
converts the nitrites into nitrates (E., p. 186). The green plant is
thus supplied with that form of nitrogen food which it can utilise
best. Animals then consume the plants as sources of protein, and
so the circle is completed. But not entirely, since during the
conversion of the residues to ammonia some of the nitrogen
is lost, apparently by the agency of oxidising bacteria, becoming
58 INTRODUCTION TO, GENERAL PHYSIOLOGY
atmospheric nitrogen. Moreover, there are in the soil what are
called " denitrifying " bacteria, which cause a loss in the nitrate
unless it is rapidly used up by the plant. This loss involves
return of nitrogen gas to the atmosphere.
Unless, therefore, there were some means of making use of
nitrogen from the atmosphere, there would be a continual loss
of nitrogen in the form in which alone it can serve as food for
plants and animals. The student is probably aware that there
are artificial processes by which the oxygen and nitrogen of the
atmosphere are made to combine to nitrous and nitric acids, and
others which combine nitrogen and hydrogen to form ammonia,
which is oxidised to nitric acid by a further process (p., p. 253).
But there is a natural process. There are bacteria in the soil which
are able to utilise nitrogen from the atmosphere to form the
material of their own bodies. When they die, this material serves
as a source of ammonia to the soil. The actual chemical reactions
by which nitrogen is made use of by these bacteria are not known,
but it is clear that a supply of energy is required. This is provided
for by oxidation of carbon compounds in the soil. Bacteria, with
similar powers, are present in the nodules on the roots of the
plants belonging to the order of the beans, clovers, etc., the
Leguminosae, and in rare instances in other orders (E., p. 187).
This last case is one of those known as "symbiosis? where
organisms join together for mutual assistance. The leguminous
plant supplies the bacteria with a carbon compound to oxidise, and
receives in return material which serves it as a source of nitrogen.
Readers, of the " Georgics " will remember that Vergil advises
farmers togrow vetches on their fields before sowing wheat Another
interesting case of symbiosis is that of a marine worm, in whose
tissues cells of an alga containing chlorophyll are present. The
animal's waste nitrogen serves for the plant cell, and this in turn,
by aid of its chlorophyll, supplies carbohydrate to the animal
(p., p. 295). We may learn a lesson from this. Much advantage
\is to be gained by mutual co-operation in making use of what
\s put at our disposal in the outer world. Waste of energy is
involved in contest for its possession. Claude Bernard, the great
French physiologist, has pointed out how much more inspiring
it is to regard living beings as adapting themselves to surrounding
conditions, rather than as being in perpetual conflict with them.
The life of an animal, as he says, is part of the total life of the
universe.
Sulphur and Phosphorus
The supply of these elements in organic combination is involved
in that of nitrogen, since some of the proteins contain, as parts
FOOD— DIGESTION AND RESPIRATION 59
of their molecules, compounds containing them, so that they will
be taken as food by the animal along with the other parts. The
higher plants are able to make these compounds for the animal
from inorganic salts, sulphates and phosphates in the soil. The
animal also, to some extent, uses inorganic compounds of sulphur
and phosphorus.
We may note again here that there are certain constituents in
protein food which cannot be made by the animal organism itself,
and must be supplied. They are probably required for the
replacement of particular parts of the cell machinery, although
it is also possible that important chemical products need certain
chemical groupings to be provided, the animal cell being unable
to make them.
Salts
Inorganic salts are found to be present in living cells, and in the
food taken by living organisms. But the question may be asked,
Are they necessary, or only present because food materials always
contain them ?
The relationship of colloids and salts, briefly discussed in the
preceding chapter, indicates that salts must play an important part
in the colloidal changes of the cell. It has been found by
experiment that certain inorganic elements are necessary, not only
for growth, but for the proper working of the activities of the
living cell.
The heart of a frog can be made to continue beating if supplied
with a solution of inorganic salts only. We have learned in our
study of the osmotic pressure of cell contents that such a solution
must possess a particular osmotic pressure, otherwise the tissue
cells either swell or shrink. We can give this value to our solution
by sugar or by sodium chloride. It is usual to do so with the
latter, because of its convenience ; but there is good evidence that
sodium chloride is somewhat toxic, and that a part of it may, with
advantage, be replaced by its osmotic equivalent in cane sugar.
Even if we do this, however, we find that we cannot keep the heart
beating normally for more than a short time (E., p. 187). We find
that both calcium and potassium are necessary in small amounts,
and that there is a certain proportion between the three cations
that gives the best results. It may have been noticed that no
mention was made of any particular salt of these metals ; in fact,
the anions may be of various kinds indifferently. This must not
be taken to imply that the anion plays no part, but rather that its
function is one common, more or less, to all anions, apparently due
to the sign of the electrical charge. Certain special properties,
on the other hand, are required in the cations. Calcium ions,
60 INTRODUCTION TO GENERAL PHYSIOLOGY
as we saw, are active in the way of aggregating or precipitating
colloids, but there are probably other properties to be taken into
account. Potassium appears to be of importance on account of its
radio-activity.
There is an interesting and suggestive fact about the salts
necessary in a solution to take the place of blood. Suppose that
we take sea water and dilute it so that its osmotic pressure is the
same as that of blood. We find that it serves excellently as an
artificial fluid, so far as the salts are concerned. Examining it
more closely, we notice that the proportion of sodium, potassium,
and calcium salts is practically the same as that found to be the
best in a mixture made for the purpose. Is this merely accidental ?
The blood, as we find, contains the salts of the ocean such as they
would be if sea water were less concentrated than it is now. But
we know that it has been, through geological ages, continually
increasing in salt content, because rivers are always adding salts
dissolved from the land by rain, whereas it is only water that
evaporates from the ocean. At some period, then, its composition
was similar to that of the present land vertebrates as regards
inorganic salts. When the ancestral vertebrates, which were formed
in the ocean, left it for the land, there is every reason to suppose
that the salt content of their blood would be the same as that of
the ocean, and that their cell mechanisms would have been adjusted
in relation to it. Hence it remained at this point. The geologists
tell us that this taking to life on land occurred about the end of the
Cambrian period. This period was one of great length, judging
by the thickness of the rocks ; so that ample time had passed for
the adjustment of the cell mechanisms to the composition of the
ocean. We may take it that the blood represents the salt content
of the ocean at the end of the Cambrian period. There is, however,
one point which requires some further explanation, namely, the
high content of the sea at the present time in magnesium salts,
which is out of proportion to the other constituents, as compared
with blood. There are reasons for believing that magnesium has
increased more than the other salts, but further discussion would
lead us too far (p., p. 210).
A further conclusion is suggested. Perhaps the salt content of
the cells, which is not identical with that of the blood, may
represent the composition of the ocean at a still earlier period.
But there are difficult questions involved here.
The variety of salts required for growth, at all events in the
case of plants, and as far as we know in that of animals also, is
greater than this. The ordinary mould, Aspergillus, requires for
its most rapid growth magnesium, potassium, zinc, and iron as
cations ; phosphate, sulphate, and silicate as anions. A sea weed
FOOD— DIGESTION AND RESPIRATION 61
was found to require at least sodium, potassium, calcium, and
magnesium.
Accessory Factors
We have seen that we must provide for a supply of carbon and
nitrogen in certain forms, and also salts. But there is something
else to be considered.
Suppose that we give to a growing animal a diet of pure protein,
pure fat and pure carbohydrate, together with salts, that is, all the
actual chemical compounds required and in sufficient amount, we
find that it does not grow. But if we add a very small quantity of
milk or of turnip juice it grows as well as on its normal food.
There is evidently something wanting in the pure materials, of
which only a small amount is needed, but which is, nevertheless,
indispensable.
We do not know yet what this " accessory factor" is. It has
been called " vitamine," owing to a mistaken view of its chemical
nature. There is ground for believing that there are several kinds,
because different diseases develop in the absence of particular
constituents present in some foods, not in others. For example,
ben-beri, after having made its appearance, can be cured by the
addition of a small quantity of the outer layer of rice, whereas
scurvy cannot be cured by this, but needs the juice of oranges or
other fresh vegetable. This latter fact was known to Captain Cook,
who discovered, in his second voyage round the world, that scurvy,
then so serious a difficulty in long voyages, could be prevented by
adding fresh vegetables to the preserved diet whenever a chance
presented itself. Some other diseases are turning out to be, in all
probability, " deficiency diseases."
Although there seems to be some variety in these factors, they
fall into two main groups, one soluble in fat (" fat-soluble A-factor "),
the other soluble in water (" water-soluble B-factor "). That con-
tained in butter is typical of the former, that in wheat-germ, of the
latter. Both are necessary for normal growth.
These factors are somewhat easily destroyed by cooking, es-
pecially if heated in alkaline solution, and by preservation with
salt. Hence the importance of fresh food, especially fruit and
vegetables. Fruit contains an unusually large amount of the
anti-scorbutic factor, which seems to belong to a special class.
Notwithstanding their great importance, very little is known
about the nature of these substances or the way in which they act.
They seem to behave like those agents called "catalysts," about
which we shall learn more presently. They are not subject to
chemical change in the course of their activity (P., pp. 258, etc.).
62 INTRODUCTION TO GENERAL PHYSIOLOGY
The Supply of Energy
Up to the present we have chiefly considered the things
necessary to make new structures or to replace those worn away.
Only incidentally have questions concerning energy been touched
upon.
It was stated above that, in the adult, there is very little loss in
the wear and tear of the cell machinery in its normal work. There
is some loss by destruction of cells on the surface of the body and
elsewhere.
The fact that the machinery does not wear away in its normal
function has been shown most clearly in the case of muscular
work. Since the cell structures contain nitrogen, if there were
disappearance of their material there would be found an increased
amount of nitrogenous compounds in the urine, since this is the
way in which they are got rid of. The most careful investigation
has shown that no increase is to be detected.
The necessity of taking more food than that indicated by such
considerations as those above is a matter of general experience, so
that there is another purpose which actually requires the provision
of something other than the material itself. As already pointed
out, this is energy, which has been lost in the performance of work,
and must be replaced.
The food we take is, chemically, of such a kind that by oxida-
tion, or burning with oxygen, energy is given off. If we burn
sugar or fat in the air (E., p. 189), we notice that heat is produced,
and if we collect the gas given off, we find that carbon dioxide and
water have been formed. We saw previously that these are also
produced by the living organism when it makes use of the same
substances. The chemical energy which appears as heat when the
substance is burned in air is therefore available for the needs of the
organism when similarly burned in its cells. But the mechanism
of the living cell is so arranged that it can seize upon this energy
before it has become degraded to heat, and so make more
economical use of it.
In the preceding chapter it was shown that any form of energy
can be converted into heat, and therefore measured in terms of heat
units. This is a matter of some convenience in respect of materials
used for food. We can burn them with oxygen and measure the
heat produced, thus obtaining their value as sources of energy, on
the assumption, of course, that they are such as the body is capable
of using in this way. Charcoal or coal gas are useless, because the
living cell does not possess the means of burning them. In the
case of carbohydrates and fats, the values are those actually
obtained by the organism, since these are completely oxidised by
FOOD— DIGESTION AND RESPIRATION 63
it. But the nitrogenous part of the protein used for energy purposes
is not completely oxidised. Urea, when burned with oxygen, gives
a certain, not very large, amount of energy, but it must be allowed
for in calculating the energy value of proteins.
The unit of heat in which the energy value of food is expressed
is the large calorie, in physiological discussion usually spoken of as
the calorie simply. This is the amount of heat required to raise
the temperature of one kilogram of water by one degree centigrade,
or more precisely, to raise its temperature from o° to 1°, since the
specific heat of water varies somewhat at different temperatures.
The performance of a given amount of work by our muscles
requires the provision of its equivalent quantity of energy in food.
This is obvious as regards work done on objects in the outer world.
But a living organism differs from a machine which consumes no
energy when at rest, in that the heart must go on, breathing must
go on, and some other functions require energy even when the
body appears to be at rest. It may rather be compared to such a
machine as a circular saw, which is kept running when not actually
occupied in sawing wood. A certain quantity of energy is used in
friction and in fanning the air, but the consumption is greatly
increased when a beam of wood is sawn.
The consumption of energy for the internal needs of the
organism is known as the " basal metabolism" or basal consumption.
This is the first time that we have used the word " metabolism,"
and it needs definition. It is simply a convenient word to express,
briefly, the series of chemical^cjianges undergone by chemical
compounds inTFie organism. ThuTthe metabolism of carbohydrate
means the various stages through which it passes before final
oxidation to carbon dioxide and water.
The actual amount of energy required for basal needs varies,
naturally, with the size of the organism. It is almost exactly one
calorie per hour for each kilogram weight in man when asleep ; so
that, for a man of average weight (70 kilos or 1 1 stone), it amounts
to 1700 calories per day. The amount required when external
work is done differs according to the amount of this work, but the
following table shows the approximate accepted values :—
Basal, in sleep - - 1,700 calories.
Do. awake, but at rest - 2,100 „
Sedentary occupation - - 2,500 „
Light work - 3,000 „
Moderate work - - 3,500 „
Heavy work - - 4,000 to 9,000 or more calories.
Carbohydrate alone might give the energy required ; but we
have seen that protein is necessary to afford nitrogen for replace-
ment of wear and tear, and since it also gives energy, a diet might
64 INTRODUCTION TO GENERAL PHYSIOLOGY
be composed of it alone. But it would be wasteful, because the
greater part of the nitrogenous component is excreted unused.
There is some popular misconception implied in the name some-
times given to proteins as being " flesh-formers," as distinguished
from carbohydrates, which are said to be "heat-givers." It is
unnecessary to say that protein must be supplied when new tissue
is being formed ; but this is a very different thing from the sugges-
tion that it will form flesh (f.e.t muscle) of itself alone. If the
muscles are exercised, they may increase in bulk, and to do this a
small amount of nitrogen is required. On the other hand, a food
giving heat is equivalent to saying that it gives energy in general,
and the name applies to protein as well as to non-nitrogenous food.
It was, indeed, supposed at one time that there was some special
value in protein as a source of energy, but exact observations have
been unable to confirm this view. The names " flesh-formers " and
" heat-givers " are quite unscientific, and do not correspond to any
real distinctions. They should be given up altogether.
Not very much has been said, as yet, with regard to the third
class of substances used for food. These are the fats. They con-
sist of carbon, hydrogen, and oxygen, like the carbohydrates, but
the hydrogen is present in larger proportion than required to com-
bine with the oxygen to form water. Accordingly they afford,
when equal weights are oxidised, more energy than carbohydrates
do. Otherwise there does not seem, so far as can be made out,
any physiological necessity for fat as there is for protein and
carbohydrate. There is, undoubtedly, a desire for it, but this may
be for reasons of making dishes attractive to the palate. It has
been found possible for strong, healthy men to live without fat for
two years. The presence of a particular accessory factor, the " fat-
soluble A," in some fats makes their use advisable, although the
factor is not confined to what are generally called fats.
As to the chemical nature of fats, they are what are known as
" esters" a large class of compounds in which an alcohol residue is
united with an acid residue. Alcohols are characterised by the
presence of a CH2OH group, united with carbon and hydrogen. If
the alcohol group is combined with hydrogen alone, we have methyl
alcohol, CH3OH ; adding CH2, we get ethyl alcohol, C2H5OH, and
so on up to a large number, when the compounds become solid.
The additions are not necessarily made in a way to form a straight
chain, hence we have different alcohols with the same number of
carbon atoms. The group CH2OH may be attached to other more
complex groups than in the fatty acid series above referred to, but
it is this series that interests us more especially here. There may
also be more than one alcohol group, as in glycerin (more correctly,
glycerol, since the termination ol has been agreed upon as that of
FOOD— DIGESTION AND RESPIRATION 65
an alcohol). Glycerol has three alcohol groups united together by
loss of hydrogen from the middle one : —
CH2OH
CHOH
CH2OH
The acid in an ester may- be either organic or inorganic, and the
combination takes place with the elimination of the elements of
water. Ethyl alcohol and acetic acid unite thus : —
C2H5(OH~~ ~H)COOCH3
forming what is often called ethyl acetate ; but since it is not a salt,
its correct name is acetic ethyl-ester. The acidic properties of the
acid have disappeared, although not by combination with a base.
Ordinary fats and oils are esters of glycerol with fatty acids of
a large number of carbon atoms. Thus, olive oil or oleiri is the
tri-glyceride of oleic acid, having three oleic acid residues united to
the alcohol. Oleic acid contains eighteen carbon atoms, while the
stearin of mutton fat is the corresponding ester of an acid also
containing eighteen carbon atoms. The well-known difference in
their properties is due to the smaller number of hydrogen atoms in
oleic acid, some of the carbon atoms being unsaturated. Reference
will be made to this again later.
There is an interesting group of complex fatty substances called
"lipines," which form important constituents of cells, although their
function is not altogether clear. One of their uses is doubtless to
take part in the production of the cell membrane, but there must
also be some more distinctively chemical part to be played by
them. Probably they take some share in the oxidative mechanisms.
Some of these contain phosphorus and nitrogen, such as lecithin,
which may be regarded as phosphoric acid combined with a fat
and an organic nitrogen base. Others are devoid of phosphorus,
but contain fatty acid, a nitrogen base and a sugar, and are found
chiefly in the brain.
The relative proportion of the three kinds of constituents of a
diet may be varied to a large extent without injury to the healthy
individual, so long as the energy value is not decreased. The
Royal Society Food Committee recommends the following as the
basis of calculation for the food supply of a nation :—
Protein 70 gm. = 280 calories
Fat - 90 „= Sio „
Carbohydrate - 550 „ =2,200 „
Total =3,290 calories
per day per 7o-kilo man, doing moderate work.
66 INTRODUCTION TO GENERAL PHYSIOLOGY
Tables have been constructed, giving the composition of various
articles of food and their calorie values.
Digestion
The fats, carbohydrates, and proteins contained in the various
articles of our diet are not in the form in which they can be made
use of by the living cells, whether for growth or energy purposes.
Carbohydrate must be in the form of glucose or fructose ; protein
in that of amino-acids. That in which fat is required to be is
unknown, but it cannot be absorbed in the form found in articles of
food. The necessary changes in all these cases are made in the
alimentary canal.
The primitive form of the alimentary canal is that of a tube
passing through the body, open to the exterior at both ends. At
the anterior end, the mouth, the food is taken in. In its passage
it is subjected in turn to the action of various fluids and the
products absorbed. Finally, the constituents which resist the
action of the digestive juices are expelled through the anus. The
processes to which the food is subjected are essentially the same
in all animals, so that we may take the arrangements present in
one of the higher vertebrates for description of the whole series of
events, which can be analysed in such a case much more accurately
than in small animals.
We need spend but little time on the mechanical disintegration
necessary in the case of certain materials, on account of their being
united together in more or less dense masses. This is done by
the teeth in mammals ; in birds which eat hard grains there is
a muscular organ, the gizzard, which contains small stones, serving
to grind up the food. In the mouth cavity the food is moistened
by a liquid, the saliva, which is poured in along tubes leading from
special organs which secrete it, as the process of its formation is
called. In some animals, including man, saliva contains an agent
which brings about the conversion of starch to sugar. This is the
first of a series of agents acting in a similar way as the food
passes along the alimentary canal. They are known as " enzymes"
and before we proceed further we must learn something about the
manner in which they act.
Enzymes
These substances are sometimes defined as the catalysts pro-
duced by living cells. But what are catalysts? In brief, we may
say that their action is to make chemical reactions proceed at
a faster rate than they naturally do, and that they do this without
FOOD— DIGESTION AND RESPIRATION 67
themselves suffering any permanent change. They reappear un-
altered when their work is finished.
That chemical reactions vary greatly in the rate at which
they proceed is familiar (E., p. 189). We have already noted that
comparative slowness of reaction is characteristic of carbon com-
pounds. In the processes of the living organism, it is of importance
that reactions should not go on at a perceptible rate except when
required. Hence the value of catalysts to quicken them up at
appropriate times. In the cell these catalysts, or enzymes, are
produced or brought into an active state as wanted.
When we say that what we have to do with is an acceleration
of chemical changes which proceed of themselves, although but
slowly, we must remember that this rate may be so slow as to
seem not to take place naturally at all. Since, as we stated above,
the enzyme appears at the end unaltered, it is obvious that it does
not give up energy to the reacting substances, and therefore that
the final result must be the same as it would have been without
the presence of the enzyme. There is a certain qualification, how-
ever, which must not be passed over, although it does not alter
the general principle. Owing to the fact that enzymes, as we
shall see, form a separate phase of the system in which they act,
and the conditions at boundary surfaces differ from those in
homogeneous systems, the final state of equilibrium arrived at is
not necessarily the same in both cases. The explanation of the
fact is still obscure, and need not detain us at the present stage.
An important consequence follows from such considerations,
and is found experimentally to be the case. An enzyme acts in
opposite ways, according to the state in which the reagents are
when subjected to its influence. This will be clear if we take
a special case. Fats, as we saw, are esters in their chemical nature,
and there are enzymes which bring about a splitting of esters in
general into their component acids and alcohols. This they do
by introducing water. Thus, taking ethyl acetate and putting
Et for ethyl (C2H5) and A for acetyl (CH3COO) we have :-
Et A + HO
Although this reaction only takes place rapidly under the
action of a catalyst, it proceeds at a detectable rate if allowed to
proceed by itself, but never completely. If we start with ethyl
acetate and water in the proportions in which they combine, that
is, if we take an equal multiple of the molecular weight of each,
we find that after some days we can detect the presence of acetic
acid (E., p. 190). After a long time it will be found that no
further change is going on, and that we have a state which we
call equilibrium, .By estimating the amounts of the four com-
68 INTRODUCTION TO GENERAL PHYSIOLOGY
poi>ents present, it is found that there are twice the number of
molecules of the ester and water that there are of alcohol and
acid. Now, suppose that we begin with alcohol and acid, we find
that they combine to form ester until a certain amount has been
produced. The reaction then stops, and on examination we find
that the proportion of the four components is the same as in the
former case. When the reaction is accelerated by a catalyst,
similar conditions hold, since no energy is supplied by it. There-
fore the same catalyst may appear to have either synthesising
or hydrolysing powers, according to the substances on which it
acts (E., p. 190).
The position at which no further change takes place is called
an equilibrium, or the reaction a balanced one, because what is
happening is that the two opposite reactions are both proceeding
at equal rates at this point.
Our problem is, then, one of velocity of reactions. The funda-
mental law concerned is that of mass action, which states that the
rate at which a particular reaction takes place is proportional to
the mass of the reagents present in a certain volume. It follows
that the relative amounts of the original reagents and of their
products present in the final equilibrium is also proportional to
their original masses. Thus, if we increase the mass of water in
the example given above, we correspondingly increase the rate of
the hydrolytic reaction, and since the rate is maintained greater,
relatively to the opposite one, also in the equilibrium position
itself, there will be present finally a larger proportion of acid and
alcohol than if there were less water present to start with. When
the proportion of water is great, the synthetic reaction may be so
slow that it is difficult to detect it, and the final result may appear
to be one of complete hydrolysis.
In stating the law of mass action correctly, it is necessary to
say that the rate of the reaction is proportional to the concentration
of the active masses of the reagents. The reason for inserting the
word "active" will be clear if we suppose that we are dealing
with a reaction brought about by hydrogen ions, and that the
acid used is a weak one. The active mass is not that of the
acid added, but that part of it which is electrolytically dis-
sociated.
The word "concentration," or mass in unit volume, is necessary,
as can be seen by realising the kinetics of the process. The
reaction takes place because, as the molecules shoot hither and
thither, some of them hit those with which they enter into com-
bination. It is only a certain number of these encounters that
actually result in combination, but it is obvious that the number
of effective meetings that take place in unit time, that is the rate
FOOD— DIGESTION AND RESPIRATION 69
of the reaction, is the greater the more molecules there are in the
way to be hit against.
We saw above that the essential property of a catalyst is that
of increasing the rate of a reaction. Their importance is therefore
greatest when the reaction is, by nature, a very slow one, and this
applies in a marked degree to those of living systems.
There are two ways in which catalysts act, according to whether
they are in true solution, forming homogeneous systems, or present f
in a distinct phase, as a solid or in colloidal solution, forming a
heterogeneous system. In the former case the effect is known
to be due sometimes to the formation, first of all, of a compound
between the catalyst and the components of the reacting system.
This compound is then decomposed rapidly with separation of the
products of the reaction and the reappearance of the catalyst itself.
The whole process takes place more quickly than the reaction
proceeds by itself. It must be admitted, however, that there are
cases of homogeneous catalysis to which this explanation is not
readily applied, and although the production of intermediate
compounds is not altogether excluded as a stage in the action of
enzymes, it does not concern us here, because our catalysts, the ,
enzymes, are in colloidal solution and act, in some way, at the I
surfaces of contact with the solution containing the reacting
substances. Can we form any conception of what happens ?
By the law of mass action, if we can increase the concentration
of the reagents, we shall increase the rate of the reaction. Further,
we have seen that if substances decrease surface energy, they are
concentrated by adsorption on the surface. What evidence is there
in the case of enzymes or other heterogeneous or insoluble catalysts
that adsorption plays a part ? Let us examine a fairly simple case,
that investigated by Faraday, where platinum surfaces were found
to bring about rapid combination between oxygen and hydrogen
gases. Strong evidence was brought in support of the view that
this was due to condensation of the gases on the surface of the
platinum, and in this way brought within range of the forces causing
their combination with each other (p., pp. 306 and 326). It was
shown by Faraday that the surface must be perfectly clean ; that
is, if another substance had already obtained possession, oxygen
and hydrogen were kept out to a great extent. Moreover, chemical
reaction with the platinum, forming intermediate compounds, was
excluded. No matter how the surface was cleaned, by mechanical
or by oxidising or reducing agents, the method was effective.
Chemical reaction between platinum and oxygen is also excluded
by the facts that nitrous oxide and hydrogen are caused to combine,
and that similar effects "are produced by most, if not all, solid
bodies."
70 INTRODUCTION TO GENERAL PHYSIOLOGY
Is there any evidence of similar action in the case of enzymes?
What evidence there is, is naturally more or less indirect ; but it is
in favour of a preliminary adsorption of the reacting substances
on the surface of the enzyme. It lies, in great part, in the facts
concerning the rate of the reaction in relation to the amount of
the enzyme present, and cannot be discussed here (P., p. 315).
There is also an interesting retarding effect of certain inert sub-
stances, such as saponin, whose action, as a whole, can only be
explained by its great lowering of surface energy, thus obtaining
possession of the enzyme surface, and displacing more or less the
other constituents of the system from that surface. This is similar
to the effect of impurities on Faraday's platinum, and "it has the
negative temperature coefficient characteristic of surface tension.
But there are a great number of enzymes known to us, each
of which causes acceleration in one particular kind of reaction only,
a fact at first sight difficult to reconcile with mere condensation
on a surface. It might be supposed that one enzyme only would
suffice for all purposes. At present, however, we have not sufficient
knowledge of the numerous properties of surfaces to be able to
exclude the probability that surfaces of different chemical structure
have different powers of adsorption with regard to the various
chemical natures of the components of the reactions which they
influence. It is, moreover, not improbable that the rate of a
reaction may be increased on a surface in a way additional to that
of mass action. In the act of condensation, molecular forces may
be brought into play, which raise the chemical potential of the
reacting substances ; in other words, increase their affinity for each
other.
On the whole, the following view is that which is most in
agreement with the facts. Owing to the nature of its surface,
each variety of enzyme has the property of causing condensation
thereon of the components of a particular system. This results
in a greatly increased rate of attainment of the natural equilibrium
that system ; whether merely by mass action or by increase of
chemical potential in addition, is as yet uncertain.
Changes in Carbohydrates
We may now return to the fate of starch after being subjected
to the action of an enzyme in the mouth, which brings about
its ultimate conversion to glucose (E., p. 193). The enzyme in
question is called ainylase, because it acts on starch (amylum).
This action, although a rapid one, has not time to be completed,
and does not progress further than the formation of some com-
pounds intermediate between starch and glucose, namely, dextrin,
FOOD— DIGESTION AND RESPIRATION 71
which is a condensation of several molecules of glucose, but less
than the number contained in the molecule of starch, together
with the sugar called maltose, consisting of two molecules of
glucose united by the elimination of H2O, and is therefore one of
the di-saccharides, or more correctly, bi-hexoses.
After the food is swallowed, it passes quickly down the
oesophagus, or gullet, and reaches the stomach. This is a large
bag in the course of the alimentary canal, and has muscular rings
at both ends, so that the food is not allowed to leave it until it
has been thoroughly exposed to the action of the digestive juice
secreted by the lining cells. This "gastric juice" is strongly acid,
and stops the further activity of amylase as soon as the mass in the
stomach has become penetrated by the acid. But this does not
take place very quickly, so that the salivary amylase continues its
action for some further time. The digestion of starch is, however,
not completed by the time that the food is passed on to the small
intestine, a long tube in which a variety of processes takes place.
Although the stomach is an important organ for the digestion of
proteins, as we shall see presently, the process is incomplete, even
in this case. In fact, there is no absorption into the blood of any
digestive products from the stomach. They are not yet in the
state required by the tissue cells.
Maltose, dextrin, and unaltered starch pass, then, into the
small intestine, and are next subjected to the action of the
pancreatic juice, a mixture of powerful enzymes, one of them
being an amylase which completes the digestion of starch and
dextrin, certainly as far as maltose and probably, to some extent,
as far as glucose. Maltose is acted on by an enzyme, maltase,
which completes the hydrolysis to glucose. This enzyme is
produced by the cells lining the intestine. Pancreatic juice is
a liquid formed by a secreting gland by the side of the intestine,
and poured into the intestine by a duct.
The glucose so formed passes by diffusion into the blood
vessels, with which the wall of the intestine is richly supplied. It
is carried by the blood to the liver, through which it passes before
reaching the rest of the body.
Since the whole of the glucose produced by the digestion of
a particular meal is not wanted for use at once, a part of it is
stored in the liver, and also in the muscles, in the form of an
insoluble starch-like substance called *jjjJKgg*n? The liver itself
contains an amylase, which changes the stored glycogen into sugar
when required. Moreover, what we have learned about the
reversible action of enzymes tells us that this same enzyme is
able to form glycogen from sugar, when the conditions are
favourable. Since the equilibrium position in such cases depends
72 INTRODUCTION TO GENERAL PHYSIOLOGY
upon the relative concentration of the reagents, it will readily be
seen that if one of them is insoluble, it is deposited out of solution
as fast as it is produced, and takes no further part in the equilibrium.
The result of this is that more of it continues to be formed in
order to supply that constituent of the system necessary for
equilibrium. In this way, even if the equilibrium position is such
that a very small amount of the synthetic product suffices to
maintain it, yet a notable quantity may be formed even in a
moderate time, if it is removed from the reacting system in any
way. Thus it may be removed by its insolubility, or by com-
bining with some other substance, or again by being carried away
by diffusion into the blood stream.
Similar considerations apply to the deposition of starch from
the sugar formed in the photo-chemical assimilation of carbon
in the leaf of the green plant, and to its removal to other parts
of the plant, stem, or root during darkness.
But the capacity of the liver and muscles to store glycogen
is somewhat limited, and there is another way in which sugar not
needed for use at once is stored. This is as fat. The fact is well
known to those who grow cattle or pigs for food, although the
chemical changes involved are unknown, except in a general way
(p., p. 278). A similar change occurs in plants, as shown by fatty
seeds, such as linseed.
Although we have hitherto spoken only of starch, it is familiar
to the reader that all our carbohydrate food is not in this form.
We take cane sugar, maltose, and lactose (milk sugar), which are
compounds of two molecules of simple hexoses. These must be
split by hydrolysis before they are of use to the cells. We have
seen that there is a maltase in the intestine, and appropriate
enzymes are also produced there for the other two sugars. Inver-
tase causes the rapid hydrolysis of cane sugar to glucose and
fructose ; lactase hydrolyses milk sugar to glucose and galactose.
The final destiny of all these sugars is to be burned up to give
energy. This takes place chiefly in the muscles, because, as we
shall see later, this is the situation where most supply of energy
is wanted. The machinery by which this combustion is effected
is not completely known, but there is reason to believe that the
sugar does not become chemically combined with the actual
protoplasmic molecules. We may call to mind that, in a petrol
motor, the fuel does not become a constituent part of the mechanism,
but is burned up in such a way in relation to this mechanism that
its energy becomes available for use.
Although this is what finally happens to glucose and other
foods used for energy purposes, there are a number of intermediate
products formed before they become carbon dioxide and water.
FOOD—DIGESTION AND RESPIRATION
73
Many of these are of importance, because they play a part in
various chemical reactions in the cell, and in the abnormal changes
of disease. Lactic acid is one of the most important intermediate
I
•o
CO
S
FIG. i. — Diagram of the Carbohydrate Cycle from Carbon Dioxide through
Starch to Carbon Dioxide again.
products of carbohydrates on account of its relation to muscle,
as will be seen later.
In Fig. i a diagram is given of the carbohydrate cycle.
Fat
We have already learned something of the chemical nature of
fats as esters of glycerol and, incidentally, the fact that there is an
74 INTRODUCTION TO GENERAL PHYSIOLOGY
enzyme that accelerates the hydrolysis and synthesis of esters.
This enzyme is known as " lipase"
It is not found in any appreciable quantity in the saliva or in
the gastric juice. The first change that takes place is when the
pancreatic juice is met with. This contains a powerful lipase, and
effects the hydrolysis of fats into glycerol and the particular fatty
acid. These higher fatty acids are insoluble in water, but are
dissolved by the bile. This liquid is a secretion made by the liver,
and poured into the intestine by a duct close to that of the
pancreas. Bile contains salts of two related complex acids, which
have a great effect in lowering surface_tension. From what has
been said in the previous* chapter in regard to the effect of lowering
surface tension in increasing the dispersion of colloids, we shall not
be surprised to find that the activity of lipase is increased in a
marked degree by the presence of bile. Indeed, some kinds of
lipase appear to be practically inert in its absence. The increase in
activity is, no doubt, due to the increase in the surface of the
enzyme, owing to its greater dispersion ; but it may also cause a
greater dispersion of the fat itself.
Fatty acids and glycerol are absorbed together and, somewhat
remarkably, at once recombined in the wall of the intestine to
the original neutral fats, which can be seen in the cells of the
intestinal surface (E., p. 195). Although the actual proof is difficult,
there can be little doubt that this synthesis is effected by lipase,
acting in a system in which the water has been reduced by some
means.
The fine globules of fat which then pass out of the cells do not
enter the blood at once, but are taken into a space which is in
connection with a system of branching vessels, called lympliatics.
These have their origin in the spaces of the tissues of all kinds of
organs, and finally unite in a large vessel which enters the veins at
the root of the neck. The fat thus enters the blood in very fine
, globules, and can be detected there after a meal containing fat.
What fat is not required for current use as a source of energy is
stored up in subcutaneous and other connective tissue in numerous
parts of the body. This is the special advantage it possesses over
other kinds of food, since the capacity of storage is practically
unlimited.
Like carbohydrate, there are intermediate products formed
before fat is completely oxidised to carbon dioxide and water. The
process is not completely understood, and involves more chemical
details than can be given here.
FOOD— DIGESTION AND RESPIRATION 75
Proteins
We have learned that these compounds are the form in which
our nitrogen supply is provided. They also serve as a source of
energy.
In order that the tissue cells may be able to make use of these
substances, they must first be split up into their constituent amino-
acids. The first step is taken in the stomach, where the gastric
juice contains an enzyme, pepsin, which acts only in a fairly acid
medium. The acid present in the stomach is hydrochloric acid,
which is secreted by the glands in the walls of the stomach, but not
by the same cells that produce the pepsin. Pepsin does not, how-
ever, carry the hydrolysis as far as amino-acids, but only to certain
polypeptides, called peptones, which still consist of several amino-
acid molecules. Peptones are not absorbed in the stomach, but are
passed on to the small intestine where the " trypsin " of the pan-
creatic juice converts them almost entirely into amino-acids,
although some of the dipeptides, formed near the end, are difficult
to hydrolyse. The operation is completed by another enzyme,
"erepsin," which is produced by the cells lining the intestine.
Trypsin acts in a faintly alkaline solution, resulting from the
mixture of the acid products from the stomach with the alkaline
pancreatic juice (E., p. 193).
Although we can only follow the series of changes in the
vertebrates, there are indications that the process is common to all
animals, even including the amoeba. There is a preliminary action
of an enzyme acting in acid solution, followed by another in faintly
alkaline solution. Since trypsin can act upon the original protein,
it is not quite clear why there is a preliminary action by pepsin.
It is doubtless a means of hastening the process, because the
products of gastric digestion are more rapidly hydrolysed by
trypsin than if the process had not already been partially
performed.
The amino-acids thus formed are absorbed by the blood
vessels of the small intestine, and carried to the liver. The greater
part are subjected there to a chemical change, which will be
described presently. The smaller part passes on to the tissue
cells, which select the particular amino-acids which they require
for the repair of their structure, or for growing new structures.
We have already seen that proteins can be burned and used
as sources of energy. The process, however, as it takes place
in the living organism, is incomplete. The NH.2 groups are not
oxidised, and are lost as urea. How are these groups split ofT from
the amino-acids? So far as we know, what takes place is as
follows. There is an enzyme or enzymes in the liver which
76 INTRODUCTION TO GENERAL PHYSIOLOGY
removes the NH2 group and the hydrogen from their combination
with the carbon atom next the carboxyl group (the a-position), and
replaces them by oxygen, thus forming a ketonic acid. In the
same process the carboxyl is reduced to an aldehyde group. Taking
alanine (a-amino-propionic acid) as an example : —
HCNH2 becomes CO + N H3
I I
OCOH OCH
A ketonic aldehyde is produced, in this case pyruvic aldehyde
corresponding to the particular ammo-acid affected by the process.
known as " examination" Further changes are produced in these
compounds by one of three kinds of reaction, under the control
of enzymes. Most of those enzymes with which we have been
concerned so far have been those dealing with the removal or
addition of the elements of water. If a ketonic aldehyde is
hydrolysed, we obtain the corresponding hydroxy-acid. In our
example it would be lactic acid : —
CH3 CH3
C0 + H20 = HCOH
I I
OCH COOH
But there are also enzymes, as we shall see later, which bring
about oxidation or reduction. In the first case we get : —
CHo CHo
i i
CO + O = CO (i.e., pyruvic acid)
I I
OCH OCOH
In the latter case we have : —
CH3 CH3
I I
CO + H2 = CH2 (i.e., propionic acid).
I I
OCH OCOH
Any one of these three acids, passing on to the tissues is
oxidised to carbon dioxide and water, giving off energy.
The ammonia from the deamination reaction combines first with
carbon dioxide to form the carbonate, which is then converted into
FOOD— DIGESTION AND RESPIRATION 77
urea in the liver by the removal of water, a reversal of the action
of the enzyme urease, described elsewhere (E., p. 182).
C02+H20
Loss, probably by bacterial action.
FIG. 2. — Diagram of the Nitrogen Cycle from the Atmosphere through
Plants and Animals back to the Atmosphere.
In Fig. 2 we have a diagrammatic representation of the
nitrogen cycle, as it may be called.
The Large Intestine
The importance of this last part of the alimentary canal varies
in different animals ; in those living on flesh, nearly all of the useful
constituents have been absorbed by the time the large intestine
is reached. The chief process in it is a further taking up of water
from the semi-liquid indigestible contents, so that they become
more solid before final rejection. In those animals which take a
large proportion of vegetable food, a certain further process of
digestion goes on in the large intestine, mainly by the action
;8 INTRODUCTION TO GENERAL PHYSIOLOGY
of bacteria. In the course of the changes brought about by these
organisms, intermediate products, such as glucose from the cellulose
of the plant tissues, are formed, and these are to a large extent
absorbed before they have undergone further destruction.
Movements of the Alimentary Canal
The various digestive enzymes, whose action has been briefly
described above, are formed either by cells lining little pits on the
wall of the cavities or by separate organs called glands, which pour
in their products by means of ducts. The process of formation
of these juices is known as "secretion? to which we shall return
presently.
But it will be clear that the effective mixing of these secretions
with the food requires a process similar to the kneading of dough
to make bread. Moreover, the absorption of the digested material
needs the bringing into contact with the wall of the intestine of
all parts of the mass in turn. And again, the useless indigestible
portion must be carried along to make room for the new material
arriving from the stomach, while the new material itself needs
exposing to the action of different parts in turn.
The mechanism by which these things are done will be better
understood after the later portions of this book have been read.
The process is a complex one, involving nervous and muscular
factors. It will be best described in its essential features by taking
the case of the small intestine, where the simple tubular form shows
it in a way most easily followed.
Let us first see what actually happens, and then attempt to
explain how it is brought about. We will suppose that an animal
has been fed with a meal containing an insoluble powder which is
opaque to X-rays, such as bismuth subnitrate. The shadows of
the food masses can then be observed on a fluorescent screen in
the usual way and their movements followed. Looking at the
shadow of a loop which happens for a moment to be at rest, we
notice that it is filled with a long column of regular diameter like
a sausage. Presently this column is nipped together in several
places at the same time, and divided up into a number of small
portions. Each of these is next divided up and the parts forced in
both directions, so as to join with similar parts of neighbouring
masses. The process is repeated many times, and then a powerful
nip starts at the end of the mass next the stomach, forcing the
whole onwards into the intestine beyond, which becomes lax in
order to receive it. This nipping passes along the mass from end
to end and empties the loop, much as if it had been seized by the
finger "and thumb, and the contents pressed out by sliding the finger
FOOD—DIGESTION AND RESPIRATION 79
and thumb along. This last kind of movement is known as
"'peristaltic contraction"
Now as to the way in which these movements are produced.
It must be assumed that the reader is aware that there are
structures called muscles, which have the power of shortening when
required. If arranged in the form of a ring, the ring must be
narrowed when the muscle shortens, or even closed up entirely.
The wall of the intestine consists in part of muscular tissue, which
can, by shortening at particular points, empty out the contents of
the intestine at these points into the adjoining part. Further,
there are two kinds of muscular tissue found in the body, one kind
forming the muscles most familiar to us in our arms and legs, and
so on. These are at rest, until ordered to activity by the brain
(E., p. 195) ; the other kind, which forms a large part of the walls
of hollow organs and tubes, such as the heart, intestine, and
blood vessels are, when left to themselves, in a state of moderate
activity, which shows itself in two ways.' It may be present as a
steady, uninterrupted state of "contraction," as the shortening of
muscle is usually, but incorrectly, called. This state is, as a rule,
only partial, since it can either increase or decrease. Or the
spontaneous activity may be shown by a series of contractions and
relaxations, following one another at regular intervals. The
intestinal muscle, when removed from any connection with the
nervous system, manifests both these properties at the same time.
The series of contractions starts at a point and passes as a wave
from point to point along the course of the intestine.
It is clear that before any ordered movements, such as those
decribed as part of the digestive mechanism, can take place, a
means of control of spontaneous activities must be arranged. It is,
in point of fact, usual to find that muscular tissue of this kind,
which is known by various names, "smooth," "non-striated," or
" involuntary," is under the influence of two kinds of nerves, one
of which restrains its activity, the other increases it. We do not
know yet how the two effects are actually produced, except that
they are due to the different ways in which these two kinds of
nerve fibres end in the muscle cell itself, not to any peculiarity
of the nerve fibre itself or the process in it. The restraining nerves
are called "inhibitory," these increasing activity, "excitatory." In
the case of the intestine, the excitatory nerve, a part of that nerve
known as the vagus, has also inhibitory functions of a certain
kind. Its fibres are not connected directly with the muscle fibres,
but with a layer of nerve fibres and cells in the wall of the intestine.
Secondary fibres are supplied to the muscle itself from this layer.
Now we saw that when the digestive process at one place has gone
on for a certain time, the backward and forward movements cease
So INTRODUCTION TO GENERAL PHYSIOLOGY
and a wave of contraction passes along, preceded by a wave of
relaxation. This peristaltic wave occurs also in the intestine after
removal from the body, if distended locally in any way. The most
satisfactory way of explaining the fact, which has been called the
" law of the intestine," is that the nerve cells with which the vagus
fibres join give off, each one, two nerve fibres, one of which goes
backwards to supply muscle nearer the stomach, and is excitatory ;
the other goes forward, and is inhibitory. When, therefore, some-
thing happens in the interior of the intestine opposite one of these
cells, of such a nature as to set this cell into activity, a contraction
will be started behind the place in question and a relaxation in
front, both these processes travelling onwards as the food mass is
pushed between them. We must suppose that something, perhaps
of a chemical nature, happens at a certain stage of digestion, and
that the movement is brought about which sends the mass onwards.
One of the properties of smooth muscle is that it is stimulated
to contraction by being stretched. We understand, therefore, how
the distension of an intestinal loop by food passing into it will
result in contraction, at this place, and that when a contraction
occurs over the food mass, this will be forced into a neighbouring
region and in turn cause contraction here. But it is not quite so
easy to explain why a number of points along a column of food
material enter into activity at the same time, as we saw happens.
It is possible that small differences in degree of distension may
determine where a contraction is to take place. But we must also
remember that any of the activities described can be stopped by
the inhibitory nerve fibres in order that a different kind of activity
may take its place. These fibres are contained in the nerves called
" splanchnic," which issue from the central nervous system, so that
it is not impossible that there may be control of co-ordinate move-
ments by the higher centres in response to messages received.
The inhibitory fibres in the splanchnic nerves are readily set into
activity by stimulation of nerve fibres in the intestine which carry
messages to the nerve centres, producing what we shall learn later
is a "reflex action." Thus, injury to the intestine, as by gun-shot
or shell wound, results in a long-lasting cessation of movement, a
reflex " paralysis" of the muscular wall, clearly a beneficial prccess,
especially if the injury has resulted in perforation of the wall of
the intestine.
The feeling of hunger^ as distinct from appetite, is caused by
contractions of the empty stomach, by which nervous structures
are pressed upon and stimulated.
Appetite, on the other hand, may be described as pleasure
anticipated from the taste of food, and may, of course, be present,
independently of hunger or actual need for food. It is, however,
FOOD— DIGESTION AND RESPIRATION 81
a powerful stimulus to the secretion of the gastric juice, which is,
therefore, already present in the stomach when the food arrives
there.
Secretion
The digestive juices, as pointed out, are formed by the cells of
organs called " glands," specially devoted to this purpose. Like
all the other cells of the organism, these cells derive from the blood
the material from which they manufacture their special products.
These products are not present in the blood, whose composition
we shall have to discuss in other places. While all cells form their
own structure, some of them prepare also things for the use of
other parts of the body, giving them off by means of a current of
water, which washes them out, as it were. The production of this
current of water is one of the problems concerning secretion, to
which we shall have to give attention.
As will have been evident from what has been said about the
process of digestion, glands are not always in action. If we
examine under the microscope the condition of secreting cells after
a period of rest, we notice that they are filled with a number of
granules, differing in kind according to the nature of the gland
(E., p. 196). These granules are clearly the material out of which
the constituents of the secretion, or some of them, are produced
when the gland is excited to activity, because they disappear to a
greater or less extent during activity. When the stimulus to
secretion ceases, new granules are formed by the cells automatically
and, as it appears, by a chemical process of the nature of a balanced
reaction, because, when the cell has stored a certain quantity, the
formation ceases, the gland being then ready for renewed activity.
This stage of preparation requires the expenditure of energy, as
shown by the fact that there is an increased consumption of oxygen
for some minutes after secretion has ceased. During continued
activity, the using up and production of new material keep pace
with one another, unless very great demand is made on the gland.
In the latter case, the granules are used up faster than they are
formed, and they disappear first of all from that part of the cell
furthest from the side in connection with the duct.
Before we pass on to discuss 'what happens in the cell when it
enters into activity, we may spend a little time on the ways in
which a gland is excited to secrete.
Some glands are supplied with nerves, and when these nerves
are stimulated, changes are produced in the cells leading to a flow
of liquid. This mode of setting into activity is particularly evident
in the salivary glands, and is also present in the gastric glands
The nerves are stimulated by reflex action from various nerves o
6
82 INTRODUCTION TO GENERAL PHYSIOLOGY
sense. Thus, not only is the taste of food effective, but also the
sight and smell. Hence, as we saw, the importance of the
possession of appetite.
But in the case of the stomach we find that another mode of
stimulation begins to show itself. Glands can be excited by
chemical agents, as we know from the fact that there are drugs
which have the property of causing them to secrete. Such a drug
is pilocarpine. In the course of digestion, certain substances are
produced by the action of the contents of the alimentary canal on
the cells of its walls. These substances are absorbed into the
blood and arrive in this way at some gland which is sensitive
to them. The most obvious of these mechanisms is that of the
pancreas. When the acid contents of the stomach pass into the
small intestine, they cause the formation in the cells of a substance
which has been called " secretin," but whose chemical nature is still
unknown. This passes into the blood, and arriving at the pan-
creas, excites this organ to secrete. It is a remarkable fact that
the trypsin contained in the pancreatic juice is inactive until it has
been acted upon by another substance, " enterokinase," apparently
an enzyme, secreted by the walls of the intestine.
So far as can be made out, what happens in the secreting cell
when stimulated is somewhat as follows. We have seen that the
cell at rest contains a store of material, which has been produced
by the expenditure of energy. Its molecules, or some of them,
are large ones, and onset of activity is associated with a splitting
up of these molecules into smaller ones. The consequence of this
is that the osmotic pressure of the cell contents rises, water is
absorbed from the blood and, supposing that the cell membrane is
impermeable to these solutes, the cell merely becomes distended.
But now, supposing that the end in relation with the duct becomes
permeable, then the pressure will cause a flow of liquid into the
duct, carrying with it in solution the constituents of the secretion.
Sometimes granules appear to escape bodily from the cells, be-
coming dissolved in the ducts later. This process will continue as
long as any osmotically active material is present in the cell and
the membrane at the duct end remains permeable. It can be
imitated in a model (E., p. 197).
In some secreting mechanisms of plants it can be shown that
changes of permeability occur at the end of the cell at which the
liquid appears. A similar state of affairs is at the basis of the
root pressure. If the stem is cut through, a flow of liquid, under
fair pressure, occurs from the cut end, being absorbed from the
soil.
In the animal cell, proofs of increased permeability in activity
are more indirect. The cells take up certain dyes more readily.
FOOD— DIGESTION AND RESPIRATION 83
But the most interesting phenomenon showing a change of per-
meability is the electrical change which takes place in activity.
We may spend a short time on this, since the explanation applies
to muscle and nerve as well as to secreting glands.
We saw above that a membrane may be impermeable to an
electrolyte because one of the ions of this electrolyte cannot pass
through, although the membrane may be easily permeable to the
other ion. This opposite ion goes out only as far as the attraction
of the oppositely charged ion inside the membrane will allow it.
We have here the Helmholtz double layer in one of its forms. A
simple illustration may enable this important conception to be
grasped. Imagine two large pastures separated by a fence, and
that the spaces between the bars of this fence are wide enough to
allow lambs to get through, but too narrow for their mothers.
Membrane. . , Protoplasm,
(A)
+¥^r-+ + + -f -+
(B)
FIG. 3. — Diagram of the electrical state of the resting cell (A) and
that of an active cell (B).
Introduce into one of these pastures a flock of sheep, each ewe with
one lamb. In the course of their wanderings they will arrive at the
fence. The propensity of the lambs to wander further will take
them through the fence, but the ewes must remain behind. How-
ever, the attractive forces of their mothers, especially that of food,
will prevent the lambs from being in any number far from the fence
at any time. Similarly, the presence of the lambs in the adjoining
field will prevent the ewes from wandering far from the fence. It
may be said that the thickness of the layer would be somewhat
great, but if we imagine molecules magnified to the size of sheep
the proportions would not greatly differ from the molecular one.
Let us suppose that the membrane of a particular cell is
permeable to the cations of some salt contained within the cell,
impermeable to the anions of this salt. The cell will be surrounded
by an electrically positive sheath, the other component of the
double layer with a negative charge being on the inside of the
membrane. Fig. 3 shows the arrangement at A. If we connect
84 INTRODUCTION TO GENERAL PHYSIOLOGY
the two ends to a galvanometer, we should not be able to detect
any electrical difference between them ; any two points on the
surface are at the same potential. Imagine next that one end loses
its semi-permeable nature, as in B, so that the two kinds of ions freely
mix by diffusion. This end will become neutral, and merely serve
as a conductor to the internal member of the double layer.
Accordingly, we observe a deflection of the galvanometer. At the
same time the way is open for the escape of solutes from the cell.
It will be clear that another membrane might be permeable to
anions only. Or again, during activity a reversal of the sign of the
permeability might occur. There are, indeed, many possibilities.
The fact of the occurrence of an electrical change in gland cells on
activity is referred to here as one of the pieces of evidence in favour
of an increased permeability (E., p. 197).
We saw in our experiments on blood corpuscles that the osmotic
pressure of the salts in blood amounts to several atmospheres.
Now if the liquid produced by the secreting glands were a solution
merely of the important organic constituents in water, its osmotic
pressure would be low, and a large amount of work would have to
be done to hold back the salts. Accordingly, we always find the
salts of the blood present in secretions, although sometimes in
lower concentration than in the blood.
It is to be remembered that secretions are produced in other
situations besides those in connection with digestive processes.
We have the sweat glands of the skin, for example, for the purpose
of getting rid of excess heat by evaporation of water. Further,
the secretion of some organs does not flow away in a duct, but the
products diffuse into the blood and exert an action on other organs
by this means. These are called " internal secretions " or " hor-
mones," and will be dealt with later.
Thirst. — This feeling is due to dryness of the throat, owing to
deficient secretion of saliva. When the blood has lost water, its
osmotic pressure rises, consequently the cells of the secreting
glands have to do more work to extract the watery saliva from it ;
or the expenditure of the same amount of energy results in less
secretion. The body is continually losing water from the lungs,
skin, and kidneys ; unless this is replaced, the blood becomes con-
centrated. The osmotic pressure of the blood is higher than that
of saliva, hence the natural direction of flow of water would be from
duct to blood vessels. To counteract this, the expenditure of
energy is required.
Respiration
In order that energy may be obtained from food materials,
they must be burned or oxidised by combination with oxygen.
FOOD— DIGESTION AND RESPIRATION 85
Thus it is not, strictly speaking, correct to refer to the energy
value of fat, for example. The system possessing the potential
energy is fat plus oxygen.
Animals of small size or of flat form obtain their oxygen by
free diffusion to the tissue cells. But as soon as larger dimensions
and more complex forms appear, the necessity of special arrange-
ments for conveying oxygen to the tissues becomes evident.
In insects, \ve find a peculiar branching system of fine tubes,
called " tracheae," which contain air and are distributed to all
organs (E., p. 200). The air is changed by squeezing movements
which press it out through certain openings on the side of the
body, while fresh air enters when the pressure is relaxed.
In Crustacea, molluscs, and vertebrates, a liquid, the blood, is
carried to all parts of the body by a tubular system. The
arrangements of this system will be described in a later chapter.
For the present, it will suffice to remember that a supply of
oxygen is conveyed to all tissues in this way. But oxygen is only
slightly soluble in water, and a very copious current of such a
solution would be necessary to provide enough oxygen for the
vigorous movements of the vertebrates ; in fact, so great a
current would be required as to be mechanically impossible.
Accordingly, we find in the blood certain very small red bodies,
the red corpuscles, which contain that remarkable compound,
haemoglobin, about whose chemical nature we have already
learned some facts. The most important one is that it takes up
oxygen when the pressure of this gas is about that which it has in
the atmosphere, and gives it up again when the pressure is lower,
as in the tissues, where the gas is being continually used up
(E., p. 200).
We must at this point understand what is meant by the tension
of a gas, since the expression is often to be used. Suppose that
we have a mixture of air with carbon dioxide, such that in one
hundred volumes of the mixture there are 95 of air and 5 of carbon
dioxide. And further, that the mixture of gases is at atmospheric
pressure. The five volumes of carbon dioxide are diffused through-
out the space, and if we imagine the air removed, it is clear that the
pressure would be only five-hundredths of an atmosphere, since
the five parts fill the space of one hundred. The pressure of the
carbon dioxide in the mixture is therefore five-hundredths of 760
mm. of mercury ; that is, 38 mm. This is known as the partial
pressure or tension of the carbon dioxide in the mixture. Similarly,
the tension of the oxygen, which forms 21 per cent, of air, will be,
in our mixture, 21/100x95/100x760=151.6 mm. A liquid in
contact with such a mixture will dissolve carbon dioxide until the
tension of the gas in the liquid is the same as that in the gas phase,
86 INTRODUCTION TO GENERAL PHYSIOLOGY
since there must be as many carbon dioxide molecules leaving the
surface of the liquid in a given time as there are molecules entering
it; otherwise there could be no equilibrium. But although the tension
must be the same whatever the liquid taken, the actual amount of
carbon dioxide dissolved may vary greatly. Thus, water dissolves
more than a strong salt solution does. Further, the amount of
different gases dissolved by water varies greatly at the same
tension.
If now we take a solution of haemoglobin and expose it to the
air, that is, to 160 mm. tension of oxygen, we find that it takes up
a much larger amount of oxygen than water does. Next, expose
it to a vacuum or an atmosphere of pure nitrogen, that is, to a zero
tension of oxygen ; the oxygen which it contains is given off
again. What happens, however, if we expose it to a tension of
oxygen of one-eighth of that in the atmosphere, namely, to 20
mm.? We find that it takes up oxygen, but less than at 160 mm.
Moreover, at all tensions between o and 100 mm. the amount of
oxygen taken up has a definite value ; and, if a graph be made,
it will be seen that these values are not directly proportional to
the tension, but the curve rises more steeply at the lower tensions,
so that very little more is taken up at 100 mm. than at 80 mm.,
while above 100 mm. haemoglobin is practically saturated with
oxygen, a further rise in tension resulting in no further amount
being taken up.
Although this union of oxygen with haemoglobin is usually
looked upon as a chemical compound, it must not be overlooked
that there are difficulties in this view, and that it is held by some
that the case is more analogous to the adsorption of gases by
surfaces such as that of charcoal, which played so great a part
in the protection from poison gases in the War. The question
is not yet decided, and more complete discussion may be read
elsewhere (P., pp. 613-625). But there are some facts of interest
that may be mentioned briefly here on account of their interest
and importance.
We saw above that when haemoglobin has taken up a certain
amount of oxygen, any further rise in tension does not result in
any more being taken up. In speaking of the chemical nature
of haemoglobin, it was pointed out that each molecule contains one
atom of iron. Hence, by comparing the amount of oxygen taken
up in saturation with the amount of iron, it is possible to find out
the number of molecules of oxygen that unite with a molecule
of haemoglobin. It is found to be precisely one molecule. This
is a fact strongly indicating a definite chemical compound. But
a difficulty arises at once as to the nature of the compounds
present when the haemoglobin is only partially saturated. It
FOOD—DIGESTION AND RESPIRATION 87
should be kept in mind that haemoglobin is in colloidal solution
in water, and about 5 per cent, is the most that can be dissolved.
The red corpuscles contain 35 per cent, of haemoglobin, and 63
per cent, of water, so that the haemoglobin must be in the form
of a moist solid. The following considerations will show that if
oxy-haemoglobin (as the oxygenated form is called) is a definite
chemical compound, it is unique. There are various compounds
which give off oxygen or carbon dioxide at a certain tension of
these gases, and combine with it again at a higher tension. But
the point is that there is no half-way state. Above a particular
"dissociation tension," according to temperature, the whole is
in the form of the complete compound. Just below this tension
the whole of the gas is given off. It may be stated to be a case
of " all or nothing." To get over the difficulty, it has been
suggested that there is a series of compounds of haemoglobin with
oxygen of the composition HbO2, Hb2O4, Hb3Oc, etc. These would
each obey a different form of the law, deduced from mass action,
of the rate of combination in relation to concentration of oxygen,
that is, the tension of oxygen. Of course, the difficulty is not
present in such a case as that of adsorption by charcoal, where the
amount condensed on the surface is in proportion to the tension
of the gas, up to the point of saturation.
Again, it is found that the amount of oxygen taken up by
haemoglobin is less at a higher than at a lower temperature,
although the rate at which it takes it up or gives it off is greater
at the higher temperature. This is one of the peculiarities of
adsorption also, owing to the negative temperature coefficient
of surface tension. It might be supposed to imply a disadvantage
on the part of warm-blooded animals, but it seems to be more
important to obtain the oxygen quickly than to have the larger
reserve, which is made up for by rapid replacement of the blood
by vigorous circulation.
Haemoglobin, being colloidal, is subject to aggregation by
electrolytes, and the fact shows itself in an effect on the form of the
dissociation curve such that when acids or salts are present, less
oxygen is taken up at a given tension of the gas. The difference
is not great at the higher tensions (90-100 mm. of mercury), but
marked when it is 15-20 mm.
The darker colour of blood which has lost oxygen is familiar in
the appearance of the veins. Although they look bluish, as seen
through the skin by reflected light, the colour of the blood itself
may be more correctly described as crimson, when compared with
the bright scarlet colour of fully oxygenated arterial blood. The
scarlet colour in the arteries is due to the fact that the blood has
taken up oxygen in the lungs. As it passes through the tissues,
88 INTRODUCTION TO GENERAL PHYSIOLOGY
it becomes crimson as the oxygen is given up to the active cells,
which use it for combustion purposes (E., p. 201).
A fact rather difficult to reconcile with chemical combination is
that oxygen is not the only gas or vapour taken up by haemo-
globin. Carbop monoxide, .niti^cJ>xide,€arbojxcHpjdde_^and chloro-
form are absorbed, apparently in a similar way to oxygen, although
in different numerical proportions.
The taking up of carbon dioxide by haemoglobin leads, naturally,
to the question of the carriage of this gas in the blood, so as to
remove it from the tissues, where it is being continually produced
by oxidation of carbon compounds. As the arterial blood reaches
the tissues, oxy-haemoglobin gives up part of its oxygen, since the
tension of oxygen in the cells is low. The tension of carbon
dioxide in the cells becoming higher than in the blood, owing to
the above-mentioned combustion process, it passes into the blood,
and is taken up by the haemoglobin. Experiments show that
carbon dioxide drives off a part of the oxygen from the
haemoglobin. Or, put in another way, under a given reduced
tension of oxygen, less is held by the haemoglobin in the presence
of carbon dioxide than in its absence. The advantage of this is
clear, since more oxygen is set free for use. We are reminded of
a similar state of affairs in the case of enzymes, wrhere one sub-
stance can drive another out of adsorption on the surface.
The Lungs
The blood returning to the heart by way of the veins contains
then less oxygen and more carbon dioxide than when it arrives at
the tissues. How does it replenish its oxygen and get rid of the
excess carbon dioxide? It is sent by the heart to the lungs for
this purpose. These organs consist essentially of an elaborate
system of little bags full of air, on the walls of which there is a fine
network of minute blood vessels (capillaries). The blood in these
vessels is separated from the air by a very thin membrane, so that
the haemoglobin is quickly exposed to a tension of oxygen high
enough to saturate it, and to one of carbon dioxide low enough to
remove a great part of the carbon dioxide from it. But there must
clearly be some means of renewing the air in the lungs. This is
done by alternately expanding and contracting the cavity of the
chest, in which the lungs are contained, a process known as breath-
ing. There are muscles which raise the ribs from an oblique to a
more horizontal position, and- there is a muscular partition between
the chest and the abdomen, the diaphragm, which at rest is in the
form of a dome projecting into the chest. When it contracts, the
top is pulled towards the abdomen, since the lower edge is fixed
FOOD— DIGESTION AND RESPIRATION 89
to the ribs and spinal column. By these two means air is sucked
into the lungs. When the contraction of the muscles ceases, the
chest returns to its position of rest, expelling part of the air con-
tained in it. Under vigorous respiration, however, there are
muscular movements which assist in pressing air out. The whole
of the air is, of course, not driven out, so that in the depths of the
small air sacs, where the exchange between blood and air is
effected, the tension of oxygen is not as high as in the atmosphere,
although it is high enough to saturate the haemoglobin ; while
the tension of carbon dioxide is not so low as in the outer air, but
is lower than in the venous blood.
The way in which the supply of air by respiratory movements
is regulated in accordance with the needs of the organism is by
the fact that the nerve centre, which is responsible for sending
periodic discharges to the muscles acting on the chest, is extremely
sensitive to a slight rise in the hydrogen-ion concentration of the
blood. When more oxygen is being consumed by the activity of
cells, more carbon dioxide is being given off. This becomes an
acid when dissolved in water, and raises the hydrogen-ion concen-
tration of the blood to a slight degree. The respiratory centre is
stimulated, more oxygen is supplied, and the excess of carbon
dioxide removed (E., p. 202). It is important to remember that the
production of any acid by the tissues has the same effect. Since
acids are produced in the course of the normal metabolism of fats
and carbohydrates, and subsequently oxidised, it is clear why they
appear in the blood in states where there is deficient supply of
oxygen, as in lowered rates of circulation of blood, and so on. A
rise in hydrogen -ion concentration of the blood results, owing to
the fact that these acids drive off carbon dioxide from the bicar-
bonates contained therein. This rise stimulates the respiratory
centre, and tends to automatic benefit by a more copious supply of
oxygen. It will also be clear that treatment of this so-called
"acidosis" by giving alkalies is inappropriate, because the stimu-
lation of the respiratory centre is thereby prevented, and
spontaneous supply of the necessary increase in oxygen is
retarded.
The Mechanism of Oxidation
The supply of oxygen to the tissues is provided by the means
described. But the mere presence of ordinary oxygen is not
sufficient, as will be evident when it is remembered that glucose is
one of the foods most largely burned for the supply of energy, and
that glucose is not oxidised by the air, or so slowly as to be useless
for the purpose in view. What we need is to raise the chemical
potential of the oxygen so that it shall attack substances refractory
90 INTRODUCTION TO GENERAL PHYSIOLOGY
to it in its ordinary form. Although this can be done in various
ways, we do not as yet understand completely what is the change
that takes place. Sir W. Ramsay taught that its activity was
manifested during the change from the quadrivalent to the
bivalent form, that is, in the process of losing electrical charge. It
is interesting to connect this view with what was pointed out
previously in our general discussion of the energetics of living
organisms, namely, that it is in the process of transfer of energy
that those activities which we recognise especially as manifestations
of life are to be found.
The way in which the " activation " of oxygen takes place in the
living cell may be described briefly, as follows.
Although most of the materials oxidised in the cell are
refractory to ordinary oxygen, certain constituents are slowly
oxidised by it. Such are the unsaturated fats and lipines. These
are said to undergo a process of " autoxidation " (E., p. 202). Now,
in this process, investigation has shown that a rather curious thing
happens. When a part of the substance is oxidised to a simple
oxide, energy is given off, as in the ordinary process of combustion.
But this energy is not entirely lost as heat in the case of autoxida-
tion. Simultaneously with the oxidation of one molecule to a
lower oxide, another one is converted to a peroxide, which requires a
supply of energy to put in the extra atom of oxygen. Peroxides
have higher powers of oxidation than ordinary oxygen has ; they
supply oxygen to oxidisable substances at a higher potential
than it possesses in its ordinary molecular form.
A rough idea of this process of raising chemical potential
may be obtained by thinking of the increased destructive effect
of a weight when dropped from a greater height. To this
greater height it must have been raised by the doing of work
upon it.
But even peroxides are not powerful enough to oxidise sugar
or lactic acid. Hydrogen peroxide does not cause the evolution
of carbon dioxide from lactic acid (E., p. 203). There is, however,
a catalytic means by which hydrogen peroxide and similar
peroxides can be made to afford oxygen at a higher potential.
The addition of a trace of a ferrous salt (Fenton's reaction) results
in the complete oxidation of lactic acid to carbon dioxide and
water (E., p. 203). What the exact mechanism of this reaction is,
has not been completely explained.
We may ask, has there been found in the living cell any agent
similar in action to that of the iron salt in the above reaction ? In
the following description, when certain enzymes are stated to be
obtained from particular sources, it is not to be understood
that they are only present there, but that from this source they
FOOD— DIGESTION AND RESPIRATION 91
can conveniently be prepared free from admixtures which obscure
their typical action.
A preparation can be obtained from the root of the horse-
radish and elsewhere which has the same action on hydrogen
peroxide as ferrous salts have (E., p. 203). That is, it enables the
peroxide to oxidise lactic acid, etc. It has been called pcroxidase,
having the general characters of an enzyme. Its action on the
peroxide is quite different from that of catalase, another enzyme
of very wide occurrence. While the latter causes the evolution
of gaseous oxygen, and does not increase the oxidation potential
of the peroxide, peroxidase causes no evolution of oxygen, but has
a marked effect in raising the oxidative power. Although the
composition of peroxidases has not yet been definitely established,
there is evidence that they consist essentially of the colloidal
hydroxide of a metal, such as iron, copper, or manganese, which is
capable of existence in two forms, one produced from the other
by an oxidation. In the actual enzyme these hydroxides are
associated with some stable organic colloid — gum, protein, etc.
(P., p. 585). The function of this colloid appears to be to protect
the hydroxide from aggregation and loss of active surface by the
effect of electrolytes.
Thus, the concurrence of four factors is required — (i) oxygen ;
(2) an autoxidisable substance ; (3) a peroxide, produced by the
action of the first on the second ; and (4) a peroxidase. In
many cases we can separate from cells complexes containing
peroxides and peroxidases ; these are often called oxydases.
A convenient reagent in the investigation of such systems
is an acid contained in the gum-resin, guaiacum. This is
oxidised by active oxygen, not by ordinary oxygen, or only
very slowly. When oxidised, a blue pigment is formed. No
effect is produced by a peroxidase alone, nor by an organic
peroxide alone ; only when combined. If placed on the cut
surface of a potato, a blue colour is produced. Hence there
must be both peroxide and peroxidase present. In some
cases a substance is naturally present which changes in colour
when oxidised. This is the origin of the brown tint seen
to form on the cut surface of a living apple. When the cells
are cut across, and exposed to oxygen, an autoxidisable sub-
stance gives rise to a peroxide, which is then acted upon by
a peroxidase. The active oxygen, thus available, oxidises a
colourless compound, also present in the cells, forming a brown
pigment (E., p. 203).
There are many subsidiary details concerning these oxidation
mechanisms for which space is not available here. The account
given is a brief summary of the state of knowledge at present, which
92 INTRODUCTION TO GENERAL PHYSIOLOGY
has been arrived at after numerous investigations, at first
apparently complex and contradictory.
From previous remarks in various places of this book, it will be
clear that reduction processes also play an important part in cell
life. Some of these processes, up to the present, have been shown
to be under the control of enzymes, and substances similar to
aldehydes take the place of peroxides. But the mechanism is still
somewhat obscure (P., p. 586). It will be clear that when one
substance is reduced, another has to be oxidised in order to take
away the oxygen from the first (E., p. 203).
CHAPTER III
WORK— THE MUSCLES
ALTHOUGH movement is not the only way in which the energy of
food is used up, it is the most striking and obvious way. It is
perfectly clear that work is done when we raise a weight or throw
a ball.
On the other hand, it is unnecessary to remind the reader that
energy is expended in many other ways, as in the overcoming of
osmotic pressure, the formation of chemical compounds of a higher
potential than those from which they arise, and so on.
In the present chapter we have to learn something about
muscular activity and its mechanism.
Suppose that we have a set of fibres attached at one end to a
bone and at the other end to another bone, which is capable of
moving by a hinge joint at the end of the former, and that the two
bones are placed so as to be in line with one another. It is plain
that if the fibres shorten, the two bones will be moved so as to form
an angle with one another, since in this position the line joining
a point on one to a point on the other is shorter (E., p 204). In
general, the action of a muscle when it enters into activity is
to bring closer together the points to which its two ends are
attached. One of these points is usually fixed, and is called the
".origin" of the muscle; the other is movable, and called the
"insertion." But, for special purposes, the parts may be reversed.
For example, the arm muscles may move the arm itself when the
body is fixed, or they may raise the body when the hands are
holding a fixed bar. A very great variety of movements is
rendered possible by the numerous muscles and bones connected
by joints, found in the vertebrate body.
The first point to notice is that the designation " contraction "
is not really a correct one. The muscle does not change in volume,
but in shape. It becomes shorter and thicker. The increase in
thickness can easily be felt in the biceps muscle on the front of the
upper arm.
We know, further, that if we attempt to move a very heavy
object our muscles enter into great activity, but are unable to
93
94 INTRODUCTION TO GENERAL PHYSIOLOGY
change in length unless the object yields to our efforts. The fact
becomes obvious when we break a string by pulling it (E., p. 204).
Take a piece of fine, non-extensible string and find the weight
necessary to break it. Then take another piece and break it by
pulling with the muscles. No change in length of the muscles can
occur until the string breaks, but a force equal to the weight in the
previous experiment must have been exerted on the string in order
to break it, and this was done before the string broke and the
muscles shortened.
The fact is expressed in the statement that the muscle develops
a state of " tension " if not allowed to shorten. This is, indeed, the
more fundamental fact, since it is the production of the state of
tension that causes the muscle to shorten and to do external work.
If we take a coiled steel spring, hang it vertically and increase
its length by pulling upon it, a state of tension is produced in it,
and, by virtue of this, if a weight is attached to its lower end and the
hand pulling it is removed, the tension of the spring does work by
raising the weight (E., p. 204). It is somewhat difficult to realise
the state as applied to muscle. If we take a coil of lead wire
similar to the steel spring and stretch it to the same length, no
tension is developed, because lead has not the elastic properties of
steel. It may be said, then, that a muscle, when it " contracts,"
changes its state from that of a stretched lead coil to that of a
stretched steel coil, without necessarily altering its length.
The details of the way in which this happens and the origin of
the energy set free belong to one of the most difficult parts of
physiological science, and are by no means clear, as yet.
In order that a muscle may be put into a state of activity, we
may apply what is called a " stimulus," either to the muscle itself
or to the nerve which enters it. The most convenient form of
stimulus is an electrical one, since it can be adjusted in strength in
an accurate and simple manner (E., p. 205). But other forms of
stimulation may be used — a tap, heat, or application of salt.
Let us make what is called a " nerve-muscle preparation " from
a frog (E., p. 204). We can make the muscle do work by raising a
weight, although, being cut out of the body, it is impossible for it
to receive any supply of energy from outside itself. It must, there-
fore, contain a store of energy within itself, and may be compared to
a wound-up clock spring, a raised weight used to drive some
mechanism, or again, the cordite charge in a cartridge.
If, by repeated stimulation, we make an isolated muscle perform
a long series of contractions, we exhaust its store of potential
energy ; it becomes " fatigued," in one sense of the word. This
store of energy is not replenished under the conditions of our
experiment ; but we know from experience that a muscle recovers
WORK— THE MUSCLES 95
when in its natural situation, and is supplied with blood. It is
clear that its store of energy is made up again. This is found to
be by the oxidation of some material brought to it by the blood.
It is also found, experimentally, that the supply of energy obtained
in this way follows the act of contraction itself. While there is no
consumption of oxygen in the act of contraction itself, nor any
carbon dioxide given off, both of these take place in the period
following the contraction. That oxygen is not used in the act of
contraction itself is readily proved by the fact that a muscle
can execute a long series of contractions in an atmosphere of
nitrogen.
Some food material is burned, therefore, to supply the potential
energy which a muscle has lost in doing work and to prepare it
for more work. It appears that glucose is used preferably when
available, but that fat or the non-nitrogenous part of protein can
be used. The same amount of food energy is used for a given
supply of muscle energy in each case. We may note here that
the fact that either carbohydrate or fat can be utilised, places a
difficulty in interpreting the muscle system as being a chemical
one, in the strict sense.
The method by which it is discovered whether carbohydrate or
jat is being used in muscular work in any particular case is of interest.
Since the former may be looked upon as having all its hydrogen
already completely oxidised, all the oxygen used is taken up in
oxidising the carbon to carbon dioxide, and the volume of carbon
dioxide produced is equal to that of the oxygen taken in. If, then,
we determine, during a period of muscular work, how much oxygen
is taken in and how much carbon dioxide is given off, and compare
the ratio with that before the work, we shall find this ratio increased
if a larger proportion of carbohydrate is being burned. If nothing
else but carbohydrate is burned the ratio, obviously, is unity. This
ratio is known as the " respiratory quotient." On the other hand,
fat requires oxygen to burn its hydrogen as well as its carbon, so
that the carbon dioxide given off in proportion to the oxygen used
is much less than unity, and the respiratory quotient would be low
when fat is being burned in the organism.
The only chemical change definitely known to occur in the
contractile process itself is the production of lactic acid (E., p. 205).
It is clear that this must arise from some source in the muscle, but
what this is we do not exactly know. In the second stage, which
succeeds the contractile one, and that in which the muscle recovers
its energy by the aid of a combustion process, this lactic acid
disappears, and there Ts evidence that it js_burned in order to give
the energy. Glucose must be taken up in some way in order to
afford the lactic acid produced in a subsequent contraction, but it
96 INTRODUCTION TO GENERAL PHYSIOLOGY
is at present impossible to say how the energy produced by the
oxidation of lactic acid is stored in the muscle.
Some light is thrown on the nature of the mechanism which
causes the characteristic state of tension by two experimental
facts :—
I. It is found that the magnitude of the tension developed, and
therefore of the work done, is greater the longer the fibres of the
muscle are at the moment when the state of tension is brought
about. If the muscle is stretched, a more vigorous contraction is
obtained. This applies, naturally, only within such limits as not
to affect the muscle fibres injuriously (E., p. 206). The fact shows,
in the first place, that change of volume of some elements of the
structure is not the determining factor in the process, because the
volume is not altered by stretching the muscle. In other words,
we cannot look for an explanation of the origin of the tension in
osmotic forces. What has been increased in the experiments
referred to is the length of the fibres and certain constituents in
them. This means that there has been an increase in' the area of
certain surfaces arranged longitudinally. We think at once of that
property of boundary surfaces that results in surface tension, and
that it is by changes in this surface tension that the state of tension
of the muscle, as a whole, is produced. Now, what does the
structure of muscle suggest? Examination of the microscopic
structure of that kind of muscle with which we are dealing, the
voluntary or skeletal muscle, which is under the control of the will
and moves parts of the bony skeleton, we find that it is composed
of long narrow fibres of a protoplasmic material ("sarcoplasm "), in
which are embedded a number of very fine threads of somewhat
complicated nature (" fibrillse ") (E., p. 206). We have provision,
therefore, for the boundary surface between phases demanded by
the surface tension theory. When hydrogen ions make their
appearance in consequence of the formation of lactic acid, or other
( acid, at this contact surface between sarcoplasm and fibrillse, a
1 change in surface tension results. The surface energy provided
^ by this is in proportion to the area of surface on the fibrillae or to
their length, as found by experiment.
II. The second fact which tends to confirm the view that
surface forces are responsible for the tension of muscular contrac-
tion, is that the tension developed is higher at a low temperature
than at a higher one (E., p. 206). As we saw in an earlier chapter,
this negative temperature coefficient is a peculiarity of surface tension,
so far as concerns those various phenomena which could play a
part in the process.
The fact itself is shown also by the behaviour of smooth muscle,
WORK— THE MUSCLES 97
such as that of the alimentary canal, whose state of " tonic " con-
traction is relaxed by warming.
The effect of temperature excludes another explanation which
has been suggested, namely, that acid increases the amount of
water taken up in the swelling of colloidal structures, and that the
arrangement in muscle is such that the swelling causes the shorten-
ing of the fibrillae. This imbibition, however, has the usual positive
temperature coefficient ; is greater as the temperature rises.
There is yet much to be learned about the intimate nature of
the process of muscular contraction, but further discussion would
not be profitable here (P., pp. 436-458).
Gradation of Contraction — " All-or-nothing"
Practical experience teaches us that we can cause our muscles
to contract with different degrees of strength. Since any individual
muscle consists of a large number of fibres, the adjustment might
in theory be made in two ways, either by causing all the fibres to
contract, but with less than their maximum force, or by causing only
a certain varying number to contract, but each always with the same
maximum degree of intensity. If we call to mind the similarity of
a muscle to the propelling charge in a cartridge, we realise that
the former method is less probable than the latter. Although a
certain small expenditure of energy is required to move the trigger,
this has no relation to that set free in the explosion of the charge ;
and whatever the strength with which the trigger is pulled, the
energy set free is the same. The movement of the trigger
corresponds to the stimulus applied to a muscle, and this has no
relation to the energy set free in a contraction. Direct experi-
mental proof, however, shows that the changes in degree ^ of
contractile strength in a muscle are actually due to the putting
into action of a varying number of individual fibres, each work-
ing at its greatest capacity. Of course, this does not mean that a
fatigued muscle can exert the same degree of tension as a fresh
muscle. It means that, so far as any fibre is concerned, whatever
the strength of the stimulus, if it has any result at all, the force
of the contraction is the greatest that this fibre can exert in its
state at the time.
We shall see later that the same statement applies to any
individual nerve fibre, so that it is impossible to vary the strength
of the stimulus to a muscle fibre. Thus, even if the latter were
capable of different degrees of contraction, there is no means of
altering the strength of the normal stimulus so as to make use of
the property. In the nerve, as in the muscle, it is a question
of "all-or-nothing." As in the muscle, adjustments are made
by altering the number of fibres in action.
7
98 INTRODUCTION TO GENERAL PHYSIOLOGY
Refractory Period
A word must be said next about another important property
of muscle and other excitable tissues. If a second stimulus arrives
at a brief interval, a fraction of a second, after a previous one, the
second stimulus produces no effect. This interval of time during
which the muscle is inaccessible to stimulation is known as the
"refractory period," and is exhibited while the muscle* is in the
initial stages of giving effect to the first stimulus (E., p. 208). If
we remember the evidence that an essential part of the process
of excitation consists in an increase of permeability of the cell
membrane, we see that it cannot be repeated until the membrane
has recovered its normal state of semi-permeability.
"Staircase"
Another interesting phenomenon is that of the " staircase." If
a muscle has been at rest for some time, it -will be found that the
strength of the contraction increases for each successive stimulus
during a few contractions (E., p. 207). It appears that a certain
very small degree of acidity is that best adapted for maximum
contraction. As we saw, lactic acid is produced in contraction, and
a trace is left after each contraction, gradually increasing until it is
oxidised as fast as it is formed.
Tetanic and Voluntary Contraction
The simple form of contraction which follows a single electrical
shock lasts an appreciable time, varying with the particular muscle
in question. In the frog it lasts about a tenth of a second. Now
the refractory period referred to above lasts only something over
a thousandth of a second. If, therefore, a second stimulus arrives
later than this, but during the time in which the muscle is shorter
than at rest, a further shortening takes place, and another stimulus
and shortening may be superposed on this. Each succeeding
stimulus after the first, however, has somewhat less effect on the
length of the muscle than the one before it, so that, after a certain
number, the height becomes practically steady, but much higher
than that produced by a single stimulus (E., p. 206). This state is
known as a ''tetanic" contraction, and is similar to that resulting
from a normal discharge from the nervous system, which consists
of a series of stimuli, varying in number according to the length of
time that the muscle is required to remain in contraction.
WORK— THE MUSCLES 99
Muscular Mechanisms of Various Kinds
It is not only for the purpose of bringing about effects in the
outer world that muscles are made use of. Those concerned with
breathing and in speech, and those by which the eyes are moved,
may be referred to. The importance of the latter will be seen
later.
That kind of muscle called smooth or involuntary has been
mentioned already in connection with the movements of hollow
organs, such as the alimentary canal, and its general properties
have been described. The heart and blood vessels will be dealt
with in a subsequent chapter.
Posture Phenomena
There are some rather remarkable phenomena exhibited, especi-
ally by involuntary muscle, but also in a certain way by voluntary
muscle. They are not yet completely understood, but are of much
importance. If we try to keep a weight raised with the arm out-
stretched, we soon become aware that a continuous expenditure of
energy is required. On the other hand, a bivalve mollusc, such as
an oyster, is able to keep its shell firmly closed, even when continu-
ally pulled upon by a weight, for a long time without signs of fatigue
or evidence of consumption of material. There appears thus to be
a possibility for certain muscles to maintain themselves at various
lengths, which oppose resistance to stretching, but without the
presence of a state of tension. It is as if they had become fixed at
a particular length, as by freezing, and that a kind of thawing pro-
cess was necessary in order to restore them to their original state.
We may picture the state as being analogous to the holding up of
a weight, after it has been raised to a height by the expenditure of
energy, by slipping a support underneath it. It does not fall again
until the support is removed. The process of relaxation in the
muscle, corresponding to the removal of the support, is brought
about by the stimulation of a nerve, and does not take place other-
wise ; this nerve is a different one from that which induced the
shortening. Thus, if certain nerves supplying the closing muscle
of the mollusc be cut while the muscle is in a state of contraction,
it remains permanently at this length, unless the end of the nerve
in connection with the muscle is stimulated, and then relaxation
occurs. These properties are exhibited by the urinary bladder
of the vertebrate in a striking way. If this organ were like an
india-rubber ball, the greater the filling the higher would be the
tension of the walls and the pressure inside it. But this is not
the case. It may possess very various degrees of tension with the
ioo INTRODUCTION TO GENERAL PHYSIOLOGY
same degree of filling ; or conversely, various degrees of filling
may coincide with the same tension. The muscle of its walls has
\ the power of altering its length to accommodate the contents
1 without changing its tension, just as we can voluntarily adjust
the grasp of the hand so as to exercise the same pressure on a
large or on a small ball. There is also reason to believe that
the muscular coat of the small blood vessels, which prevents their
over-distension by the pressure of the blood, has properties of the
same kind.
Something of the same kind is shown by the voluntary muscle
of the vertebrate ; but in this case it is more directly brought about
through the nervous system. After removal of certain higher parts
of the brain, it is found that a limb offers resistance to a change in
position, because some of its muscles are in a state of shortening.
When this resistance is overcome, the limb remains in the position
in which it has been placed, although its own weight may have to
be held up against gravity. This reaction is due to the stimulation
of certain nerves in the muscle substance, which convey messages
to the nerve centres, and the result is a reflex stimulation of nerve
fibres causing the peculiar form of contraction. There is evidence
that this " postural " state requires the expenditure of much less
energy than the voluntary production of the same degree of
shortening. It appears that conditions of this kind are to be met
with in some forms of " contracture," met with after injury, although
not directly due to it (P., pp. 333, etc.).
The suggestion has been made that it is the sarcoplasm of the
muscle that is responsible for the phenomena spoken of in the pre-
ceding paragraph. But the proof is not complete.
Energy for Other Purposes
From various statements in the previous pages of this book, it
j will be realised that a supply of energy is needed for such purposes
as raising osmotic pressure, chemical reactions in which potential is
raised, and so forth.
A useful index of the amount of energy required by an organ
is the oxygen consumed by it, since oxidation is the source of the
energy. This can be found by comparing the oxygen present in
the blood going to the organ with that in the blood leaving it in
a given time. This has been done in the cases of the secreting
glands and the voluntary muscles already mentioned.
All living cells are found to consume oxygen, although it is
not always obvious for what purpose they require energy. It has
been suggested that it is to prevent diffusion, to maintain the
integrity of membranes, and other purposes of this kind.
WORK— THE
The Maintenance and Regulation of Temperature
In the first stage of muscular contraction, in which the
potential energy is converted into tension, no loss in the form of
heat is to be detected. That is, the whole of the potential energy
lost appears in the form of mechanical tension, which can perform
external work. If no external work is done, on the other hand,
this energy becomes heat, and there is always heat produced in the
restitution phase, since only a part of the energy obtained by com-
bustion is stored as potential energy in the muscle system. We see,
therefore, how the temperature of warm-blooded animals is kept up
by muscular activity. The advantage of having a raised tempera-
ture is that the numerous processes, physical and chemical, go on
at a faster rate, the former being less affected than the latter. It is
even a debateable question whether the raised temperature in fever
is not beneficial in the destruction and elimination of the bacteria
and the poisons they produce.
The heat produced in muscular activity serves, then, to main-
tain the raised temperature in warm-blooded animals. But in
muscular exercise too much is produced, and we become too hot.
How do we get rid of the excess ? The most effective way is the
familiar one of sweating, since the evaporation of water requires a
large amount of heat energy, which is drawn from the skin and
indirectly from the blood. Evaporation of water from the lungs
must also be added. A less effective way is by widening the blood
vessels in the skin and allowing more loss by radiation, and by
heating the air by conduction.
In hot weather we make use of yet another means, that is, by
reducing muscular activity as far as possible.
In cold weather we diminish loss by narrowing the blood
vessels of the skin, and we increase production by greater muscular /
activity. One form of the latter is "shivering" — an automatic
method of keeping warm. A hibernating mammal, on waking up,
raises his temperature in this way with rapidity.
Thus, the most effective way of lowering the temperature is by
sweating ; of raising it is by muscular activity.
Since the source of our energy is food — and we need less heat
energy in hot weather because we lose less to the surroundings — it
is clear that less food is required in the summer.
There are, as we see, several factors involved in the regulation
of our temperature, so that the necessity of a co-ordinating centre
is obvious. Such a centre has been found in a part of the brain,
situated between the highest intellectual parts and the more
automatic parts. This centre is so arranged as to be sensitive to
the temperature of the blood passing through it. If this tempera-
TO GENERAL PHYSIOLOGY
ture is raised, the various means for increasing loss of heat and
decreasing its production are set to work. If the temperature is
lowered, those for decreasing loss and for increasing production are
set to work.
The mechanism for controlling loss of heat appears to have
been developed later in the course of evolution than that regulating
production. It seems natural that an animal, finding itself getting
too hot from exercise, should diminish first of all the amount of
heat being produced, if circumstances permit becoming quiet.
CHAPTER IV
STIMULATION— THE SENSES
IN order that any organism may be able to make use of, or adapt
itself to occurrences, in the outer world, it must possess means
of obtaining knowledge of what is going on there. The various
things that happen must, in some way, produce changes in the
outer surface of the organism that is accessible to their influence.
In other words, there must be structures capable of being "stimu-
lated," or changes produced in them, by the forms of energy that
strike upon them.
When this has taken place, the nerves connected with these
" receptors " or organs of sense, as we may now call them, convey
messages to the brain. They are then perceived in consciousness
in a manner at present inexplicable, and may, sooner or later,
result in muscular activity adapted to take advantage of the
information received.
It will be seen that we cannot properly separate the discussion
of the senses from that of the nervous system, and we might have
taken the latter into consideration first. But whichever order is
chosen, it is impossible to treat either one without assuming
or forestalling what must necessarily be described later. Indeed,
although for convenience it is usual to subdivide physiological
phenomena into sections, they are, in reality, all parts of one system
acting as a whole. This will have been manifest to the reader
already, and for this reason no physiological text-book can be
understood by reading it through once.
If we take a frog whose central nervous system consists of the
spinal cord only, a "spinal frog" as it is called, we shall find that
by stimulating the skin in a variety of ways we can produce
movements (E., p. 209). These are called " reflex," because the
message conveyed to the nerve centre is " reflected back " along
another set of nerves, and causes muscular contractions. The name
" reflex " is thus given to those movements which result from a
stimulus without necessarily involving conscious perception of the
stimulus. The phenomena of consciousness are only present when
the highest part of the brain, the cerebral hemispheres, are intact.
104 INTRODUCTION TO GENERAL PHYSIOLOGY
There must be channels along which the messages are conveyed
to the nerve centres and back again to the muscles. These are
the white threads called " nerves " (E., p. 209), which consist of
a number of separate fibres, each carrying its own message apart
from the rest. Nothing can be seen to happen either in the nerve
or in the nerve centres. In the nerve-muscle preparation which
we made previously, a stimulus applied at the far end of the nerve
caused the muscle to contract, although there was no sign of
anything passing along the nerve.
The student should examine the general arrangement of the
central nervous system in a frog or rat (E., p. 209). But, at the
present stage, details are unnecessary.
Let us next see what are the different kinds of sensations we
receive from various external agencies. If the skin is pinched, we
feel pain. If touched gently, there is no pain, but a sensation
of a different kind. If a warm object is held near the skin, we have
a sensation of heat. A cold object produces a sensation which is
distinct from that of heat. All these are from the skin. By the
eyes we perceive light. By the ears, sound. By another receptor,
anatomically associated with that for the perception of sound, but
having no physiological connection with it, we are informed of
changes in our position in space, or our relationship to the direction
of gravity. By the nose we smell, and by the tongue we taste.
There are, thus, nine different kinds of sensation, each corresponding
to some distinct property of external nature. The receptors which
enable these sensations to take place must therefore each possess
a structure which is appropriate to some particular form of external
energy, so that a change may be effected in it by that form of
energy when it obtains access to the receptor. A structure
sensitive to light would be unaffected by sound waves, and so on.
A not inappropriate illustration, as we shall see later, would be
a photographic plate, in which chemical changes are produced
by light, but not by sound. The change brought about in the
receptor must be of such a nature and magnitude as to act as
a stimulus to the ends of the nerves which arise from this receptor.
We saw that pressure is able to stimulate a nerve when applied
directly, but, in order to do so, it must be far greater than the
degree of pressure involved in the sense of touch. It would seem,
in this case of touch, that all that is necessary is some form of
mechanical magnification of the action of the external agent. In
other cases, as those of sound and light, the nerve itself is unaffected
by them (E., p. 210), and it is necessary that they shall set into
activity some, mechanism which has the result of producing a form
of stimulus to which the nerve is sensible. It appears that the
energy value of an actual stimulus to which a sense organ can
STIMULATION— THE SENSES 105
respond is not great enough to stimulate the nerve endings, even
when converted into an appropriate form. It must act, therefore,
as a trigger, or an electrical relay, setting off some store of
potential energy present in the receptor mechanism.
All evidence available goes to show that, so long as a nerve
fibre is stimulated at all, the process set up in it, and passing
as a disturbance along it, is the same in all kinds of nerves, and
always of the same magnitude. We have seen this to be the case
with muscle, and it has also been shown, experimentally, to be the
same with motor, efferent nerves. But the direct experimental proof
is yet wanting for sensory, afferent nerves. The way in which the
fact applies to the phenomena of sensation is expressed in the law
known as that of " specific sense energies," a somewhat unintelligible
phrase. What is meant is that, whatever the manner in which
a nerve connecting a special receptor with the brain is stimulated,
the sensation is always that associated with stimulation of this
organ by its appropriate form of external energy. It matters not
how the nerve from the eye is stimulated ; the sensation is that of
light. The clearest case is that of one of the nerves of taste, which
passes through the ear in a way accessible to stimuli. Whether
these stimuli be electrical, mechanical, thermal, or chemical, the
sensation is one of taste, and nothing else. The object of each
receptor mechanism is then to provide a stimulus of some sort
to its nerve, no matter what. All that is necessary is that the
arrangement shall be such that the external influence shall effect
a change which actuates a stimulating agency.
The process may be illustrated thus : the nerve may be com-
pared to an electrical circuit which can be connected up to a battery
by closing a switch. It does not matter how this switch is closed.
But, if light be the agent, it is clear that something sensitive to
light must be present and be made to close the switch, say, by a
current produced in an electro-magnet by a photo-chemical cell.
If by sound, something similar to a microphone, and so on. These
examples are not to be understood as implying that such are the
actual means adopted in the eye and the ear.
In physical measurements we can convert any form of energy
into an electrical current by a proper means, and in the physiology
of the senses any form of outside stimulus is converted into one
and the same form of nerve impulse.
But, it will naturally be asked, how can we distinguish sights
from sounds, taste from touch, if the messages differ only as
regards the particular nerve by which they arrive at the brain?
We here come into contact with the mysterious relation between
consciousness and the physiological changes in the brain. ^ All that
can be said is that when a particular region of the brain is set into
io6 INTRODUCTION TO GENERAL PHYSIOLOGY
activity, we experience something which differs in quality frorr
that associated with the activity of another region. And this
applies down to the individual cell at the end of each nerve fibre.
It is somewhat as if a man lives in an office in which electric
bells are fitted in various positions on the walls. The bells are al
alike, but each is connected with a different kind of factory in the
town. When the bell in one corner rings, the man knows that i
silk factory is at work ; when that in another corner rings, a brass
foundry starts work, and so on. But we have also to suppose thai
the visualising power of the man is good enough to picture the
factories as if he were there.
When a message comes along a nerve fibre from the foot or the
hand, we refer it correctly to its place of origin, although there i<
no reason to suppose that the process in the nerve fibre itsel
differs in the two cases. It is merely that it passes to a difTereni
place in the brain. The psychological reader will recognise thai
we are concerned with what has been called " local sign."
We may now proceed to discuss, more or less briefly, the
different kinds of receptors.
Physiologically, the most primitive and simple is the sensatior
of pain, associated with the action of something that is likely tc
cause actual injury. In this case there are no specialised receptors
The nerve fibres come to an end between ordinary cells, and the
stimulus acts directly upon the nerve itself. The sensitiveness
is therefore not great. It would indeed be a disadvantage if ii
were, since the muscular reactions due to pain are usually powerful
and it would be undesirable to provoke them unless there were
actual risk of injury. The protective function of pain would be
defeated if innocuous contacts excited it. The skin contains nerve
endings of this kind, along with specialised receptors. The eel
layer covering the front of the eye contains no other kind of sense
organ, and is sensitive to pain only.
The sense of touch is associated with special recepton
adjusted to be responsive to very slight degrees of deformation
These receptors are localised in spots in the skin, usually arounc
hairs. Although the presence of the hairs increases the sensibility
apparently by some kind of lever action, the sensation of touch i<
still present when the hairs are removed. The whiskers of the cal
are extremely sensitive organs of touch, and their roots in the skir
are copiously supplied with nerves. The structure of the various
receptor organs for touch does not throw much light on the way ir
which they act.
Heat and Cold. — If an object, applied to the skin, is at a highei
temperature than the skin itself, we call it warm; if at a lowei
temperature, it is said to be cold. Like touch, there are separate
STIMULATION— THE SENSES 10;
spots sensitive to temperature, and, a rather curious thing, there are
different receptors for heat and cold (E., p. 21 1). The nature of a
specialised receptor may, to some extent, be realised by stimulating
with an electrical current a spot sensitive to cold, for example. A
sensation of cold, and no other, is produced, but the strength of the
stimulus necessary is very much greater than when it is the normal
one of cold. This means that the mechanism is specially adjusted
to be affected by the withdrawal of heat. In what way this is
done we cannot say. It has been suggested that it may be by
some chemical reaction which is very sensitive to change of
temperature, or some effect on volume may be concerned.
Taste and Smell. — These may be called "chemical senses,"
because they depend on the properties of substances acting in
watery solution on the receptors. But it must be remembered that
the properties are not ordinary chemical ones, since there are a
number of compounds which taste sweet, although there is nothing
in common in their chemical nature.
The skin of fishes has a kind of generalised chemical sense,
such as would naturally be expected to make its appearance at an
early stage in evolution, in response to the variety of chemical
substances given off to water by other animals and plants. It
seems probable that the senses of smell and of taste of the higher
animals have developed from this. It should be remembered also
that the sense of smell plays a large part in the life of water animals.
In one case, that of acid substances, the taste is definitely in
relation to the hydrogen-ion concentration.
With the exception of smell, the senses hitherto described
require the actual contact of objects with the surface of the body,
and they give us no warning of the approach of distant influences.
Although touch gives valuable information of the properties of
objects, and guides us in muscular movements, while smell,
especially in certain organisms, is of value in warning of distant
occurrences, it is by sight and hearing that accurate information is
obtained of such things. It is to these " distance receptors " that we
owe the greater part of our higher intellectual life. The mode of
action of the receptors in these cases is a complex one, but, never-
theless, it may be said that we know more about it, up to a certain
point, than about the apparently simpler cases.
Hearing or the Receptor for Sound.— The phenomenon in the
outer world that arouses in us the sensation of sound is an alternate
condensation and rarefaction of the material of which bodies are
composed, transmitted in the form of waves. If we confine our
attention to one point in the air, for example, we notice that the
air becomes alternately denser and rarer. What is known as \bzpitch
of a note is the number of times per second that this process takes
io8 INTRODUCTION TO GENERAL PHYSIOLOGY
place. The number of vibrations in what we call musical sounds
lies between about 40,000 and 30 per second. What we call the
loudness of a sound depends on the degree of changes in density,
or, what comes to the same thing, to the amplitude of the back-
ward and forward movement of the particles of the vibrating
substance, since the more they have congregated together at one
moment, the further have they come. There is another property of
sound, shown most markedly by the difference between the same
note played on the violin and on the flute. This is called quality,
and will be referred to presently.
What the ear has to do, then, is to transform periodic changes
in density of the air into something of the nature of an actual
pressure or pull upon the endings of nerves, in such a way as to
stimulate them. When these air vibrations enter the ear, they
come against the " drum," a membrane stretched across the passage.
The membrane is caused to move in and out by the periodic
changes of pressure upon it. The important point is that it moves
equally well to any rate of vibration, on account of the fact that
it has no particular rate of its own, as an ordinary drum has. This
is partly due to the shape of the membrane and partly to the fact
that it is connected to a series of small bones which prevent its free
vibration. The result is that it follows exactly the smallest
changes in air pressure and passes the movement on to the end of
the chain of bones, unaltered in wave form, but, owing to the lever
action of the bones, .diminished in amplitude and correspondingly
increased in force. The further end of the bony lever is fixed to a
small membrane covering an aperture at the end of a canal in hard
bone. This canal has a spiral form, like a snail's shell, hence
called "cochlea," from the Latin name. It contains liquid, and on
this liquid a periodic series of pressures is exerted by the end of the
chain of bones. Suspended in the liquid is a complex structure in
which the auditory nerve ends, the " organ of Corti." The details
of this organ can only be given here in a general way, so far as
necessary to understand its mode of action.
But, first of all, what is the nature of the vibrations set up in
the liquid by the periodic changes of pressure upon it? Let us
see what would happen supposing that the sound waves in the air
hit directly the end of such a column of liquid. We know that
they are transmitted, and may be transferred to air again at the
opposite end of the column. Sound can only be transmitted owing
to the elasticity and the compressibility of the material conducting
it. If this material were devoid of elasticity, the particles in
vibration would not return after being displaced, and if it were
incompressible, the alternate states of condensation and rarefaction
would be impossible. Although liquids are almost perfectly elastic
STIMULATION—THE SENSES
109
in the physical sense, they are, compared with gases, only very
slightly compressible. The result is that the amplitude 'of the
sound vibrations in a liquid is excessively minute, but the force
involved is of correspondingly increased magnitude. We have
seen that the changes of pressure exercised by the chain of bones
FIG. 4. — Mechanism of the Organ of Corti.
Upper diagram — at rest. Lower diagram — when displaced by vibration.
A, represents the basilar membrane.
B, the arch of Corti.
C, the reticular membrane.
D, one of the hair cells.
E, the tectorial membrane.
F, a fibre of the auditory nerve.
on the liquid in the cochlea correspond exactly with those in the
air, in contact with the drum, except that their amplitude is
decreased and their force increased. This would clearly be an
advantage in transmitting them to liquid. But, in other respects,
they must be precisely similar in the liquid to what would have been
the case if the sound waves had impinged directly on the liquid
itself. In this liquid we have, then, waves identical with those of
sound.
i io INTRODUCTION TO GENERAL PHYSIOLOGY
Next, supposing that we have immersed in water a spring
which is capable of vibration at a certain rate, like the wires of a
piano, and that we send, by some means, sound waves of this rate
into the water. The spring will be set into sympathetic vibration
by resonance in the way previously explained (p. 52 above). But
if its rate is not that of the sound waves, it will remain at rest.
Have we then anything that might act in this way in the cochlea ?
One of the component parts of the receptor structure immersed
in the liquid of the cochlea is a membrane, the " basilar membrane,"
to which other parts composing the organ of Corti are attached.
This membrane is a strip narrower at one end than at the other,
and is stretched transversely by being attached to the bony walls
at both sides. In a longitudinal direction it is lax. It also con-
tains fibres arranged transversely. Such a membrane can be
shown mathematically to have a series of different rates of vibration
in order from one end to the other, so that a narrow section
would respond by resonance to a higher note than a broader
one (E., p. 212).
All that we need further is a mechanism by which the
vibrations of each section can be made to stimulate a particular
nerve fibre and we have the means of distinguishing between notes
of a different pitch or rate of vibration. The precise means by
which this is done is difficult to make out, but it seems to be that
represented as a diagram in Fig. 4. There is a series of arches,
jointed at the top, arranged along the membrane. One foot of the
arch rests on the basilar membrane near one of its attachments, so
that it is practically immobile. The other foot rests on a part of
the membrane which vibrates up and down as represented. The
result of this is a movement of the top of the arch chiefly in the
direction from right to left in the plane of the paper, and back
again in the opposite direction. Attached to these arches is a
membrane ("ret&ufar") with holes in it. Through these holes
project stiff hairs attached to cells below it. The points of the
hairs appear to be more or less fixed by being stuck against
another soft membrane (" tectorial"). When the reticular mem-
brane therefore is pulled backwards and forwards by the up and
down movements of the basilar membrane, the base of the hair is
pulled through, or together with, the cell to which it is attached,
and exerts pressure on the termination of the auditory nerve which
ramifies in or upon the cell.
It will be clear that there must be as many nerve fibres and
elements of the organ of Corti as it is possible to distinguish in
difference of pitch. It is said that about 1 1,000 different notes can
be distinguished, and the number of fibres in the cochlear division
of the auditory nerve has been found to be 14,000. The number
STIMULATION— THE SENSES in
of Corti elements appears to be sufficient also, but they are more
difficult to estimate.
So long as a disturbance is sent along a particular nerve fibre,
it does not matter whether or not this disturbance corresponds
in its form, or in the rate at which separate impulses follow one
another, with the sound vibrations in the air. When it reaches
a particular region of the brain, we have the sensation of a certain
note, in which the separate vibrations are not distinguished.
FIG. 5.— Compound Wave Forms resulting from Fusion of a Vibration of a
certain rate with one of twice that rate, in two different Phase Relations
with each other.
It should be mentioned that the view according to which the
basilar membrane responds to different rates of vibration by
resonance is due, in the main, to Helmholtz. Although it is more
in agreement with all the facts than other theories, there are some
which assert that the basilar membrane vibrates, as a whole, to all
notes, the wave form of which is held to be transmitted to the
brain in all its detail, so that the analysis is performed there.
A few words are necessary on the perception of quality in
musical sounds. Why is the same note played on the violin and
on the flute so different? It is because, in the first case, the
ii2 INTRODUCTION TO GENERAL PHYSIOLOGY
fundamental note itself is accompanied by a large number of other
rates' of vibration due to the subdivision of the string into various
numbers of parts of shorter length, each giving rise to a note
of higher pitch, in some multiple of the fundamental. The presence
of these harmonics, as they are sometimes called, can be detected
by the use of appropriate resonators. The form of the air waves
resulting from the combination of these harmonics with the
fundamental note is usually represented by compound sine curves,
such as those of Fig. 5. But it is to be remembered that although
such curves correctly represent the movements i of particles in
transverse vibration, or the ether waves of light}, they are only
diagrams of the changes of pressure in sound wa^es. The height
of the ordinates of such curves is to be taken as/representing the
series of pressures at a given point. One may/ realise, to some
extent, the kind of thing that would correspond to the upper curve
of Fig. 5, by imagining what would be shown py a manometer.
The pressure would rise quickly and fall gradually. Whereas
in the lower curve it would rise slowly and fall quickly. In some
other cases it would, after having fallen somewhat, rise again and
then continue the fall, and so on. Looking atjsuch curves it is
difficult to believe that they are composed of a Dumber of simple
vibrations, and that an appropriate resonator can pick out any one
from amongst them.
It might be supposed that the comparison of two such wave
forms as those of Fig. 5 would serve as a test of the correctness of
the resonator theory of the cochlea. If this theory be correct,
there should be no difference in quality if the phase relation is
altered, because the two resonators are independent, and each picks
up its own rate of vibration regardless of the other. If, on the
other hand, the basilar membrane vibrates, as a whole, in a wave
form corresponding to that of the sound, and transmits this by the
nerves to the brain to be analysed there, then phase difference
should be appreciated. Unfortunately, there is no agreement on
the fact. But it seems rather doubtful whether the methods used
by those who state that phase difference is of importance were such
as to exclude other effects on the components of the complex waves.
The Eye— Receptor for Light
The eye may be said to be the most accurately adjusted of all
our receptor organs. It is adapted by its movements and great
sensibility to give us more correct and valuable information about
things that are happening, both near and at a distance, than any
other organ of sense.
As previously mentioned, light consists of transverse waves in
STIMULATION— THE SENSES 113
a medium which is not material, at least, not matter in the ordinary
form.
Some idea of the kind of vibration in question may be gathered
by watching a sea-gull floating on the sea. It will be seen to rise
and fall, as a wave passes under it, without permanently altering its
position in relation to objects around it. Such a vibration is in
one direction only, the vertical, and in the case of light would be
called a polarised beam. Ordinary light consists of vibrations in
all directions at the various angles with this.
We have already seen that large quantities of energy are trans-
mitted to us from the sun by wave motion of this kind, and that it
is only a limited range of wave lengths or rates of vibration that
we perceive as light, although the longer wave lengths can be per-
ceived by the heat receptors of the skin and the shorter ones have
powerful chemical effects. Waves of a greater length than the
longest of the solar spectrum can be produced by electric dis-
charges, and form the basis of wireless telegraphy. Waves much
shorter than the ultra-violet of the spectrum are known as X-rays
or Ron tgen -rays', which have remarkable powers of penetrating
substances opaque to ordinary light.
The manner in which the vibrations of wireless telegraphy,
electric waves, are produced, reminds us that light is an electrical
disturbance, although there are still difficulties to be explained in
connection with the relationship between the moving electrons and
the transmission in wave form.
The first question that arises in connection with the perception
of light is, what effects capable of being used to stimulate nerve
fibres does light produce when it falls upon material objects ?
Although, as would be expected, there are certain electrical effects
to be detected, the most obvious ones are heat and chemical change.
The means of perceiving the former are not nearly delicate enough,
and there is every reason to believe that the immediate cause of
the stimulation of the endings of the optic nerve is by a so-called
photo-chemical reaction. The photographic plate shows us how
sensitive such a reaction can be made, although the mechanism in the
eye is much more sensitive than the most rapid plate. The fact of
a chemical change produced by light is readily seen in the case of
" printing out paper." The change does not go so far on the dry
plate used in the camera as to be visible, but the fact that an image
appears on development by a reducing agent shows that a chemical
effect had been brought about.
The skin of some lower organisms appears to be sensitive to
light, but such a general sensibility would only give information
of the approach of another object by the shadow cast by it, and it
is not until specialised eyes are developed that the perception of
ii4 INTRODUCTION TO GENERAL PHYSIOLOGY
light takes the important place that is attained by it. It is, how-
ever, very early in the scale of evolution that eyes are found. The
jelly fish possess them, although they do not possess the necessary
elaboration of structure required to form distinct images. The
perception of sound, contrary to that of light, seems to be a
comparatively recent development.
The layer or coat at the back of the eye in which the nerve
fibres end is called the " retina " (E., p. 214). It contains a substance
called "visual purple," which is sensitive to light (E., p. 212). We
saw, in discussing the action of chlorophyll, that in order that light
energy should have any effect it must be absorbed, and the
magnitude of the effect is naturally in proportion to the amount
of light absorbed. Investigations of the properties of visual purple
have shown that its absorptive power for different parts of the
spectrum agrees with the sensibility of the retina to these parts.
Further, the effect of light in bleaching the pigment follows the
same course, and also does the apparent brightness of the different
parts of the spectrum. We are therefore justified in regarding this
pigment as the seat of the photo-chemical reaction at the basis
of vision (see P., p. 521).
But a mere sensibility to light would be of comparatively
little value. It is necessary to have a means of producing a
picture of external objects on the retina, so that different parts
of this picture may stimulate separate nerve fibres, and a
representation of it be conveyed to the brain.
A familiar method of producing a picture on a sensitive surface
is that of the photographic camera, and it will be instructive to
compare its essential parts with the corresponding parts of the
eye. The student should examine these parts in the eye of an
ox (E., p. 213). The sensitive plate, as we saw, is represented by the
retina, and on this an image is formed by means of a lens which
gives a real image. Such a lens consists of the convex surface
of a medium having a higher refractive index than air. The ray
from each point of an external object is bent towards the centre
of the lens when it strikes it, and in proportion to the distance from
the centre of the lens at which the ray enters. At a certain
distance behind the lens an image is produced. If the object
is distant, the focal plane, as it is called, is nearer to the lens than
if the object is at a less distance, and the size of the image is less,
the nearer it is to the lens (E., p. 163). That part of the eye which
takes the place of the lens of the camera is not, as might be
thought, that structure which is actually called the " lens " of the
eye. This plays a comparatively small part in the formation of
an image, but has another function, as will be seen presently. The
actual lens is the front clear spherical surface of the eye, known as
STIMULATION— THE SENSES 115
the cornea. This can easily be seen by a simple experiment on the
eye of an albino rabbit (E., p. 213). But remembrance of the fact
that the liquid in the eye has a higher refractive index than air, and
that it is bounded by a spherical surface, is sufficient to bring
conviction. The material of the lens itself has a refractive index
not much higher than that of the liquid in which it lies, so that the
actual refraction due to it is not great. What it does is to adjust
the focal length of the dioptric system of the eye, so that sharp
images of objects at various distances from the eye may be formed
on the retina. This it does by altering its curvature. The greater
the curvature of a refracting surface, the shorter its focus. Many
photographic lenses are double, so that each part can be used
separately. One part is often of shorter focus than the other, and
can easily be seen to have a more curved surface. The mechanism
by which accommodation to objects at different distances is effected
in the case of the eye of the higher vertebrates is, briefly, as follows : —
The lens is an elastic body, which has, when released from its
position in the eye, a particular natural curvature. In its normal
position in the eye it is pulled flatter by the way in which it is held
stretched between membranes in front of it and behind it, which
are kept in a state of tension. There is, further, a ring of muscle,
the ciliary muscle, whose fibres are arranged in such a way that
when they contract they pull the place to which the suspension of
the lens is attached nearer to the lens itself, and thus lessen the
tension on it, allowing it to approximate more or less to its natural
curvature. Its focal length is diminished, and the image of a near
object, which would otherwise be formed beyond the retina, is thus
brought to lie nearer to the lens and on the retina itself.
This is not, of course, the way in which the photographer adjusts
the focus of his camera ; he moves the lens backwards and forwards,
since its curvature is fixed. In some of the lower animals, indeed,
a means of accommodation like that of the camera is adopted, a
muscle being present to change the distance of the lens from the
retina.
There are two further arrangements common to the camera and
to the eye. The diaphragm, which enables sharper images to be
formed by limiting the part of the lens used to the middle, naturally
with loss of light, is represented by the iris, the coloured screen
with the aperture, the pupil, in front of the lens. The iris contains
muscular fibres arranged in a radial direction, which enlarge the
pupil when they contract, and others in a circular direction, which
narrow it. In the eye, however, the chief use of the iris is to
prevent excess of light from reaching the retina, and the improve-
ment in sharpness of vision is secondary, although advantageous
when the light is strong enough to permit it.
ii6 INTRODUCTION TO GENERAL PHYSIOLOGY
The other arrrangement corresponds to the bellows of the
camera, to keep out stray light from acting on the plate. It is
represented by the eye-ball itself, which is lined by a layer of cells
containing black pigment. This pigment layer is to be found in
the very simplest eyes, and is clearly of much importance.
The retina in the vertebrate is a very complex structure of
several layers of different kinds of cells (E., p. 214). But several of
these layers properly belong, not to the receptor organ itself, but
to the nerve centres. In the cuttle-fish they are in a separate
nervous mass, outside the eye. The actual receptive layer is that
of the rods and cones. That the cones are the elements concerned
with accurate vision is obvious from the facts that this is in direct
relation to the number of cones present in a given area, and that
the central part of the retina, where the most accurate vision is
present, contains cones only. The function of the rods is somewhat
obscure, but their nervous connections are very similar to those of
the cones, and it seems that they must also be percipient elements
of some kind. The rods and cones lie in a solution containing
visual purple, and when a bright part of an image is formed at a
point on the retina, the photo-chemical change in the sensitive
substance causes the cones, and perhaps the rods, with which it is
in contact to be affected in such a way as to stimulate the nerve
fibres in connection with them. Whether this is by a chemical
action or by the resonance of molecules to particular wave lengths
is not yet clear, but the phenomena of after-images, to be referred
to below, suggest that the former is the case. We saw, in discussing
the chlorophyll system, that light energy is absorbed by a system
for the reason that a certain molecular group has a vibration rate
which is in unison with that of the light which it absorbs. The
resonant vibrations may be great enough to result in chemical
decomposition.
That a change is produced in the visual purple such that a
certain time is necessary for a return to normal is familiar in the
negative after-images, where a part of the retina, on which the
image of a bright object has fallen, remains for a time less sensitive,
thus causing the appearance of a dark patch in the field of view.
The regeneration of the visual purple is of interest, because a
similar phenomenon is met with in some of the simpler photo-
chemical reactions, such as that of silver chloride. Suppose that
we have some of this compound in a sealed tube and allow sun-
light to act upon it. It turns purple, chlorine being given off,
and metallic silver in a finely divided, colloidal form being left.
Now chlorine and silver have a strong affinity for one another,
and if the tube be placed in the dark they recombine. But
this recombination takes place whether light is acting or not, so
STIMULATION— THE SENSES 117
that it must always be going on, even while light is acting. Hence
there is a balance between the decomposing action of light and the
recombination of the products, such that the composition of the
system depends on the intensity of the illumination. Since this
balance only lasts as long as external energy of light is being
supplied, it is not a true chemical equilibrium. The reason why
the image formed on a photographic plate does not disappear after
exposure is because there is gelatin present, and the chlorine or
bromine liberated combines with the gelatin, and is not available to
recombine with the silver.
Other phenomena, whose meaning is not yet clear, are produced
in the retina by light (P., pp. 519-525). Among these there is an
interesting electrical change.
The perception of colour is a question about which opinions are
somewhat at variance. When we look at the spectrum there are
to most of us six distinct colours in it — red, orange, yellow, green,
blue, and violet. A few people, like Newton, see a distinct colour,
indigo, between blue and violet. All other colours can be formed
by combinations of these with each other and with black or white.
Whether there is a distinct variety of visual purple for each of the
six primary colours, or whether each of these affects the same
substance in a different way, is unknown. It is certain, however,
that visual purple, as we know it, absorbs light of all parts of the
spectrum ; but this may be due to its being a mixture of six
substances.
Position-receptors
These receptors are of two kinds, and they give us information
of our position in relation to the direction of gravity or of the
direction in which our bodies have moved. They may be called
position-receptors, and make use either of the weight of particles
to stimulate nerve endings, or of the inertia of liquid in its refusal
to take up suddenly the movement of a vessel which contains it.
Practically all multicellular animals, and plants too, have organs
by which the direction of gravity is made known to them, and
reactions set up to bring them into a definite position in relation to
it. We find sacs full of liquid containing one or more solid
particles. Projecting into the liquid are hairs attached to cells in
connection with nerve fibres. Such organs are known as statocysts
in animals (E., p. 214). In plants, starch grains in cells appear to
perform similar functions. When the organ or cell is in the normal
position in relation to the vertical, the grains lie on the lowest part
of the sacs and stimulate the nerves or protoplasm in that region.
If the position changes, the grains stimulate hairs in a different place,
and the change of position is known and corrected. This mode of
nS INTRODUCTION TO GENERAL PHYSIOLOGY
action was made clear by an ingenious experiment with the crayfish,
which sheds the inner lining of its statocysts along with its shell.
The cavity of the statocysts is in communication with the outside
by a small pore ; grains of sand are normally taken in to replace
those lost when the lining is shed. If nothing but iron filings is
available, these are taken in and can then be caused to press upon
various parts of the wall of the statocyst by bringing a magnet near
the animal, which then proceeds to turn over in the way it would
have done if this part of the statocyst had been brought in the
normal way to be the lowest part of the sac. In the vertebrate,
these functions appear to be undertaken by those parts of the
internal ear known as utricle and saccule^ which have structures
similar to those of the statocysts of lower animals, although some
observers hold that the former have auditory functions. It must
be remembered that the touch and pressure receptors in the skin
and the muscles serve to give us information of that part upon
which pressure is being exerted, or of what part is being stretched,
and thus indirectly of the relation of our bodies to the vertical
direction.
The second kind of position-receptor is that interesting organ
known as the labyrinth or semi-circular canals, present only in
vertebrates. There are three of these on each side of the head,
forming a part of the internal ear, although they have nothing to
do with the perception of sound. Each is in the form of a hollow
ring, and the three are connected together at one part, so that there
is communication between their internal space. The plane of each
ring is at right angles to that of the two others, so that the three
canals are situated in the three dimensions of space, corresponding
to the length, breadth, and thickness of an object (E., p. 214).
Suppose now that the whole arrangement is moved quickly in
a plane which coincides with that of one of the canals. The liquid
contained in it will not partake, to any great extent, in the move-
ment of the walls, because the friction between the layers of the
liquid is not sufficiently great to convey the motion to the whole
mass at once. In other words, the walls are moved along, leaving
the liquid behind. Those canals which are at right angles to the
plane of movement will, naturally, not be affected in such a way as
to cause relative displacement of the liquid and the walls. But if
the direction of movement is such as to have components affecting
more than one canal, the effect on each will be inversely propor-
tional to the angle which its plane makes with that of the
movement.
How is this effect made to stimulate. nerve-endings? At one
end of each canal there is a dilated portion, and on one part of its
wall there is a protruding mass of cells with long hairs reaching
STIMULATION— THE SENSES 119
into the liquid. These hair-cells are connected with nerve fibres
and, when the apparatus moves, the hairs are dragged through the
water, since this does not move with them. The result is that they
are deflected, and their bases press upon the cells and nerves. The
process may be compared to " catching a crab " in rowing, the hairs
corresponding to oars, the nerve cells to the oarsman, and the walls
of the canal to the boat.
It will be seen that the function of this apparatus is chiefly for
the perception of more rapid movements in space than could be
detected by the statocyst. If the movement is slow there will not
be much relative displacement of the liquid and the hairs, and when
at rest in any position there will be no stimulus at all. The stato-
cysts, on the other hand, are arranged to indicate permanent
changes of position, and are not very sensitive to rapid movements.
As mentioned above, we do not depend entirely on the laby-
rinth for information as to position. In addition to differences of
pressure on that part of the skin in contact with solid objects, there
are receptor organs in the muscles themselves, which indicate
changes of tension in them, according to their position and that of
the parts to which they are attached. Moreover, the joints are
provided with nerves. Together, these form the receptor organs for
the so-called muscular sense.
It will be noticed that we have here a distinct class of receptors,
affording information of the state of parts in our own bodies, as
distinguished from that of external objects. The former class is
known as that of the intero-ceptors ; the latter are the extero-ceptors.
Of the intero-ceptors, the most important are the proprio-ceptors
of the muscles, which afford information of the state of activity of
these organs. The centres receive messages, as it were, as to
whether the command has been obeyed, clearly of great import-
ance in the carrying out of complex movements, which depend on
a series of acts.
The fact that certain sense-organs, especially the eye and the
hand, are provided with muscles capable of moving them in any
direction, is of much importance in the perception of direction in
space, and of the forms and distances of external objects. It is by
the co-ordination of these two organs that we learn, by experience,
how to interpret the information given by either. The size of the
image of an object on the retina would not inform us of the actual
size of the object unless we had, at some previous time, moved the
hand over it or some object of the same apparent size, and found
the muscular effort necessary.
CHAPTER V
ADJUSTMENT— THE NERVOUS SYSTEM
IN its simplest aspect, the central nervous system may be said to
be concerned with the adjustment of the organism to external
changes. We have seen how these outer changes are enabled, by
appropriate receptors, to impress themselves and how the appro-
priate responses are made by muscular movements. We have now
to try to understand something of the way in which the connec-
tion between them is made. In the physiological discussion of the
functions of the nervous system, we are not concerned with the
fact that the activity of the highest parts of the brain is associated
with what we call the mind, with conscious knowledge of their
activity. By the " highest " parts of the brain we mean those parts
which are the most removed in anatomical relationship to incom-
ing stimuli from that which we know to be the seat of the
simplest reflex movements and to be devoid of consciousness,
namely, the spinal cord (E., p. 215). It may be remarked here that
the parts in question, the cerebral hemispheres, are developed in
relation more especially to what we have recognised as the distance-
receptors, the eye and the ear ; and it may be noted that these are
the receptors, together with the hand, chiefly concerned with the
development of speech and the use of written language, without
which intellectual growth would have been impossible. It need
scarcely be said that in animals of high mental development, a
large number of processes and much lapse of time may intervene
between the reception of a message and the execution of the
response appropriate to it.
When an impulse arrives in the nerve centre along a fibre from
a receptor organ, what happens to it? We find, by histological
examination (E., p. 216), that the fibre divides, and that its
branches are connected to a cell containing a nucleus, usually to
fibres proceeding from the cell.- In the simplest conceivable case,
this cell is the " motor centre" of some particular muscle That
is, the nerve fibre given off by it passes to a muscle, and when set
into action causes contraction of that muscle. This is the most
elementary form of "reflex action," and is rarely met with. It may
be represented by the parts H and n of the diagram in Fig. 6.
120
ADJUSTMENT— THE NERVOUS SYSTEM 121
A reflex is then the physiological unit of the central nervous
system, but not the anatomical one. We see that at least two nerve
cells are concerned, in addition to the receptor and the muscle,
which latter may be called the u effector." These structural units
of the nerve centres are known as " neurones," because the use of
the name " nerve-cell " was found to lead to confusion. A neurone
is a peculiar type of cell in that part of it consists of a fibre, the
"nerve fibre," which may be of great length, sometimes several
FIG. 6. — Diagram of the General Arrangements of the Central Nervous
System.
A, excitatory association neurone.
B, E, and II, excitatory afferent (sensory) nerve fibres.
C and G, inhibitory afferent nerve fibres.
D, motor (efferent) neurone, ending in a muscle.
F, inhibitory association neurone.
The excitatory synapses are white, the inhibitory ones black.
feet ; but it is, nevertheless, as truly a part of the cell as the pro-
truded pseudopodium of an amoeba is. Its structure degenerates,
and it loses the power of conduction after it has been cut off from
the part of the cell containing the nucleus. This fact enables us to
obtain some information as to the nature of the connection between
the two or more neurones forming a " reflex arc." If the sensory
or " afferent " fibre taking a message to the centre be cut, the
portion beyond the place of section degenerates, but only so far as
the place where it joins another neurone. The process does not
extend beyond the junction, which is called the "synapse" It is
122 INTRODUCTION TO GENERAL PHYSIOLOGY
clear that there is not protoplasmic continuity, and that the two
neurones are independent of one another, as far as their nutrition is
concerned. The usual cell membrane intervenes, and is here called
the "synaptic membrane," having some special properties. The
physiological process must clearly be transmitted through this
membrane or no reflex would result. This transmission is perhaps
an electrical effect in which ions change places, but the process is
still obscure in its details.
In the greater number of reflexes there are one or more inter-
mediate or " association " neurones, as in the arc E, A, D of Fig. 6.
As the nervous system increases in complexity, we find that the
number and length of these association neurones increases, so that,
while in the earthworm they only extend to one or two adjoining
segments, in the higher vertebrates they reach to the cerebral
hemispheres themselves. These higher parts are formed entirely
of such neurones.
A word may be said here as to the nature of the protoplasm
composing the substance of the neurone. If examined, while still
alive, under dark ground illumination, the protoplasm of the body
of the cell is seen to be filled with numerous granules in Brownian
movement, which appear to congregate together for a time in
various parts of the cell, so that larger and more brilliant particles
are formed locally. These granules are said to show signs of the
possession of a fatty sheath which stains with methylene blue.
The movements show that they are suspended in liquid, so that the
structure of the neurone is in general similar to that of other
protoplasm. There are also reasons for believing that the nerve
fibre process, "axon," as it is called, is also of a liquid nature.
It may be asked, what is the function of the nucleated part of
the neurone ? It may possibly act as a kind of relay, adding
energy to a nerve impulse which has become weakened by passing
through the synapse. But this is by no means certain. It has
been shown that reflexes can take place after these parts of the
neurones have been removed, as is possible in some animals. It is
clear, however, that the continued life of the neurone depends on
this nucleated part, as would be expected from the statements
made in our first chapter with regard to the functions of the
nucleus. If a part of the neurone is separated from the "trophic"
influence of the nucleus, it dies, disintegrates, and ceases to carry
impulses.
Comprehension of the general principles on which the central
nervous system is constructed may be assisted by a short account
of its evolution. In the lowest multicellular animals, the sponges,
there are no structures comparable to nerves, although they possess
effectors in the form of muscle cells under the layer of amoeboid
ADJUSTMENT— THE NERVOUS SYSTEM 123
cells of the outer skin. The muscle cells are evidently stimulated
to contraction by mechanical influences on the amoeboid epithelial
cells, which may in a sense be regarded as receptors, but of a very
simple kind. In the sea-anemone there are more highly specialised
cells in the outer epidermis, which have long, thin projections
inwards, forming, with other similar fibres, a felt work between the
outer layer and the muscular layer. These fibres ultimately end
on muscle cells at a greater or less distance from the cell giving
origin to them. Since they serve to elicit muscular movements at
a distance from the point stimulated, they may with justice be
called nerve fibres and, together with their epidermal cell bodies,
form primitive receptor or afferent neurones. There is still no
indication of nerve centres. The next stage is met with in the
earthworm and elsewhere, and is the beginning of the synaptic
system, which enables so much advance in adjustment and co-
ordination to be made. We find that the nerve fibre does not
proceed straight to a muscle cell, but it enters a nervous mass or
"ganglion," and forms a synapse with processes of a neurone,
whose cell body is found here. The axon of this neurone passes
to a muscle cell, and is hence called a motor neurone. The advan-
tage of such an arrangement is that the same muscle can be put
into action from different sources, since more than one afferent
neurone can form a synapse with it. Thus commences what is
called the principle of the "final common path;' where the neurones
supplying a particular muscle serve as a common channel for the
many reflexes in which this muscle takes part. In the earthworm
there are also association neurones. Here the afferent fibre does
not form at once a synapse with the motor neurone, but with
another one which is entirely confined to the nerve centre. The
axon of this neurone ends either directly on a motor neurone^ or
only after the interposition of one or more further association
neurones, which may end in a more distant part of the nervous
system. As complexity and variety of adjustments increase, we
find a more and more copious growth of these association neurones,
extending to a greater and greater distance, so that the organism
becomes a connected whole. Thus the general arrangement is that
of a series of alternative loops or arcs (P., pp. 468 and 478), by
which an impulse received, say in the foot, may either pass across
as a spinal reflex in a neighbouring part of the spinal cord or by
various other paths in the brain itself, including the cortex of the
cerebral hemispheres.
There is a circumstance with respect to the receptor neurones
in the vertebrate in which they differ from those of the invertebrate.
It was mentioned above that cells in the epidermis of the latter
organisms become specialised so as to act as more sensitive
124 INTRODUCTION TO GENERAL PHYSIOLOGY
receptors. In this process their outer ends become elongated into
protrusions of various kinds, so that the nucleated cell body recedes
from the surface somewhat. But this does not proceed far until
we arrive at the vertebrate, in which the cell bodies of the receptor
neurones have receded nearly as far as the spinal cord itself,
forming what are known as the dorsal root ganglia. In this
way the ends of their axons are either merely situated between
cells at the periphery, forming pain receptors, or are connected
with the cells of specially developed receptors, such as were
described in the preceding chapter.
Those association neurones which form the arcs extending
through parts of the brain itself represent the complex co-ordinated
activities in which thought and memory take part.
We must now return to consider some aspects of reflex action
hitherto unmentioned. First of all, we should realise, by the
examination of some of these, that they take place without the
necessary participation of consciousness. The spinal frog (E., p.
216) serves well for certain experiments. In ourselves, the quick
withdrawal of the hand, when it touches a hot object, is done with-
out the conscious intention of doing so, although the sensation and
the fact of the reflex taking place are present in our consciousness.
Although the afferent impulses from the skin receptors have
travelled across by short arcs, branches from them have also pro-
ceeded to the brain by the long arcs.
The variety of reflexes in which the same muscle or group of
muscles take part will probably be noticed in the experiments on
the spinal frog. The importance of the final common path is
shown here. Instead of having the receptors for each of these
reflexes separately joined up to the muscle, they have merely to be
connected to the motor centres of the muscles, directly or through
intermediate neurones, and one set of out-going or efferent fibres
suffices. But it is clear that the same final common path cannot
be used for different reflexes at the same time, and if it is to be
used quickly for a new reflex, the preceding one must be cut short.
The discharge of a reflex arc lasts longer than the stimulus pro-
ducing it, and it is frequently necessary to stop it more rapidly
than it would cease if left to itself. This is done by a process of
inhibition. We do not know what this actually consists in, beyond
the fact that certain nerve fibres end on a muscle or nerve cell in
such a way as to lessen or stop its activity, instead of increasing
it, as the ordinary motor or excitatory fibres do. We have seen
an example of it in the case of the intestinal muscle, and a very
important one is that of the action of the vagus nerve on the heart
(E,p. 217).
It must be understood that the process of inhibition is an actual
ADJUSTMENT— THE NERVOUS SYSTEM 125
effect on the discharging neurone, making it for the moment in-
capable of discharging. When the inhibitory influence ceases, the
neurone has been put into a state of rest in preparation for taking
its part in a new reflex act. Suppose that a motor neurone is dis-
charging under constant stimulus from a receptor. Inhibition
does not mean putting a block in the path of the stimuli, since the
motor neurone would continue its discharge for some time after-
wards. It is actually caused, by a direct influence, to stop dis-
charging practically instantaneously. The muscle contracting under
its discharge relaxes to its full length suddenly (p., pp. 410 and 414).
Some of the ways in which inhibition works in nerve centres
may be realised from Fig. 6. The fibre C, when stimulated, inhibits
the motor neurone directly. A reflex through an intermediate
neurone can also be stopped by inhibiting this intermediate
neurone, as by F. An interesting case is when a reflex is being
elicited by stimulation of E. If, at the same time, H is being
stimulated, it sets the neurone F into activity, and this stops the
reflex. But the reflex can be restored if the inhibiting fibre G is
also stimulated along with the other two, since it stops the activity
of the inhibiting neurone, and leaves the neurone A free to convey
the exciting impulses from E. Thus an inhibitory nerve may
appear to start a reflex.
Since the two processes of excitation and inhibition are
opposite, it is possible, stimulating them both in appropriate
strength, to make them mutually abolish one another, so that no
effect results. This can be shown in the case of certain muscles of
the thigh, which are caused to contract when a particular afferent
nerve of the opposite side is stimulated, and to relax if the corres-
ponding nerve of their own side is stimulated. By different relative
strengths of the two stimuli, various intermediate states between
full contraction and relaxation can be brought about. Similar
phenomena can be observed in the case of the nerves to the heart
and the blood vessels, about which more will be said in the next
chapter.
An important aspect of muscular movements is that known as
reciprocal innervation. Suppose that a limb can be either bent or
straightened by the action of muscles, which are therefore antagon-
istic in their effects. It is found that when a reflex or voluntary
movement involves the contraction of one set, the antagonists are
concurrently relaxed by inhibition of their motor centres. It is
clear that for the exact performance of delicate movements, such
as those of the eye and the hand, the relaxation of the antagonistic
muscles must proceed step by step with the contraction of the
muscles producing the movement, and that, by this means, a very
accurate adjustment of the movement can be made.
126 INTRODUCTION TO GENERAL PHYSIOLOGY
Inhibition plays a very important part in the functions of the
higher nerve centres, as we shall see presently. It is a matter of
every-day experience that we can stop a movement suddenly, if
necessary. This is effected not only by bringing antagonistic
muscles into play, but by inhibition of those producing the move-
ment which is required to cease.
Fatigue
If we perform the same movement many times in succession,
it is possible to arrive at a state in which we cannot make the
muscles contract any more. We say that they are fatigued. But
we must be careful to distinguish this state from one of exhaustion.
The store of material yielding energy to the muscles has not been
used up, since an electrical current applied to them directly pro-
duces vigorous contraction. Something has happened at a synapse
in the course of the nervous arc by which it ceases to be able to
conduct. It may be either the using up of some material in this
situation, or the production of some chemical substance that has
not been removed with sufficient rapidity. But, in any case,
the synapse recovers very quickly, and the fact of fatigue has the
useful function of preventing the possession of any particular final
common path by a reflex for an undue time. But why do we say
that it is the synapse and not the whole neurone that is fatigued ?
It is because of the remarkable fact that the fatigue of a reflex
arc, using a certain final common path as motor neurone, does
not affect the use of this same motor neurone by another kind
of reflex. The motor neurone itself seems to be very difficult
or impossible to fatigue. It is also impossible to fatigue nerve
fibres, except in the absence of oxygen, and even this has not
been altogether satisfactorily demonstrated.
Fatigue of voluntary muscle itself, such as can be brought
about by prolonged direct stimulation of excised muscle, is un-
doubtedly due to the accumulation of lactic acid, which is not
oxidised as quickly as it is formed. If the muscle so fatigued is
placed in pure oxygen, it recovers to a notable degree as the lactic
acid disappears. But recovery is not complete, so that we must
admit a partial exhaustion of the store of potential energy, due to
inability to replace it in these abnormal conditions. The muscular
fatigue of normal exercise is doubtless due in part to some excess
of lactic acid. We know that in vigorous exercise the lactic acid
is not oxidised completely as rapidly as it is formed, since some of
it diffuses into the blood and is excreted in the urine.
ADJUSTMENT—THE NERVOUS SYSTEM 127
The Cerebral Cortex and Conditioned Reflexes
We may, from the standpoint of physiology, regard the
responses in which the higher parts of the brain take part as a
particular kind of reflexes. But they are more modifiable by
effects influencing them by way of other parts of the nervous
system than the machine-like spinal reflexes are. Thus, the con-
tact of the hand with a hot object is always followed by with-
drawal of the hand, but not necessarily by the use of "strong
language." Here we see the intervention of inhibitory processes,
which play so great a part in the functions of the cerebral hemi-
spheres. The surface of these organs, known as the cortex, is the
seat of the highest intellectual activities. In the investigation of
its functions, a method has been developed by the eminent Russian
physiologist, Pavlov, in which that aspect referred to above, namely,
their variable nature, has been made use of in a systematic manner.
Although a detailed account is beyond the scope of this book (see
P., pp. 502-507), a brief consideration will help towards a general
comprehension of the mode of action of the central nervous system.
This system has often been compared to a telephone exchange,
and the resemblance is in many ways a striking one. Any one
subscriber can be connected up with any other subscriber, just as
a particular muscle can be used in reflexes from many various
receptor organs. In this way the line to any one subscriber may
be regarded as analogous to the final common path in relation to
all other subscribers, and the costly and ineffective method of
having this person separately connected by a special wire to each
of the others is avoided. It is also possible for a subscriber to be
permanently connected by a separate wire to another, so that these
two can talk at will without having to be put into communication
through the central .exchange. This represents that kind of reflex
with which we are familiar in the spinal reflex, but which is also
to be found in parts of the brain intervening between the spinal
cord and the cerebral cortex. It may be called unconditioned,
because no special conditions need be present for it to be
manifested. But the usual method is for a subscriber only to be
temporarily connected with another, and the possibility of any
resulting conversation depends on this condition. The contrast
between the conditioned reflex, as it is obtained from the cerebral
cortex, and the unconditioned one of drawing away the hand from
a hot object, may be illustrated by supposing that one agrees with
a friend to meet at a certain place at a certain time. We
expect to do so under the conditions arranged. But a subsequent
passing by the same place is not expected to have the same result.
The association, as we may call it, is merely a temporary one.
128 INTRODUCTION TO GENERAL PHYSIOLOGY
And in the use of the word " association " it is to be understood
that we imply that an actual physiological process of connecting
up in some way has taken place. It is clear that in such more or
less complex activities of the higher centres there is a temporary
functional union of neurones, which are not joined up in the
ordinary course of affairs.
In the actual investigation of such processes, a reflex to the
salivary glands in the dog was chiefly made use of. Although the
fact has not been specially referred to, it is scarcely necessary to
remark that any organ supplied with nerves which set it into
activity may be so activated by a reflex from sense receptors.
We saw in our discussion of digestion that the presence of food in
the mouth almost invariably results in a secretion of saliva. This
is the more primitive, unconditioned reflex. But it was also found
that many kinds of external phenomena could, by appropriate
means, be made to result in such a secretion, through the inter-
vention of the higher centres, although these stimuli had previously
no relation to food. Such a temporary association could be formed
in the following way : — Food actually given to a dog produces
secretion of saliva. Suppose that every time that the food is given,
a particular bell is rung. After a number of repetitions of the com-
bination of bell and food, a new connection has been set up
between the sound of this particular bell and the presence of food,
so that now the sound of the bell alone, which previously had no
effect of the kind, excites secretion of saliva. This simple form of
conditioned reflex allows many experiments to be made on the
effect of various concurrent stimuli. The important part played
by inhibition becomes very obvious. If during the production, or
education, of the reflex to the bell some other extraneous stimulus
intervenes, that to the bell is for a time obliterated. It can be
shown also how the formation of the higher response overpowers
the more primitive one. The application of an electrical current,
strong enough to excite signs of pain, to a particular spot on the
skin is made the signal for food, in the same way as the sound of
the bell in the preceding experiment. After a time its application
results in secretion of saliva in the absence of food, and under these
conditions no signs of pain are shown. Whereas, if moved to a
spot of skin a short distance away, the same electrical stimulus
causes pain but no saliva. Such an experiment as the following
has several points of interest. It was noted above that an
extraneous influence is apt to prevent the manifestation of the
proper conditioned reflex. So that if we have a secretion to the
sound of a bell, a flash of light produced at the same time inhibits
the reflex. Now, in the production of the reflex to the bell,
suppose that the food is not presented at the same time as the
ADJUSTMENT— THE NERVOUS SYSTEM 129
sound, but two minutes afterwards, the process being repeated until
the conditioned reflex is duly formed. We then find that the
sound of the bell is not followed immediately by secretion of saliva,
but only after two minutes have elapsed. It is obvious that some-
thing in the nature of an inhibition must have been going on
during these two minutes. That this is the case can be shown by
the application of a stimulus in this interval, the stimulus being
one that does not produce secretion of itself, but has an inhibiting
action on other stimuli, such as the flash of light above mentioned.
The effect of this indifferent stimulus in the interval of two minutes
before the secretion normally appears is to cause the appearance of
saliva at once. The previous inhibition is itself inhibited, so that
a positive result shows itself.
The inhibitory influences are spread over a wide area of the
cortex ; in fact, during a conditioned reflex it appears that practi-
cally the whole of the cortex, with the exception of the part
concerned, is in a state of inhibition.
We may conclude with one more example. Suppose that a
sound and a light are made, each for itself, signals for secretion,
but that when both are presented together no food is given, so
that the reflex to the two stimuli together becomes one for no
secretion, and one stimulus must inhibit the other. Jf one of these
be afterwards presented alone, secretion follows, and if, while the
secretion is in progress, the other, also active by itself alone, be
superadded, the secretion stops.
It will be seen that we have in these new associations the
physiological basis of memory and of the formation of habits,
together with the possibility of their loss by breaking of the con-
nections.
The fact must not be passed over that we have in the cortex
certain areas whose artificial stimulation causes definite move-
ments. These are called motor areas, but it must not be supposed
that they are of the same nature as the motor neurones of the final
common path. They may rather be looked upon as the physio-
logical representatives of the ideas of particular movements,
although their activity is not necessarily associated with conscious-
ness, since the phenomena are shown in the anaesthetised animal.
The results of artificial stimulation of such cortical areas show
the complexity of the various effects produced at different times by
stimulation of one and the same point. Thus, after rest a point
usually gives contraction of the muscle in the same way as it had
previously, but if it be stimulated immediately after a previous
response, inhibition of the muscle occurs. If a point which normally
gives extension of the elbow be stimulated immediately after that
of another point which gives flexion, the former point gives flexion
130 INTRODUCTION TO GENERAL PHYSIOLOGY
instead of extension ; and so on. Inhibition is more prominent
than excitation, and appears independently of excitation of
antagonists. After-actions, such as tonic or rhythmic contractions,
are of various kinds, and affect the pairs of antagonists in a diversity
of ways.
The Nerve Impulse
As pointed out above, there is nothing to be seen in a nerve fibre
to indicate that a propagated disturbance is passing along it.
Moreover, only one physical or chemical accompaniment of the
impulse has been definitely shown to be present, that is, an
electrical change of such a nature as to indicate that a point in a
state of activity is electrically negative to one at rest. It has been
stated that an evolution of carbon dioxide occurs, but the experi-
ments are not altogether free from objection. The most sensitive
instruments have failed to show that any evolution of heat takes
place, and the absence of fatigue under normal conditions, referred
to previously, indicates an extremely small consumption of energy.
Indirect evidence suggests that what happens is a concentration of
ions of a certain sign at or near some membrane, and that this
concentration progresses as a wave along the fibre, the change at
a forward point being brought about by the electrical effect of that
behind it. The ions thus move backwards and forwards at any
particular point, somewhat as the molecules concerned in the pro-
pagation of sound waves do.
Like other excitable tissues, nerve fibres exhibit a refractory
phase, at first of loss of excitability altogether and then of gradual
return to normal, or for a moment slightly beyond it. The whole
period is very short, 0.0025 sec- m the frog, for the period of
inexcitability. In man, it is propably about one-fourth of this
value.
The rate of conduction of the impulse in man is about 120
m. per second.
We have seen that muscle fibres are only able to manifest one
degree of activity, however the strength of the stimulus is varied.
As far as motor nerves are concerned, the same fact of " all-or-
nothing" has been found, and, in all probability, it holds for afferent
nerves also, since no other difference between the two kinds of
nerve fibres has been detected.
The fact just mentioned is difficult to reconcile with a wave-like
displacement of ions or similar view of the nature of the nerve
impulse. If a nerve impulse passes through a region subjected to
the action of an anaesthetic, it may be abolished ; but if the
anaesthesia is not too deep or the length anaesthetised not too long,
ADJUSTMENT— THE NERVOUS SYSTEM 131
it may be merely reduced in intensity. In the latter case, when it
reaches a normal place again, it returns to its original strength,
since it requires just as severe a treatment to abolish it completely
as the normal impulse does. Such behaviour reminds one rather
of that of a train of gunpowder which is very narrow in one part.
If set alight at one end, the evolution of energy decreases as the
chemical reaction passes along the narrow part, but it recovers
again to its original value when it arrives at the part of the same
width as the initial part, whereas a physical change, such as a
sound wave, does not recover its original intensity after having
been diminished by passing through cotton wool. On the whole,
it cannot be said that the nature of the nerve impulse is yet solved.
The Visceral Nervous System
Those organs and tissues composed of smooth or involuntary
muscle, such as the contractile coats of the intestines, heart, blood
vessels, and so on, receive a nervous supply which differs in several
ways from that of voluntary muscle. In the first place, as we have
seen, there are both excitatory and inhibitory nerves ; and, in the
second place, these nerves are in reality the axons of association
neurones and belong to the central nervous system, since they form
synapses with a further set of neurones outside the nervous system,
sometimes situated in the organ supplied, sometimes in masses of
nervous tissue, ganglia, distinct from these organs. It is the axons
of these neurones that pass to the actual tissue cells. In this
system are also included fibres which go to secretory glands as
well as to muscle.
A definite set of these visceral fibres is known as the sympathetic
nervous system, and arises from a limited region in the middle part
of the spinal cord. Some of these fibres supply smooth muscle,
others glands, but all of them have the remarkable property of
being set into activity by the secretion of two ductless glands at
the upper ends of the kidneys, the supra-renals or adrenals. The
agent responsible for this effect is known as adrenaline, and has
been separated in the pure state.
Although the viscera have sensory nerves also, it should be
noted that these nerves are similar in their nature and anatomical
relations to the ordinary sensory nerves, so that the involuntary
nervous system of the special nature described above is efferent
only.
CHAPTER VI
TRANSPORT OF MATERIALS— THE VASCULAR
SYSTEM
IN unicellular animals and in the more primitive small multi-
cellular animals there is no need for the provision of special means
of conveying chemical products from one part to another, since
they readily pass by diffusion.. But when, for example, the
materials derived from the digestion of food are prepared in one
particular part of the organism at a distance from other parts
requiring them, special channels and means of transport are needed,
just as we saw was the case with oxygen. And, as in that case,
the means of transport is the blood. We have now to inquire
how this transport is effected. It is clear that the blood must be
sent in a current, so that its constituents may reach all organs, and
that the same blood must circulate since there is no loss of it.
At a very early stage of evolution we find a muscular tube
which, by rhythmical contractions, causes currents of a more or less
irregular nature in the liquid of the body. cavity. This tube is
open at both ends, but may be regarded as a rudimentary kind of
heart, although the fluid which it drives is not confined to any
particular channels, such as we find in the blood vessels of the
more highly organised animals. In its most perfect form, as in the
mammals, the general arrangement may be represented as in the
diagram of Fig. 7. In this figure, for the sake of simplicity, the
hollow muscular organ, known as the heart, is represented as two
separate organs, left and right. Although the two parts are united
in one mass, their cavities are quite distinct and separate. Starting,
then, from the left side of the heart, at the upper right-hand corner
of the diagram, we note that blood, which has replenished its
oxygen and got rid of a large part of its carbon dioxide in the
lungs, enters the contractile cavity, known as the left ventricle. But
immediately before it enters the ventricle it passes through another
chamber, the auricle, with thinner walls, but also contracting
rhythmically, immediately before the ventricle. By this means
the ventricle is filled up with blood. This ventricle then contracts
with force, and as there are valves between it and the auricle which
132
TRANSPORT OF MATERIALS
'33
open only in such a direction as to allow blood to flow from auricle
to ventricle, and not in the reverse direction, the contents of the
ventricle are expelled into the main arterial channel, the aorta.
Another set of valves is necessary at the beginning of this tube,
FIG. 7. — Diagram of the Vascular System of the Mammal.
in order to prevent the blood driven in, and stretching its walls,
from flowing back again when the ventricle relaxes in preparation
for another beat. The aorta gives off a large number of tubes, the
arteries, of which four are represented in the figure. Each of these
again divides into smaller vessels also with muscular walls, the
arterioles, and these are continued into a network of minute con-
nected tubes arranged around the tissue cells. These are the
i34 INTRODUCTION TO GENERAL PHYSIOLOGY
capillaries. The blood is collected again by small vessels, veins,
which join to form larger ones, and then open into the main venous
channel, the vena cava. The veins have thinner walls than the
arteries, but are to some extent muscular. The vena cava carries
the blood, which has by now given up a large part of its oxygen to
the tissues and received carbon dioxide from them, to the right
heart. The arrangement here is similar to that described for the
left heart, except that the walls are not so thick and powerfully
contractile. The blood driven out from it by a large artery passes
through the lungs. A capillary system is formed here around the
air sacs, and the gaseous exchange described in Chapter II. is
effected. The restored blood then arrives by the pulmonary veins
at the place from which we started.
The history of the discovery of the circulation of the blood is a
very interesting one (see P., pp. 668-669). The real proof was given
by Harvey in 1616, although, in the absence of the microscope, it
was impossible for him to see the actual passage of the blood from
the arteries to the veins through the capillaries. This was done by
Leeuwenhoek in 1686, by means of the microscope which he had
invented.
The general mechanics of the circulation can best be understood
by making experiments on a model (E., p. 218). The actual cir-
culation itself must be examined in the web of the frog's foot
under the microscope (E., p. 221), and the structure of the heart
with its valves by dissection of a sheep's heart (E., p. 222).
We see then that the blood is sent through a number of channels
in multiple arc, as the electrician would say, by means of a pump
with the appropriate valves. This pump, the heart, consists of a
hollow space surrounded by muscular walls, which diminish the
size of the cavity when they contract. In order that the blood
may be sent round the circulation, it is clear that a pressure must
be produced, since a liquid only flows from a place where the
pressure is higher to one where it is lower. This pressure is pro-
duced by the heart, which drives blood into the elastic arteries,
producing tension in their walls. This tension continues to drive
the blood onwards during the interval between the beats, so that the
general arterial blood pressure does not fall greatly in this interval.
The way in which the supply of blood to different parts is
regulated in accordance with their needs will be described later,
but it can be seen from Fig. 7 how the widening of one alternative
channel will result in a greater supply of blood to that part with a
diminished supply to other parts, while a narrowing of it will result
in a diminished local supply, with a greater one to other parts.
The uppermost of these parallel channels in the figure repre-
sents that of the heart muscle itself. As would be expected, the
TRANSPORT OF MATERIALS 135
continuous work done by the heart requires a copious supply of
oxygen and, in fact, a large part of the blood sent out by the beats
of the heart is used to feed itself. The next channel in the figure
is that through the stomach, intestines, and their attached glands.
We notice the fact, to which attention has been directed above
(p. 75), that the venous blood from these organs does not pass at
once to the great veins, but traverses the liver on its way. Thus
products of digestion are subjected to the action of this organ.
The liver would thus receive only venous blood, except that pro-
vision is made for its oxygen supply by a special artery which
proceeds directly to it. The third of the parallel paths represents
that of the viscera, whose venous blood does not pass through the
liver, and must be imagined to be itself composed of a number of
separate channels. Similarly, the lowest one represents a large
number of separate parallel paths.
The Blood
The blood is a liquid consisting of an immense number of tiny
corpuscles suspended in a clear liquid, the plasma. These corpuscles
are of two kinds, the red ones containing the haemoglobin, which
we have seen to be responsible for the carriage of oxygen and
carbon dioxide ; and colourless ones, like small amoebae, which are
in much smaller number than the red ones, are called leucocytes
(E., p. 223). The chief function of the latter cells is to take up, kill,
and digest disintegrating tissues, such as the tail of the tadpole
when the frog stage is reached, and also the micro-organisms which
invade the body and cause disease. The process is known as
phagocytosis, although it is just like the ordinary feeding of the
amoeba. It seems probable that these corpuscles may have other
functions, but very little is known of these.
The red corpuscles, as already mentioned, consist almost entirely
of haemoglobin, together with water. In vertebrates other than
mammals they possess nuclei, and have the general properties of
living cells. In mammals they lose the nuclei which are present
when the corpuscles are young. It was pointed out in our first
chapter that living cells when deprived of their nuclei degenerate
and die, so that it is a matter of some difficulty to know whether
the red corpuscles are actually to be regarded as living or not.
At any rate, they disintegrate after a certain number of clays.
The destruction takes place almost entirely in the liver, and the
bile pigments are formed from their haemoglobin. These pigments
contain no iron, that present in the haemoglobin being taken up
by the liver cells.
New corpuscles are formed in the red marrow found inside a
136 INTRODUCTION TO GENERAL PHYSIOLOGY
number of bones. Apparently the iron from the old corpuscles is
utilised in the process, being conveyed from the liver to the marrow
in some way. It might seem a somewhat remarkable situation for
this important process, but the reason seems to be that the channels
for the blood must be sufficiently thin-walled and delicate to
permit the newly-formed corpuscles to pass into them, and the pro-
tection afforded by the solid bones is advantageous.
The plasma contains two or three different kinds of proteins in
colloidal solution, together with various salts, of which the chief
is sodium chloride, organic foodstuffs, such as glucose and amino-
acids, and waste products, such as urea, in small amount. It also
contains fat in very finely emulsified form. The proteins do not
serve as food materials. Certain of them take part in that kind of
"setting" of blood into a jelly, which is called "clotting," and will
be referred to again presently. Other functions of the proteins
will be better understood later. The salts are necessary in con-
nection with the maintenance of the correct properties of the cell
membranes. Without them the functions of the living cells come
to an end.
Internal Secretions. — We have seen that food materials, includ-
ing oxygen, are supplied to all the tissues through the medium of
the circulating blood, and that the carbon dioxide produced by
oxidation is removed. Further, the various chemical waste pro-
ducts are carried away to be got rid of through the kidneys. But
we have also seen that amongst these chemical products there are
certain substances, made by glandular secreting organs, which
substances pass directly into the blood and have powerful physio-
logical effects on various organs and tissues. In many cases these
substances are essential to life, so that disease or removal of the
organ producing one of them results in death from various morbid
conditions. Such are the thyroid gland in the front of the neck,
the suprarenal glands above the kidneys, and others. The absence
of the normal thyroid gland leads to a swollen state of the tissue
under the skin, to mental deficiency and other abnormal symptoms.
The absence of the suprarenals results in what is called " Acldison's
disease." The absence of certain cells in the pancreas leads to
diabetes, and so on. In other cases, as in that of the " interstitial "
cells of the sexual glands, ovary and testis, profound changes
depend on their internal secretions. Details of the numerous
organs of this kind will be found in the larger text-books, but we
see that there exists an extensive series of substances which act as
chemical means of co-ordination between different parts of the body,
and have been called "chemical messengers" or "hormones." It
seems not unlikely that every kind of tissue produces some sub-
stance of this kind, but it is clear that we cannot remove the whole
TRANSPORT OF MATERIALS 137
of the muscles, for example, in order to discover whether this is the
case with them.
The most typical of all chemical messengers actually obtained
in solution is the " secretin " produced by the action of acid on the
lining cells of the first part of the small intestine. This is formed
for the express purpose of setting the pancreas into activity, as we
have seen.
Chemical messengers are also to be met with in plants. There
are, as it appears, chemical substances which diffuse from some
parts and, circulating in the sap, favour the growth of other parts.
But these may possibly be of the nature of food materials. There
are others, however, which check or inhibit the growth of certain
parts in a definite manner. Thus, the apical shoot of a fir tree
produces some substances that prevent the lower shoots from grow-
ing vertically upwards ; so that when this shoot is cut off or
injured, one of the lower ones which is growing more vigorously
than its neighbours begins to grow vertically. As it grows
upwards, it, in its turn, produces the inhibiting material and pre-
vents others from turning upwards. It would appear that the
exciting cause must be the action of gravity on some constituents
of the cells, analogous to starch grains in the more common form
of response to the stimulus of gravity.
The Kidneys. — In addition to carbon dioxide, there are other
waste products of tissue activity. The most important of these is
urea, resulting from the deamination of amino-acids in the liver.
There are also other compounds of nitrogen, arising either as by-
products of chemical reactions, or from wear and tear of cell
structures. Amongst these are uric acid and creatine. Then
again, there are substances taken in with the food, which are either
useless or only wanted in small quantity, such, for example, as
phosphates and sulphates. We notice that all these substances
which we want to get rid of are crystalloids of small molecular size,
present in true solution, so that if we filter the blood through a
membrane which has pores small enough to prevent the passage
of the blood corpuscles and the colloids, we can effect a separation.
Such a membrane is parchment paper, as we saw in the first
chapter, and the wall of the small blood vessels is a membrane of
similar properties as regards permeability. But in filtering off the
waste products in such a way, we remove from the blood other con-
stituents which are of value, namely, water, salts, glucose, and
amino-acids. Nevertheless, this is the method adopted by the
animal body to rid itself of its waste products. How the dis-
advantage is remedied will be seen later.
The kidney is the organ in which the process takes place, and
the structures in it where filtration occurs are coils of small blood
138 INTRODUCTION TO GENERAL PHYSIOLOGY
vessels, " glomeruli," suspended in cavities which are themselves the
beginnings of long-looped and twisted tubes, lined with cells, along
which the filtrate passes in its way to the duct, called " ureter,"
which conducts it to the bladder. As the filtrate from the glomeruli
passes along the " tubules," it is subjected to certain operations which
convert it into urine, the name given to the liquid which leaves the
kidney. It is to be understood that the glomeruli and tubules are
in very large number, so that a great area of surface is provided for
filtration and the subsequent operations (E., p. 223).
Fig. 8 represents, diagrammatically, what takes place in the
glomeruli.
In order that the filtration referred to may take place at a
perceptible rate, the liquid to be filtered, the blood, must be sub-
jected to pressure. The heart provides this pressure in the arteries.
Now it is found that if the arterial pressure is lower than about 40
FIG. 8. — Diagram of the Filtration in the Glomeruli.
The clear space represents water.
The dots, the crystalloids.
The small circles, the colloids.
mm. Hg. no filtration takes place at all. Why is this? We saw
(E., p. 172) that the colloids, almost entirely proteins, of the serum
possess an osmotic pressure of about 40 mm. of mercury. This
means that they attract water through a membrane impermeable to
them, unless the entrance of the water is opposed by a pressure of
40 mm. Hg. If the pressure is greater than this, then water, con-
taining in solution any substances to which the membrane is
permeable, is filtered through at a rate proportional to the height
of the filtration pressure above 40 mm. Hg.
The glomerular filters are able, then, to get rid of waste pro-
ducts without any other mechanism ; but, as remarked above, with
the loss of large quantities of water, a matter of great importance
to land animals, together with solutes which are of value, especially
sodium chloride, glucose, and amino-acids. In marine animals the
loss of water and of sodium chloride would not matter ; but even
here that of food materials is more serious. Accordingly, we find
TRANSPORT OF MATERIALS 139
developed in the cells lining the tubules of the kidney a capacity
for reabsorbing water and its valuable solutes, leaving untouched I
the waste products. In other words, a solution is reabsorbed /
containing all the normal constituents of the blood plasma, except
urea and such substances as are not wanted. Of course, the. liquid
which is reabsorbed does not contain proteins, because they are
absent from the glomerular filtrate. Now, it has been pointed out
by Professor Cushny that all the various phenomena connected
with the formation of urine can be most simply and easily explained
if we suppose that the solution reabsorbed by the tubule cells, and
passed back to the blood, is that to which the mechanism of these
cells, together with the other cells of the body, is adjusted. That
is, a solution which contains not merely the normal salts and food
materials of the blood, but also in the exact concentration which
they possess in the blood. Such a fluid then is invariably absorbed,
whatever the composition of the glomerular filtrate passing over
the cells. If the filtrate contains a notable excess of some solute,
even of a valuable constituent such as sugar, it may happen that a
part of it is lost by failure to be absorbed. Moreover, the rate at
which absorption is possible is limited, so that if filtration is very
rapid, there may not be time for absorption to take place as
perfectly as under normal conditions.
In order to see how the process works, let us take the case in
which the blood has been diluted by drinking a large quantity of
water. In the first place, the colloids will be present in lower con-
centration than normal, hence the available filtration pressure is
raised. The filtrate will, however, be more dilute than normal,
while the tubules absorb from it a solution of the normal concen-
tration. The result is that the concentration of the blood is more
or less quickly raised again.
Suppose next that the blood has become concentrated by loss
of water from the skin. It is obvious that the filtration will be
slower, but a certain amount of water must be lost in order to keep
in solution the excretory products. The liquid absorbed is of
normal concentration, so that the blood does not become so con-
centrated as it would otherwise, while the urine may be of very
high concentration.
Although the glucose and amino-acids are practically com-
pletely reabsorbed, there is always a loss of sodium chloride, which
is replaced by fresh supplies in the food. And, as already
remarked, if abnormal amounts of the former materials are present
in the blood, we find them in the urine. If the liver is disordered,
so that the cleamination of amino-acids is interfered with, we find .
these acids in the urine.
Since the filtration pressure in the glomeruli is the factor con-
140 INTRODUCTION TO GENERAL PHYSIOLOGY
trolling the rate of formation of urine, it is clear how this rate can
be modified apart from the effect of dilution of blood. If the small
arteries conveying blood to the filters are narrowed, by contraction
of their muscular coats, the pressure is reduced and the rate of
filtration decreased. And conversely, if they are dilated, the rate
goes up. Moreover, changes in the main arterial pressure will have
the same effects, independently of local changes in the renal cir-
culation. There are means of bringing about these various changes
by reflexes from the nervous system, as we shall see presently.
Lymph. — There is no reason to suppose that the blood vessels
of the glomeruli differ essentially in the nature of their permea-
bility from those of the rest of the body ; in fact, we have direct
evidence that the blood vessels generally are permeable to water and
crystalloids, impermeable to colloids. Hence, it may be asked, do
they not in other places than the glomeruli allow protein-free filtrate
to escape into the tissues, and, if so, what becomes of it? This,
indeed, is actually the case. As the blood flows from the arteries
through the capillaries to the veins by virtue of the greater pressure
in the former, in a part of its course the pressure is greater than the
osmotic pressure of the proteins. Liquid is filtered out here, and
is known as " lymph." It is the part of the blood with which the
tissue cells are in immediate relation. As the blood current passes
onwards to the veins, where the pressure is very low, at a certain
region the pressure has fallen to a value equal to that of the
osmotic pressure of the colloids, and beyond this point the internal
pressure in the capillaries is lower than the osmotic pressure of the
colloids. Accordingly, this osmotic pressure becomes active here
in attracting water, so that the lymph which was filtered off in the
previous part of the course is, to a large extent, reabsorbed.
Although the area in which reabsorption occurs is probably larger
than that in which filtration occurs, the rate of reabsorption is
insufficient to remove the whole of the filtrate, and what remains
passes away in channels which commence in the spaces between
the cells and gradually become definite vessels with thin walls,
finally joining together to form a large vessel, the " thoracic duct,"
which opens into the veins at the root of the neck.
Since it is by the osmotic pressure of the colloids in the blood
that the water is prevented from escaping into the tissues, and
causing what is known as "oedema," we see the object of adding a
colloid, such as gum arabic, to a liquid used for intravenous
injection to replace blood lost, or increase the volume in actual
circulation. Solutions containing crystalloids only have been
found useless, since they rapidly escape from the circulation.
Whereas, if 6 or 7 per cent, of gum be added, they remain in the
circulation, maintaining the volume and pressure of the blood at
TRANSPORT OF MATERIALS 141
their normal heights. Such solutions were found of great service
in the treatment of wounded men in the late war.
We saw previously that fat is absorbed in the intestine by
passing into lymphatic channels.
In part of their course the lymphatic vessels pass through what
are called " lymphatic glands.3' These are ductless glands, which
supply small leucocytes to the lymph, and thus to the blood. They
sometimes make their existence known by becoming inflamed and
painful when the tissue from which the lymphatics passing through
them arise is in a state of disease and giving off poisonous sub-
stances to the lymph.
In some situations the wall of the blood vessels appears to be
more permeable than is the rule. Thus, in the liver, proteins pass
through slowly. Such an increased permeability may be conferred
on the blood vessels in other parts by the injection of certain
substances, such as an extract made from dried mussels. It
happens also in the remarkable state called " anaphy lactic shock'"
As mentioned in our second chapter, many diseases are due to
poisonous substances given off by bacteria to the blood. These
are normally counteracted, or made innocuous, by the production
of " anti-bodies " in the blood, which act upon the foreign poisons
in various ways. A similar reaction occurs when proteins other
than those of the animal's blood are introduced. But to produce
this " immunity " in a marked degree requires the injection to be
repeated several times with certain intervals. If, however, the second
injection is delayed for more than ten days or so, it is found that a
greatly increased sensibility is produced, so that the injection is
followed by serious collapse and fall of blood pressure. One of the
symptoms present is frequently a swelling of the subcutaneous
tissue due to escape of liquid from the blood. If the animal
recovers from this state, it is found to be " desensitised," that is, any
further dose is innocuous. Satisfactory explanations of these com-
plex phenomena have not yet been given.
The Proteins of the Plasma.— We may now add a little more
with respect to the function of these constituents. They do not
act as food ; the tissue cells require amino-acids. They are, appar-
ently, the source of the anti-bodies spoken of in the preceding
paragraph. The two most important properties that they possess
are their osmotic pressure, by which the blood is prevented from
losing water and the tissues from becoming water- logged, together
with that of clotting. This is a process, involving several of the
proteins of the plasma, which occurs when the blood comes into
contact with most foreign substances, if they are wetted by it.
Clotting consists in the separation of a solid, " fibrin," in the form
of a network of filaments in which the corpuscles and liquid of the
142 INTRODUCTION TO GENERAL PHYSIOLOGY
blood are at first entangled. Liquid slowly exudes, owing to the
contraction of the network. This liquid, of course, is not the same
as the plasma, since fibrin has been separated from it, and other
products of the reaction which results in the deposition of this
solid are left in solution. The liquid is called " serum." The
importance of the phenomenon is in the spontaneous arrest of
bleeding from an injured blood vessel. If the rate of the outflow
is not great, the issuing blood clots when it comes into contact
with the tissues, and the familiar effect of accelerating the process
by the application of cobweb or such-like is merely due to the
provision of a large area of foreign surface.
The precise explanation of the changes that take place in this
coagulation process has led to various theories which it would not
be profitable to discuss here. It is to be feared that much of what
has been written on the question amounts to little more than
inventing names. There is one important fact, however, namely,
that the blood remains liquid if calcium be removed from it. as by
the addition of the appropriate amount of an oxalate, which forms
the insoluble calcium oxalate.
The Salts of Blood. — We saw in our second chapter (p. 60)
that the cells of the present land animals have become adjusted to
the presence of certain salts, probably owing to their presence in
the ocean at the time when their ancestors left it. In order, there-
fore, that a saline solution may serve as a perfusion fluid for isolated
organs, it has been found that particular salts must be present, and
in a " balanced," relative concentration. These salts are those of
sodium, potassium, and calcium, generally used as chlorides (E., p.
223). The function of the sodium chloride is chiefly to afford a
sufficient osmotic pressure to balance that of the cell contents.
Calcium seems to be necessary to maintain the properties of the
cell membrane, but it has doubtless other functions as well. Potas-
sium neutralises certain deleterious effects of calcium, and is also
said to be of importance on account of its radio-activity, since it
can be replaced by salts of other radio-active metals in equivalent
radio-active concentrations. The electrical properties of the anions
and cations naturally also come into play in the balance of electro-
lytes, but the problem is not completely solved.
Since the cell mechanisms are very sensitive to changes in the
concentration of hydrogen ions, while acids are produced in the
tissues under active conditions,a means of maintaining the hydrogen-
ion concentration of the blood at a constant value is a necessity.
The normal reaction is just about the neutrality of distilled water,
very slightly on the alkaline side. Although the proteins of the
plasma are able to combine with acids and alkalies, this capacity is
limited and scarcely comes into play within the region of the most
TRANSPORT OF MATERIALS 143
importance, that in the immediate neighbourhood of the neutral
point. The osmotic pressure of the proteins in the plasma is about
40 mm. of mercury, as we have seen, so that their molar concen-
tration is only 0.0023, and they do not possess many free NH2
groups capable of combining with acids. It appears that it is half
the nitrogen of the lysine contained in the protein molecule that is
in the form referred to. If so, the proteins of serum would be about
equivalent to a 0.0006 normal ammonia solution.
But there is a salt present in plasma which can combine with
acids. This is sodium bicarbonate, and its concentration is 0.03
molar. On investigation, solutions of this salt are found to be very
effective in preventing a rise in the hydrogen-ion concentration of
solutions when acid is added. This is because when an acid
stronger than carbonic acid is added to a bicarbonate solution,
carbon dioxide gas is given off to the atmosphere^ while that
remaining in solution is so little dissociated as to afford only a few
hydrogen ions as compared with those of the acid added. We may
look at the question from another point of view. Sodium bicar-
bonate solutions are alkaline because the salt, as one of a weak acid
with a strong base, is hydrolytically dissociated into carbonic acid
(H.2CO3) and sodium hydroxide. The latter is electrolytically dis-
sociated much more than the former, so that there is an excess of
OH' ions, conferring alkalinity. Thus, in a solution of sodium bi-
carbonate containing dissolved carbon dioxide, we may say that
the former confers alkalinity, the latter acidity, hence the hydrogen-
ion concentration is given by the ratio between the two. If they
vary in proportion, the reaction is unaltered. If the carbon dioxide
increases without the bicarbonate changing, the hydrogen ion is
raised. If it decreases, the hydrogen ion is lowered. These facts
give the clue to the most rapid and effective of the means of
regulating the reaction of the blood. Let us suppose that lactic
acid has been passed into the blood, as happens in defective supply
of oxygen. It immediately combines with a part of the bicar-
bonate, and if the content of the plasma in this salt is determined,
it is found to be diminished. The state is often called " acidosis,"
although it does not imply that the blood has become more acid.
In fact, it is easy to show, by experiment, that a large amount of
acid may be introduced into the blood without raising the hydrogen-
ion concentration. Why not ? The answer is found in the activity
of the respiratory centre. When bicarbonate combines with acid,
carbon dioxide is given off and, for a brief period of time, the
hydrogen ion of the blood is raised by its excess. But this excess
excites the respiratory centre to increased ventilation of the lungs
until the carbon dioxide tension in the alveolar air has become low
enough to reduce that in the blood to a level to compensate for
144 INTRODUCTION TO GENERAL PHYSIOLOGY
the reduced bicarbonate, thus bringing back the ratio of carbon
dioxide to sodium bicarbonate to its normal value. A converse
process accommodates to an increased alkalinity, although it
appears to be less effective, perhaps because the requisite decreased
ventilation implies a diminution in the supply of oxygen.
In addition to this method there are two other means of main-
taining the neutrality of the blood. In the first place, an increase
in the acidity of the blood causes, in some way, a retardation of
the formation of urea from the ammonia resulting from the
deamination of amino-acids in the liver (p. 77). This ammonia
then neutralises acid, and the salt formed is excreted in the urine.
It has recently been found that if excess of alkali is introduced
into the blood, lactic acid is formed in the organism. This com-
bines with the base present in excess, and the neutral lactate
appears in the urine. In the second place, the kidney itself acts
as a regulator. As the reader is doubtless aware, there are two
phosphates of sodium or potassium ; one of these (NaH2PO4) is
acid, the other (Na2HPO4) is alkaline. A mixture of the two in
certain proportions is neutral. They are contained in small
amount in the blood in this latter ratio. If the blood becomes
j more acid, a larger proportion of the acid phosphate is formed.
This passes into the glomerular filtrate. The phosphates in solu-
tion are hydrolytically dissociated, so that there are present sodium
hydroxide and phosphoric acid. Sodium salts are wanted by the
body, while phosphoric acid is only wanted in very small amount.
The tubules, therefore, absorb the sodium in the form of bicarbonate,
leaving the phosphoric acid in slight excess, so that the urine is
usually slightly acid. It will be clear that if the phosphoric acid is
in greater excess than normal in the glomerular filtrate, it will be
left behind and escape with the urine, while a fluid of normal re-
action will be absorbed into the blood.
Viscosity. — The molecules of a liquid experience friction in
moving over one another, so that if a part of the liquid is at rest
while another part of it is in motion, there is friction between the
two. This internal friction is the cause of that property known as
viscosity, familiar to all in the difference between water and glycerin
or treacle. If there are particles in the liquid, such as the blood
corpuscles, the viscosity is greatly increased, partly owing to the
fact that where a liquid is in contact with a solid, a film of it is
held stationary, so that there is more friction in the whole mass
than if the corpuscles were absent. Blood corpuscles are also
deformed in shape when passing through narrow channels, and this
serves to increase the apparent viscosity, since a part of the energy
of the current is taken up in the changing of shape. Indeed, the
viscosity of the plasma is only about half as much again as that of
TRANSPORT OF MATERIALS
145
water, whereas that of the whole blood may be as much as four
or more times that of water.
A high pressure in the arteries could only be maintained by the
heart if there were resistance to the outflow through the small
branches. This resistance is not due to friction between the blood
and the walls of the vessels, because the layer in contact with the
wall is stationary, but to that between successive layers of the
blood itself, extending to some distance from the wall. In a small
tube this distance is great enough to reach to the middle of the
lumen ; in a large one the greater part of the current may be all
J
FIG. 9. — To illustrate the relative magnitude of the region where internal
friction of the blood takes place in large and small arteries of equal total
sectional area.
moving at the same rate, and experience no internal friction (Fig. 9).
Hence the chief situation of the resistance is to be found in the fine
branches of the arteries, the arterioles. A volume of blood flowing
through a large tube experiences friction only in a small part near
the walls. The same volume flowing through a number of smaller
tubes, of the same total sectional area, experiences friction through-
out its mass. We see how the peripheral resistance, and with it
the arterial pressure, may be increased by constriction of the
arterioles or decreased by their dilatation.
The Regulation of Blood Supply. — If the diagram of Fig. 7
be referred to, it will be realised that if all but one of the parallel
channels be made narrower, this one will receive more supply ;
because if the heart continues to beat with the same strength, the
10
146 INTRODUCTION TO GENERAL PHYSIOLOGY
driving pressure will be greater. This is one way in which the
supply to any particular organ may be increased, and so far as
investigation has hitherto been able to make out, it is the only way
in which that to the brain is regulated, so far as other than chemical
factors acting .directly on the blood vessels are concerned. If the
brain requires more blood, the whole of the rest of the body has to
put up with less. In most organs, however, the arterioles have the
power of widening in response to messages from the central nervous
system, thus ensuring for each particular organ the more copious
supply that is wanted when it enters into activity. There is thus
a double supply of nerves to the muscle of the arterioles as to
smooth muscle in general, one set of nerves exciting to increased
contraction, the other inhibiting the natural tone. The former are
called " vaso-constrictor " nerves, and all leave the central nervous
system in the sympathetic outflow (E., p. 224). The latter are
" vaso-dilator " nerves, and have a more various origin. A familiar
case of reflex vaso-dilatation is the reddening of the skin known
as " blushing." Each set has a governing centre, the source of
reflexes to blood vessels, in that part of the brain immediately at
the upper end of the spinal cord, called the " bulb " or "medulla
oblongata." Certain sensory nerves produce, on stimulation, a
reflex fall of blood pressure by general dilatation of the artcrioles ;
others a rise by general vaso- constriction. The former are some-
times called "depressor" and the latter "pressor" reflexes.
The vaso-constrictor centre is normally sending out impulses in
a continuous stream down the spinal cord and through the
sympathetic to the blood vessels, so that these are kept in a state
of partial contraction. When a depressor reflex is produced, the
vaso-constrictor centre is inhibited, while the vaso-dilator centre is
excited. In fact, we have "reciprocal innervation" of a rather
more complex kind than in that of reflexes to voluntary muscles.
The converse effect in a pressor reflex is not so easy to show, since
the vaso-dilator centre does not send out a steady discharge except
under special circumstances. The excitation of the constrictor
centre is easily to be made out.
One particular reflex requires mention, that from the nerve
which has received the special name of "depressor'' The receptor
endings are situated chiefly in the beginning of the aorta, so that
when the blood pressure rises too high a dilator reflex is sent to
the arterioles in general, and the pressure lowered.
Another kind of vascular reflex is met with in several organs,
and is probably of wide occurrence. It is known as the " Loven
reflex" from the Swedish physiologist who first described it. When
a sensory nerve passing from the rabbit's ear, for example, is
stimulated so as to evoke a reflex to the blood vessels, it is found
TRANSPORT OF MATERIALS 147
that the arterioles of the ear itself are dilated, while those of the
rest of the body are constricted, so that a rise of arterial pressure
results. It will be seen that this is the most effective means
possible of obtaining a more copious blood supply to any organ.
The arterioles can also be made to dilate or constrict by the
action of various drugs or chemical substances. The constricting
effect of adrenaline has already been referred to. Some of the
chemical products of active cells have a dilating effect, the chief of
these being the hydrogen-ions from carbon dioxide. Hence an
increase of activity automatically brings about a better supply of
blood.
The Capillaries. — Although the walls of the fine network of
blood vessels that connects the small arterioles with the small
venules consist of simple protoplasmic cells and possess no muscular
coat, there is evidence that these vessels can be made narrower or
wider. This must occur owing to the constituent cells becoming
thicker and less flattened when contraction takes place, somewhat
as an amoeba or leucocyte does when stimulated. Whether they are
under the control of nerves is not yet decided, but certain chemical
agents have been shown to relax them. A substance called " hisfa-
mine," which is derived from a complex amino-acid constituent of
protoplasm by removal of carbon dioxide, has a dilating effect on
the capillaries, but a constrictor one on the arterioles. Under its
action, therefore, an accumulation of blood in a nearly stagnant
state is liable to take place in the capillaries, and such a condition,
by which blood is removed from active circulation, is of importance
in the condition of "shock" after wounds or surgical operation.
Substances with an action similar to that of histamine would be
produced in the destructive changes going on in damaged cells.
A condition in which the capillaries are dilated, but with very
little blood current through them, owing to constriction of arterioles,
is seen in the blue skin sometimes produced by cold. The fact that
the skin is dark in colour, not white, shows that the capillaries
in it are full of blood, but this blood is almost stationary. Its colour
shows thcit oxygen has been removed, and the coldness of the skin
shows that the circulation through it has nearly ceased. The skin
may also be full of, blood, but red and hot, as when exposed to
warmth or heated by friction. In this case, the arterioles are dilated,
and the capillaries are passively distended by the raised pressure
thus produced inside them. Thus a full current of blood passes
through them.
There is evidence that when organs are not in activity the whole
of their capillary vessels are not filled with blood. Some are empty
and more or less invisible. These may be filled with red blood when
the arterioles dilate, or with venous blood when they themselves
148 INTRODUCTION TO GENERAL PHYSIOLOGY
dilate without dilatation of the arterioles or with constriction of
these latter.
The Regulation of the Heart Beat. — For the sake of simplicity
of description of the mechanism of adjustment by changes in the
blood vessels, we have supposed that the heart has continued to
expel the same amount of blood in equal times. But the heart has
its own powers of regulating its output of blood, and thus of main-
taining a good arterial pressure even when the peripheral vessels
are dilated, and of moderating it if there is much increase in peri-
pheral resistance by vaso-constriction.
At one time this adjustment was thought to be of a very complex
nature, but the work of Starling and his co-workers has resulted in
reducing it almost completely to a comparatively simple "law of
the heart? It will be clear that the degree to which the ventricles
are filled by the time at which they contract depends on the amount
of blood which has flowed in during the pause between two beats.
Supposing that the heart muscle always contracted to the same
extent, then the fact that there was more blood in the ventricular
cavity would not result in more being expelled. In fact, the
opposite would occur, on account of the less mechanical advantage
of the tangential force when the curvature is less, as it is in the more
distended ventricle. When, therefore, the arterioles of an extensive
region of the body dilate, as in the muscles in running, or the
alimentary canal in digestion, the larger quantity of blood entering
the heart from the veins would only be sent on incompletely, and
the benefit of the vaso-dilatation would only imperfectly be realised
on account of the large fall in arterial pressure. Moreover, the
blood flow through the lungs would not be increased, and the urgent
need for more oxygen would fail to be satisfied. In actual fact,
however, this is not the case. It is found that the ventricles of the
heart expel more blood per beat the more they contain to begin
with, unless the initial distension is excessive.
Further, this behaviour is shown by the heart when separated
from the central nervous system, so that it is due to some inherent
property of the heart muscle itself.
Again, it is found that when the arterial pressure is raised, the
amount of blood expelled by the heart in a given time is not
decreased, although to raise the same volume to a higher pressure
requires more work. How is this to be explained ? Suppose that
a particular heart is working with such an expenditure of energy
that the arterial pressure is kept at a mean value of 80 mm. of
mercury, and that to do this 8 c.c. of blood are expelled at each
beat It is to be remembered that the pressure against which the
heart works rises from about 65 mm. at the beginning of the out-
flow to 100 mm. at the end. The peripheral resistance may now
TRANSPORT OF MATERIALS 149
be supposed to rise, so that a mean pressure of 140 mm. is required
to keep up a flow of the same magnitude as before, as actually
takes place. The next heart beat will not be powerful enough to
raise the pressure above, say, 100 mm., but this will be done by the
expulsion of less blood than before, so that a certain amount is
left behind in the ventricle. Since the same amount as before flows
in from the veins, the ventricle starts the next beat at a greater
distension than that of the previous one, and, as we saw above, its
contractile energy is greater. The increase continues with each
beat until the original output is reached.
Now, these facts and various others relating to the automatic
adjustment of the strength of the beat are readily and simply
explained by that, property of muscle to which attention has already
been directed (p. 96). We saw that the amount of energy developed
in muscular activity is in direct proportion to the length of the
fibre during the time that it is in the act of developing its state of
tension. This energy, in fact, is proportional to the area of certain
surfaces arranged lengthwise in the fibre. A greater distension of
the cavities of the heart is necessarily accompanied by a stretching
of all the constituent muscle fibres of their walls, so that when
contraction takes place it starts from a greater length of the active
surfaces, and greater energy is produced.
There is also what may be called an external control of the force
and rate of the heart beat. We have already seen that there are
two characteristics of smooth muscle in general, and the walls of
the heart behave as this kind of muscle, although they possess the
transverse striation of voluntary muscle. The first of these is that
of automatic activity, which may be manifested either in a state of
moderate contraction or by a series of rhythmical beats, or both
combined. This is very obvious in the case of the heart, which
continues to beat when cut out of the body, and will require
further consideration presently. The second characteristic is the
supply of two kinds of nerves, one increasing the state of activity
(excitatory), the other decreasing it (inJiibitory). In the case of the
heart these functions are exercised both on the strength of the
beat and on its rate. The excitatory nerve fibres come from
the sympathetic system, and are known as the " augmentor " or
"accelerator" nerves. They may be looked upon as similar to
the vaso -constrictor nerves, which we found to come from the same
system. The inhibitory nerve fibres are contained in the vagus
nerves, and have a general depressant action, not only on the
strength and rate of the beat, but also on the excitability of the
muscle to stimuli and on its capacity to conduct the wave
of contraction. Both of these nerves can be set into action by
reflexes. The depressor nerve, which we saw to produce a fall of
150 INTRODUCTION TO GENERAL PHYSIOLOGY
blood pressure by vascular dilatation, also produces slowing of the
heart through the vagus nerves.
The endings of both kinds of nerves in the heart are accessible
to chemical stimuli. Thus the vagus endings are stimulated by
acetyl-choline, the accelerator endings by adrenaline. The former
are paralysed by atropine (E., p. 225).
Origin and Transmission of the Heart Beat. — If the heart of
the frog or tortoise be observed carefully (E., p. 225), it will be seen
that the different cavities contract in a regular order, beginning at
the junction of the great veins with the sinus venosus, and ending at
the commencement of the aorta. Thus there is a place which has the
property of beating more rapidly than other parts. In fact, if the
various cavities are cut away from one another, it is found that the
natural rate of the activity of each part decreases in order of its
distance from the sinus. Since the contraction of one cavity is
transmitted to the next, it is clear that the most rapid one sets the
pace for the others. In the mammalian heart, although the sinus
no longer exists as a separate cavity, there is a mass of tissue
of similar structure to the sinus, and situated at the point where
the great vein of the head and neck joins the right auricle. From
this tissue each heart beat is initiated, and it is known as the
*' Keith-Flack or sinu-auricular node." In the frog, the contraction
progresses as a wave in the ordinary muscular tissue, which is
continuous throughout the series of chambers. There is, how-
ever, evidence that even here there is a certain degree of specialisa-
tion of a part of the connecting tube or funnel between auricles
anJ ventricle, such that this part conducts more rapidly than
other parts. In the mammal, this conducting tissue has become
a bundle of a peculiar kind of muscle cells which have developed
the capacity of rapid conduction. Consideration of the anatomical
arrangement by which the blood enters the ventricle at what is
called the base, where it is united to the auricle, and also leaves
it at the same end, leads us to realise that it must be an advantage,
especially in large hearts, if the ventricle contracts as simultaneously
as possible in all its parts, instead of in a rather slow wave
progressing from base to apex. This is provided for by the
auriculo-ventricular bundle above mentioned, which conducts about
ten times as fast as the ordinary ventricular muscle, sending out
branches to all the various regions of the ventricle. Thus con-
traction at the apex is almost coincident with that at the base,
and the contents of the cavity are expelled more effectively than
if different parts were not in the same phase of contractile stress
at the same time.
The nerves which regulate the beat of the heart are in
especially intimate connection with the node from which the con-
tractions start.
CHAPTER VII
GROWTH AND REPRODUCTION
IT is a somewhat remarkable fact that by far the larger number of
the different species of living organisms have a certain size to
which they grow, and individuals deviate but little from this size.
One cannot make any general statement as to why this is so, but
it seems likely that the causes are various, sometimes mechanical,
sometimes, perhaps, due to the digestive arrangements being
unable to supply a larger bulk with the necessary food. If a grow-
ing rat is supplied with a diet which is adequate to maintain it at
a small size, but inadequate for growth, it may remain at this size
long after it ought to have been fully grown. If, then, it be given
a complete diet, it grows to the normal size, but not beyond it.
Similarly, it is not easy to say why an organism should, sooner
or later, cease to perform those functions which we call "life," die
and disintegrate. It is to be presumed that some essential part of
the cell machinery cannot be replaced when it has worn out,
although this conception does not lead us far.
In any case, the fact of the death of the individual makes it
necessary that provision be made for the continuance of the race in
new individuals — for the production of a young and vigorous new
generation by what indeed we know as " reproduction''
In such lowly organisms as the bacteria, we find that, when an
individual has grown to a certain size, it simply divides into two,
and the process goes on at a great rate under favourable conditions.
When supply of food is limited, or the medium in which they are
growing dries up, a part or the whole of each individual organism
collects into a mass and becomes surrounded by a layer of resistant
material, apparently almost impervious to water. These are called
"spores." In this condition, bacteria are much more difficult to kill
by heat. They remain dormant, but become active forms again in
the presence of water and food. It is not, however, all kinds of
bacteria that form spores.
The nucleated unicellular organisms, including most of the
protozoa and algae, also multiply for the most part by simple
division, but a new phenomenon makes its appearance here, as we
152 INTRODUCTION TO GENERAL PHYSIOLOGY
shall see presently. In the process of the division of nucleated
organisms, the nucleus plays an important part, undergoing a series
of complex changes by which it finally becomes two nuclei similar
to the original, one in each of the new cells (E., p. 226).
We see thus how it is possible to speak of the " immortality "
of such organisms, since the substance of the parent does not
degenerate, but is divided up after increase in dimensions.
This mere increase in size does not, of course, involve the acquire-
ment of any new capacities of adaptation to the surroundings. The
new material added is built up like that from which it grows.
Accordingly, we find, even in such unicellular organisms as the
protozoa and algae, that a further process has been developed, which
is the gradual beginning of the wonderful phenomena of sex, that
play so large a part in the beauty of the world and the brave deeds
done in it.
We find, to begin with, that two similar individuals fuse together,
forming one single larger individual, which may then proceed to
divide in the old way, or it may split up into a number of smaller
individuals, each of which grows up to the normal size, and may
then continue to multiply in the simple way of division into two.
In either case the result is the union in the new generation of the
qualities of two individuals, which, however similar they may appear
to the eye, will almost invariably differ in their modes of reaction to
surrounding changes, as also in their capacity for " variation," as we
shall see later. This simple process of union of two individuals, in
which we cannot as yet speak of either of the pair as being male or
female, is known as " conjugation"
It was supposed for some time that a race of unicellular
organisms, continuing to multiply merely by subdivision, sooner
or later died out, unless conjugation occurred. The race was
thought to undergo senile degeneration, and to be rejuvenated by
conjugation. But it is not clear how this was to be brought about,
otherwise than by the combination of the capacities of two different
individuals, and it has now been shown that, if a proper supply of
the materials necessary for growth is provided, there is no need for
conjugation, and the vigour of the race remains unimpaired.
It will be clear that, since the advantage to be derived is the
combination in the new individual of different qualities, there is not
a great possibility of variety of experience in conjugation between
cells of similar situation and habit, especially when the organism is
not an actively motile one. The opportunity for conjugation occurs
only between cells that chance to be in proximity to one another
and have been exposed to closely similar conditions. The next
step, therefore, and one taken very early, is the differentiation of
the two gametes, as they are called, into two kinds of cells, one
GROWTH AND REPRODUCTION 153
stationary and relatively large, the female ; the other smaller and
motile, the male. In the unicellular organisms, the whole organism
is frequently converted into one female gamete, or into a number of
male gametes. The latter swim freely, and one of them conjugates
with a female gamete. But in the multicellular animals and plants,
special organs are formed for the purpose of producing the two kinds
of gametes. The female gametes are now called ova or egg-cells ;
while the male gametes have different names, spermatozoa in
animals, antherozooids or pollen grains in plants (E., p. 226). The
process corresponding to the conjugation of the simple organisms
is now called "fertilisation" The organism itself as a whole
becomes modified, the ova-bearing or female organisms being
different in many ways from the male; in some cases extraordinarily
so. The organ in which the ova are formed is known as the "ovary" \
that in which spermatozoa are formed is the testis.
The material from which the sexual cells are formed in the
course of the development of the young organism from the fertilised
ovum is very early separated from that which becomes differentiated
into the various organs of the body. It thus retains the whole of
the characters of the gametes from which it has resulted, and, even
in the adult, is but little affected by changes in the rest of the
organisms. In a certain way we may speak of the "continuity of
the germ plasm " from generation to generation. The question as
to how far it can be influenced by changes affecting other parts of
the organism will be discussed later.
When two similar cells conjugate there is, along with the general
admixture of cell substance, a dissolution of the nuclei, followed by
formation again of a single mixed nucleus. In the process of
formation of the male and female gametes in the true sexual process,
on the other hand, half the nuclear material is thrown off, so that
the final fertilised ovum contains the normal amount, half of its own,
the other half derived from the male element.
Although the process of fertilisation described above is the
normal one, the ovum until fertilised remaining stationary with no
cell division or growth of the new organism occurring, it is remark-
able that, in some exceptional cases, the unfertilised ovum is able
to develop. In such a case as that of the bee, this so-called
" parthenogenesis " is a normal fact, the fertilised cells becoming
the worker bees, the unfertilised cells becoming the males, called
"drones." In other cases, such as the sea-urchin, the ova, although
in the normal state of affairs requiring fertilisation, can be
stimulated to development by chemical or physical means of
various kinds.
In order that the ovum may become fertilised, it is clear that
the spermatozoa must obtain access in some way. When the eggs
154 INTRODUCTION TO GENERAL PHYSIOLOGY
are laid before fertilisation, as in the frog or fish, the male is
required merely to deposit a liquid containing spermatozoa over
the mass of eggs. In birds, although the eggs are laid and develop-
ment proceeds outside the parent organism, the presence of the
hard shell necessitates the introduction of spermatozoa into the
duct of the ovary, so that they may enter the ovum before the
outer layers are deposited on it. For this purpose, the male brings
the orifice of the tube down which the secretion of the testis is
poured into contact with the orifice of the oviduct and ejects the
spermatozoa into the oviduct. A similar process is, of course,
necessary in the mammalia, and in special cases in the lower
organisms, where the fertilised ovum remains within the mother's
body and completes its development up to an advanced stage
therein. Here the male is provided with a special organ, the penis,
for the purpose of more effective introduction of the spermatozoa
into the oviduct of the female. The act of doing this is associated
with feelings of pleasure in both sexes, as is obviously necessary
to ensure the continued existence of the race by the production
of offspring. It may be noted that a process of the kind is often
present although the fertilised eggs are afterwards laid and develop
outside the body of the female, as in many invertebrates.
The large size of the female gamete, or egg, as compared with
the male gamete, especially striking in the bird and reptile, is
miinly due to the fact that it is provided with a store of food
material for the growth of the young animal, but is also used by
ourselves for food. In the mammal, the food material is supplied
by diffusion from the blood of the mother to that of the growing
embryo. An organ, in which ramifying networks of blood-vessels
from both sources lie side by side, known as \\\z placenta, is present
for this purpose.
A word may be said here in regard to flowers. It is by no
means always realised that their beauty is, directly or indirectly,
connected with the process of sexual reproduction (E., p. 227). This
fact that they are, as has been said, " naked and unashamed," and
the absence of any suggestion of unseemliness should give us
matter for thought. Although the male and female gametes,
pollen grains, and ovules, are usually parts of the same flower, a
variety of devices exist in order to favour cross-fertilisation. The
pollen grains are not motile, but in some cases, as the grasses, they
are produced in enormous quantity, and, being very light, are blown
about by the wind. In other cases, the brilliant colouring, the
scent, and so on, serve to attract insects of various kinds to suck
the sugary solution, nectar, from the depths of the flower. In
doing this, they brush against the stamens and carry away pollen
from them. A visit to another flower deposits part of the pollen
GROWTH AND REPRODUCTION 155
on the stigma, whence the pollen tube grows down through the
tissue and fertilises the ovule.
There are many familiar differences, of no apparently serious
importance, between the two sexes in the greater number of
animals. These are the ''secondary sexual characters" and may
be illustrated by the hairs on the face in man, the mane of the lion,
the horns in some breeds of sheep, the crest on the newt, and so on.
It is found that they are due to the internal secretion of the sex-
glands, and indeed to that of a particular kind of cell, found both
in the ovary and in the testis, although, of course, the nature of
the secretion is not the same. These are known as the interstitial
cells. They are quite independent of the germ plasm from which
the ova and spermatozoa arise. The chemical substances respon-
sible for the effects seem to be sometimes inhibitory, preventing
the growth of hair on the face in women, for example ; sometimes
excitatory, as in the Herdwick ram, where removal of the testis
in the young animal stops any further growth of the horns.
Attempts to assign the difference between the properties of
maleness and femaleness to general physiological differences in
metabolism cannot be said to have met with great success. Thus
it has been suggested that the female is more prone to the synthetic
or "anabolic" changes, the male to the "catabolic," or breaking
down processes, supposed to be the bases, respectively, of inhibitory
and excitatory phenomena (see P., pp. 421-423). But the view
that food material is made into a complex protoplasmic molecule,
before being oxidised to afford energy, has been practically given
up as our knowledge of cell processes has grown. Indeed, the
conception of the universal occurrence of anabolic and catabolic
stages as parts of the same chemical reactions does not seem to
hold. In its application to the two sexes, it is pointed out that the
male is the more active and enterprising, the female slower and
more conservative. But the reader will be able to call to mind
many cases to the contrary, and the distinction, like many other
supposed sexual ones, is probably not of this nature at all, but
merely incidental.
Although the chick, when it is hatched from the egg, is able
1 3 pick up its own food, it is a familiar fact that the young of
mammals are comparatively helpless for some time after birth, so
that they depend on being fed with milk, secreted by special glands
possessed by the mother. These mammary glands, as they are
called, have been developed in the course of evolution from glands
in the skin. Milk itself contains all the constituents required by
a complete diet, such as we learned in our second chapter — a
sugar, lactose, fat in small globules, protein of two kinds, salts, and
the necessary accessory factors. It should be noted, however, that
156 INTRODUCTION TO GENERAL PHYSIOLOGY
cow's milk may not contain sufficient of the anti-scorbutic factor
to suffice for the human infant, and may require the giving of fruit
juice in addition. The mammary glands increase greatly in size
during the time that the young animal is growing, in the womb.
The stimulus to their growth is provided by a chemical hormone,
formed in a peculiar tissue which takes the place of the ovum after
it has left the ovary and been fertilised. This structure is called
the corpus luteum, from the yellow pigment contained in it (luteus =
yellow). The nature of the stimulus which excites secretion of
milk when the young animal is born is not yet clear. It has been
supposed that an inhibitory hormone is produced either by the
growing foetus itself or the placenta, so that when these have left
the body of the mother, the mammary gland is freed from the
agent which prevented its natural secretory process from being
manifested.
Heredity. — It can scarcely fail to arouse astonishment that such
minute structures as the ova and spermatozoa contain the potential
capacity of developing into organisms similar to the parent organ-
isms, even to details. It is obvious that, although the ova of the
cat and dog are so much alike, yet there must be represented in
them, in some way or other, the characteristics of the particular
animal. Various theories, resting on very insufficient evidence,
have been put forward, but they need not detain us.
There are, however, two aspects of the question which require
a brief consideration. The first of these concerns the facts of
inheritance associated with the name of Mendel^ who was abbot of
a large monastery near Vienna in the middle of the nineteenth
century. He found that certain inherited characters are subject to
two laws. For the sake of illustration, let us take the case of the
peculiar "waltzing" mice. If a normal mouse is crossed with a
waltzer, the offspring appears to be normal, and it might be thought
that the peculiar quality had been lost. But when these offspring
are bred together, it is found that the quality reappears in some of
their young. So that it was still present, but had been overpowered
by the normal quality in the first generation. Hence this quality
is called "dominant," while that of waltzing is "recessive." The
further remarkable fact is that there is a particular proportion
between the number of individuals of the normal kind and that of
the waltzers, namely, three of the former to one of the latter. How
is this to be accounted for ? Since the original pair may reasonably
be regarded as contributing an equal number of the factors in
question, we may take it that they are also so present in the total
generation of three normal and one waltzer. What happens then
is evidently that the factors must be arranged in the following way :
one individual contains only dominant factors, another only reces-
GROWTH AND REPRODUCTION 157
sive factors, while the other two contain equal amounts of both.
But in these latter, the recessive factor is prevented from showing
itself by the presence of the dominant factor. We have then some
individuals which are pure-breed, so that if mated with similar ones,
the progeny must be pure-breed. But those individuals which
contain dominant factors only cannot be distinguished from the
mixed ones, whereas those which show the recessive factor must be
purely recessive and can be depended upon to breed true. This
process is known as the segregation of pure gametes. The practical
object of breeding new races is thus to obtain the desired quality as
a recessive factor. Take the case of wheat. It is desired to obtain
a variety resistant to the attacks of the fungus known as " rust."
A race has been produced in which this character is recessive, and
hence can be bred true.
It is not to be supposed that all inherited characters obey
Mendelian laws, and there are many complications in detail which
cannot be entered into here.
The second aspect of heredity which requires notice is the
problem of the inheritance of characteristics acquired in the lifetime
of the parents themselves. It has been held by some that the
germ plasm conveys only those factors derived from distant
ancestry, and that it is in no way affected by what happens to the
individual organism which is its temporary host. There seems,
indeed, to be no satisfactory evidence of any kind of mutilation
happening to the parent being transmitted to the offspring, and the
germ plasm, as far as we know, is independent of nervous connec-
tion with the rest of the organism. At the same time, it cannot be
denied that it is accessible to chemical agents, and if such are pro-
duced by the various tissues, any alteration in them must have its
effect. It will be clear that the removal of any part which leaves
similar structures untouched cannot be expected to have any
chemical effect of the kind mentioned, while the removal of any
organ which is the only representative of its particular tissue is
usually followed by death. Thus the loss of a leg leaves similar
tissues intact, while that of the liver or suprarenals is fatal. That
the germ plasm is accessible to chemical influence is shown by the
experiments in which guinea-pigs were allowed to breathe the
vapour of alcohol. The offspring of the alcoholized animals were
deficient in strength and vitality, and often showed coarse abnor-
malities. The effects were transmitted through several generations,
even more marked in the later ones, although no exposure to
alcohol had been made since that of the original parents. No
changes in the sexual glands were visible under the microscope,
but the effects produced would be too subtle to be detected thus.
Variation.— Although the offspring are very like their parents,
158 INTRODUCTION TO GENERAL PHYSIOLOGY
it is well known that there are differences of a more or less marked
degree. These " variations " are often called " spontaneous," not as
implying that they are self-produced by a so-called inherent
tendency to vary, but that we cannot, as yet, assign an actual cause
to them. In any case, they are made use of by natural selection
for the production of new races and species. Darwin speaks of
" The Origin of Species by means of natural selection, or the
preservation of favoured races in the struggle for life," and he sums
up the general theory as follows : " As many more individuals of
each species are born than can possibly survive, and as, conse-
quently, there is frequently recurring struggle for existence, it
follows that any being, if it vary however slightly in any manner
profitable to itself, under the complex and sometimes varying con-
ditions of life, will have a better chance of surviving, and thus be
naturally selected. From the strong principle of inheritance, any
selected variety will tend to propagate its new and modified form."
The last sentence may be also put thus : An individual which shows
a new character is more likely in general to leave progeny possess-
ing this character than are those without it. If it is one that
enables its possessor to make better use of the forces of the
environment, this individual is better situated, especially by its
longer life, to leave more progeny, thus increasing the probability
of the permanence of the new character.
Adaptation. — We see that, according to the view expressed in
the last paragraph, the " adaptation " of organisms to their sur-
roundings, or their fitness to their environment is indirect. On the
other hand, certain writers have strongly advocated the existence
of direct adaptation. No really conclusive proof of any case has
yet been brought forward, and if we consider what it means, it
must be admitted that it seems very unlikely. It would mean that
the reaction of an organism to a new influence is such that the
effect of this influence is to produce either a means of making
appropriate use of it, or of meeting it, if injurious, by an appro-
priate defence. Since the reactions of an organism are necessarily
conditioned by its structure and properties, it is difficult to see
how a totally new condition would find the capacity of an appro-
priate reaction. The reaction may chance to be a favourable one,
but that is not direct adaptation. The only strong evidence is
derived from two characteristics of bacteria. The first is, that
in certain cases bacteria grown on a medium, which they
are unable to utilise as food, gradually develop in successive
generations the power to clo so. The weak point here is that we
are dealing with several generations, and we cannot be sure that in
the first culture there might not be a few individuals capable of
utilising to a small extent the new food material. If so, these would
GROWTH AND REPRODUCTION 159
multiply and the capacity be increased by natural selection in the
ordinary way. To be convincing, the development of the power
must be brought about in a single individual.
The second evidence is from the production of "anti-bodies " to
the action of bacteria (see p. 141 above). Before this can be
accepted as direct adaptation, we need to know much more about
the nature of the processes at work here. Unfortunately, hitherto,
investigations have chiefly resulted in the invention of names for
phenomena without explaining them.
Struggle for Existence. — Some incorrect and mischievous
interpretations have been made of the meaning of this phrase. It
has been taken to imply a conscious perpetual warfare between
individuals of the same species. In point of fact, those races are
most abundant which rely on social co-operation to make use of
the forces of Nature. Darwin himself points out that he uses the
" term in a large and metaphorical sense, including dependence of
one being on another."
The reader may remember that German writers defended the
bringing about of the late war, and the horrible way in which their
army carried it on, as justified by the "struggle for existence,"
being a necessity for the progress of the human race. But when
anti-social methods are made use of by a nation acting as a wild
beast, it becomes necessary for the more civilised nations to destroy
that beast or render it incapable of doing further damage, notwith-
standing the fact that much that is valuable may be lost in the
process. Moreover, when a nation adopts the savage method of
enforcing its systems and ideas by aggressive warfare, it becomes
the duty of the civilised nations to oppose it by the use of what
might appear to be the equally irrational method of defensive war-
fare. But there is no alternative, and the onus lies on those who
started the process.
Akin to the view of those who hold to the importance of
mutual aid as the chief factor in progress is that of Claude Bernard,
who points out that organisms develop by adaptation to cosmic
conditions, not by struggling against them. The living being is
not in contradiction to external forces, but is a part of the total
life of the universe. Bacon, again, said that " Nature is to be com-
manded only by obeying her."
To avoid misconception, it should be pointed out that such
views as the above do not in any way lessen the urgent necessity
for effort in discovering new means of making use of the forces of
the outer world, and of the most effective ways in which social
co-operation makes advance possible, not only in science and
industry, but in the arts that make life beautiful.
PART II
r r
LABORATORY WORK
NOTE.— The experiments described below will be found to vary greatly in their
difficulty and in the cost of the necessary apparatus. But I have not thought
it desirable to omit fundamental ones on either of these grounds, since their
performance is possible in many existing laboratories, and should become so in
the future to a much wider extent elsewhere.
The student may require the actual co-operation of the teacher in some
instances. Certain of these may become demonstrations. It is desirable,
however, that the student should, if possible, repeat the experiments for himself.
This would naturally depend on the time available.
If the course is held in a completely equipped laboratory, the teacher may
well be able to supplement the exercises given here.
CHAPTER I
The Microscope. — Since this instrument will be very frequently
in use, it is well to understand, in their main outlines, the principles
on which it is constructed. A few experiments with lenses will
also give information needed for the study of the eye at a later
time.
Take a biconvex lens of short focus, say about an inch, and note
that a large image of an object placed close to it can be projected
on to the wall of the room. The object and the image are at
conjugate foci, and there will be found to be numerous pairs of
such positions. The size of either is in proportion to its distance
from the lens.
An appropriate object is a small black cross on a piece of
ground glass. The glass is illuminated by a bright light behind it.
For many purposes, a small hand-feed arc-lamp is almost a
necessity. The best type is that in which the carbons are at right
angles to each other, since the bright crater in the positive carbon
is not obscured by the negative. The addition of a condensing
lens enables parallel rays to be obtained, and also the illumina-
tion of a small area with intensity.
The lens of short focus used in the experiment above represents
the objective of the microscope, at that end of the tube nearest the
object examined. But the great distance of the large real image
makes this an impossible method of obtaining the magnification
desired. If, however, we place a second convex lens in the course
163
1 64 INTRODUCTION TO GENERAL PHYSIOLOGY
of the rays at a few inches distance from the first, we bring the
rays to a focus at a short distance away. . A real image is formed
here, as may be seen by receiving it on ground glass. This lens
corresponds to the field lens of the ocular of the microscope, and
this lens may be used for the experiment. The eye-lens has the
function of magnifying the above image, just as an ordinary pocket-
lens does. In this case the image is not real, the eye being placed
close to the lens in the emergent beam. The image on the retina
in the eye is produced by the refractive system of the eye itself.
A graduated scale for measuring objects, or ruled in squares
for counting their number, may be placed at the position of the
focus of the field lens, and thus be seen superposed on the object.
Another form of ocular is sometimes used. In this, no real
image is formed : the eye-lens being closer to the field lens than
the position of the focus of the latter.
In making experiments on the properties of lenses, they may
be held in retort stand clamps.
The other details of the microscope stand can best be studied
on the instrument itself. Below the stage on which the object is
placed, we have arrangements for sending light through this. The
tube containing the observing lenses is provided with means for
accurately and conveniently bringing it to the correct distance from
the object in order that the image may be formed in the ocular at
the right place. Various objectives and oculars of different focal
distance or magnifying power can be placed in position in the tube.
An appropriate microscope is that sold for bacteriological
purposes, and may be obtained from several British makers. It
should possess a sub-stage condenser for illumination with high
power objectives, and also a dark-ground condenser to fit into the
same tube as the ordinary one. Objectives of 25 mm. and 4 mm.
focus are required, and a 2 mm. oil immersion is desirable. One of
the oculars should have a micrometer scale.
Nature of Protoplasm. — One of the protozoa, Amoeba, is the
best object for this study. As Professor Graham Kerr points out in
Nature (3ist October 1918, p. 166), the species required is the large
form, which is to be found in water containing abundant food
material and oxygen. Such a situation is the water trickling from
a boggy spot.
If masses of weeds be collected from a pond and allowed to
putrefy in water in a number of shallow covered dishes, amoebae
can often be found in the slime scraped from the surface of the
plants in some of the dishes. They are said to be most abundant
in about a fortnight, and then commence to disappear.
Another way is to collect the upper layer of the ooze from the
bottom of a shallow ditch or pond and allow it to settle in tall
LABORATORY WORK 165
narrow jars. The amoebae collect at the surface of the ooze as it
deposits, and the layer may be removed with a pipette and the
process repeated in a series of test-tubes. If not used at once, some
green algae, such as spirogyra, should be added to give oxygen.
They can often be obtained from dealers.
Should a source of this Amoeba proteus not be at hand, a culture
may be made from garden soil by the method described by Goodey
(Nature, 25th July 1918). Although most of those obtained are of
the smaller kinds, large ones may sometimes be found. The follow-
ing somewhat simplified method will serve our purpose. Boil some
hay or grass in water. Filter. Neutralise. Place a layer 2 to 3 mm.
deep in several Petri dishes. Add to each a gram or so of garden
earth. Keep in the light. After two or three days, according to
the temperature, amoebae may be obtained by floating a cover-glass
on the surface for a minute or two in order to allow the organisms
to fix themselves. Rinse gently with water, and a cleaner prepara-
tion will be made than if material is removed in a pipette. It is
well to place a short bit of hair on the slide in order to avoid
pressure on the organisms. If the surface film is used, a small
quantity of the liquid is to be placed on the slide before inverting
the cover-glass into position. Numbers of bacteria and ciliate
protozoa will be seen, especially if the material from the bottom be
taken. If the culture becomes nearly dry, encysted forms of amoeba?
will be found. The larger ones may be picked out and transferred
to fresh culture medium.
In addition to observing the properties of the protoplasm and
the formation of pseudopodia, the nucleus, the contractile vacuole
and the food vacuoles should be noted, especially the spherical
form of the latter. (See Huxley and Martin's "Practical Biology,"
pp. 21, 22.)
For electrical stimulation two strips of tin foil, or better, thin
platinum, are cemented on to a microscope slide with Prout's glue,
leaving a space of 2 mm. between them in the centre. Each of
these is connected to one of the terminals of the secondary coil of
an induction apparatus, as described below (p. 205), by means of a
fine wire which may be held in contact with the foil by a little lead
weight. A weak stimulus must be used to begin with, and gradually
increased until an effect is produced. A strong stimulus kills the
organisms at once ; their contents pass into the water, and are
dispersed therein. Note the significance of this fact as regards the
liquid nature of the protoplasm and the necessity for an outer
membrane of some kind.
The Leucocytes of the Blood. — If amoebae are not to be
obtained, many of the facts of the preceding section can be made
out on the colourless corpuscles of the blood. A high magnifica-
1 66 INTRODUCTION TO GENERAL PHYSIOLOGY
tion is necessary. A drop of blood is obtained by pricking the
finger or from the heart of a frog which has been killed by chloro-
form. The cover-glass should have vaseline painted around its
edges for a millimetre or so on the lower surface before placing it
on the blood. This is to prevent drying of the preparation. The
leucocytes will be found in the spaces between the columns of red
corpuscles. Movements will be slow or absent in the case of the
human blood unless the slide is warmed to body temperature.
This can be done by placing the slide on a piece of sheet copper
which has a hole to admit the light from below. This piece of
copper has a long narrow part projecting a few inches beyond the
edge of the stage. It is warmed by means of a small gas burner,
the temperature not being allowed to rise at the place where the
slide rests on it more than just feels warm to the finger.
Movement of Protoplasm. — This should be observed in the
hairs found on the stamens of Tradescantia. • The species, T.
virginica, with violet flowers, is grown in most gardens. The cell
sap, being coloured, obscures the protoplasm somewhat, but it can
easily be seen in the form of a layer lining the cell wall and fila-
ments stretching across the cell. There are continual streaming
movements, and the filaments change their position from time to
time. Two or three hairs are picked off with forceps by seizing
them at their bases. Mount in water.
There is a greenhouse species, T. discolor, with colourless flowers
and variegated leaves. The cell sap being free from colour, the
protoplasmic movements are more easily seen.
Dark-Ground Illumination. — The condenser used is sold in
various forms by dealers. Water is placed between it and the
bottom of the microscope slide to prevent the oblique rays being
reflected. The method is of especial value in observing fine
particles, with their Brownian movement and its cessation on
stimulation of protoplasm. It is very useful in observations on
bacteria. A brilliant source of light is required, and the small arc
lamp will probably be found best, using a condensing lens to make
the rays parallel. Large granules will appear too dazzling, but
the minute ones need to be well lighted. In using the method
with living cells, it is well to introduce a flat-sided glass vessel
filled with water between the lamp and the microscope in order
to cut off heat rays. It may also be advisable to add some
quinine sulphate dissolved with the aid of dilute sulphuric acid to
absorb ultra-violet rays.
Brownian Movement. — A cake of dry water-colour gamboge
is rubbed in distilled water and a drop placed on a slide. The
finer the particles the more vigorous the movements. They can
be seen quite well with the ordinary form of illumination, but much
LABORATORY WORK 167
better by the dark ground method. The object of using distilled
water is to avoid the aggregating effect of the lime salts in tap
water (see under ''colloids" below).
To see the fixation of the particles by a gel, rub the gamboge
in warm 5 per cent, gelatin, plr.ce a drop on a warm slide, cover
and watch while setting takes place. The movement will gradually
disappear.
Surface Tension. — Soap solution is made with pure sodium
oleate. Make a 2.5 per cent, solution in distilled water in the cold.
It may take a day or so to dissolve. Add one-third of its volume
of strong glycerine, and after shaking, allow to stand for a week in
a stoppered bottle in a dark place. Then remove the clear solution
from underneath the scum by means of a siphon. Add a drop of
strong ammonia to each 200 c.c. and keep in a stoppered bottle in
a dark place. Do not return any that has been used to the stock
bottle.
In the preparation of this and other solutions required later, a
chemical balance weighing 50 or 100 gm. and sensitive to I mg.
will be required. It is convenient to replace the left-hand scale pan
by a flat-bottomed porcelain basin. This is carefuly counterpoised
by a piece of lead suspended on the hook at the top of the opposite
scale suspension. A porcelain pan can be washed, so that it is
unnecessary to use paper for the weighing of chemicals, which must
never be put into the metal scale pan.
A useful tube for blowing bubbles is a wide glass tube, such as
the chimney of certain paraffin lamps. A perforated cork is fitted
in the narrow end, and a short piece of J-in. tubing inserted in the J
hole. Dipping the wider end into soap solution, a film is made
closing the end. This is blown out into a spherical bubble and the
end of the finger placed on the mouth-piece. The bubble remains
the same size ; but if the finger be withdrawn, surface tension causes
the bubble to contract, driving air out.
Make a film on the wide end of a funnel, closing the end of the
stem with the finger. As soon as the finger is removed, the film i/
proceeds to rise up to the narrowest part of the funnel. Thus,
surface energy is doing work in raising the weight of the film.
For the ring experiment iron wire serves well. A circle is made
with a projecting handle. The end of that part forming the circle j
may be soldered to the base of the handle, or merely twisted around
it. " A little loop of fine sewing silk is tied to a point on the ring, so
that the loop is suspended about the middle of the space. It saves
trouble if it be tied to the opposite point of the ring also, but the
suspensions must not pull tightly. Dipping the ring into soap
solution in a flat dish, a film is formed in which the loop floats. By
moving it about with a needle, it can be made to take any shape,
1 68 INTRODUCTION TO GENERAL PHYSIOLOGY
as long as the film is present inside as well as outside the loop. If,
now, the film inside the loop is broken by touching it with a pointed
bit of filter paper, the tension outside pulls the loop into a circular
shape.
This film on a ring may also be used to show that a needle can
be dropped through without breaking the film. The needle should
first be wetted with the soap solution.
To see the spherical form taken by drops of liquid requires some
trouble and skill. The method recommended by Boys is to pour
into a bottle a tablespoonful of olive oil together with a mixture of
nine parts by volume of rectified spirit (or methylated spirit con-
taining no petroleum oil) and seven parts of water. Shake up and
leave until the oil has separated again. It may be either above or
below the alcohol, according to the specific gravity of the latter.
Fill a beaker with a similar mixture of alcohol and water. If the
oil had risen to the top in the bottle, add a little water to the liquid
in the beaker by a pipette dipping about half-way down. Take
some of the oil in a pipette and empty into the middle of ths beaker.
If it sinks, a little more water is wanted in the lower half cf the
beaker ; if it floats, a little more alcohol is wanted in the upper half.
To see the shape truly, it must be looked at from above the liquid,
since the curvature of the sides of the beaker distorts the image.
When the right specific gravity of the alcohol has been obtained,
more oil may be added to the drop, best from a tap-funnel
slowly.
Adsorption. — A bubble is blown as above, but using a dilute
solution of saponin in distilled water (about 0.5 to I per cent).
Allowing the bubble to contract, or by sucking air out of it, it
becomes more or less rigid and goes into folds (Ramsden). Solid
particles of saponin will be seen in the film, and especially'in the
drop at the lower end of the bubble.
Add charcoal in powder to a dilute solution of crystal violet
until the colour is removed. Filter off the charcoal and wash it on
the paper with acetone^ After the water is replaced by acetone
the drops falling through will be stained violet. It is better to
make the acetone slightly acid with a drop of hydrochloric acid, on
account of the fact that it is the free base of the dye that is adsorbed.
Cell Membrane and Permeability. — Having found an amoeba
or leucocyte, which is known to be living by its movements, add a
dilute solution of aniline blue by placing a drop at one end of the
cover-slip and a fragment of filter paper at the opposite end. The
organism does not stain as long as it is alive, although any bits of
dead material become blue.
Slices of red beet are washed under the tap in order to remove
the contents of the cells injured by the knife. Left in tap water, the
LABORATORY WORK 169
slices do not give up their pigment. If boiled, so as to kill the cells,
the water becomes red.
To show that the living cells are also impermeable to sugar a
chemical test must be used. Owing to its aldehyde group, glucose
reduces copper salts in alkaline solution. The sugar in beet is,
however, cane sugar, which has no free aldehyde group. It must
be hydrolysed by boiling with acid. The test may be made with
the water in which living beet has lain, and also with that in. which
it has been boiled. Add to a sample in a test-tube a drop or
t\vo of strong hydrochloric acid, boil for a minute, or immerse the
end of the test-tube in boiling water for a few minutes. Cool. Add
a few drops of copper sulphate solution, a crystal of Rochelle salt,
and then sufficient sodium hydroxide to make a deep blue solution.
On boiling again, a red precipitate of cuprous oxide is formed if
a reducing sugar is present.
The effect of other methods of killing the cells of the red
beet may be tested by adding chloroform, formaldehyde, or acid.
Substances which lower surface energy, such as saponin, bile salts,
or amyl alcohol, also have the effect of allowing the pigment to
escape. In some specimens, it may be possible to observe the
reversible increase of permeability produced by sodium chloride.
Take a I per cent, solution in distilled water, change several times,
and the pigment may be seen to escape slowly from the cells.
The normal state is reproduced in such cases by the addition of
a calcium salt in small amount, most simply by changing to hard
tap water.
Dilute acid and alkali may also be tested, say, o.oi per cent,
hydrochloric acid and a similar strength of sodium hydroxide.
Osmotic Pressure
Blood Corpuscles. — Make a preparation of blood from the finger
as described above (p. 166). Measure the diameter of a number of
corpuscles and take the average. The measurement is done by a
scale in the eyepiece, the value of its divisions having previously
been determined by the aid of a scale divided into o.ooi mm.
placed on the stage and observed with the same objective as that
used for the blood preparation.
On allo\ving water to run under the cover-slip, the corpuscles
rapidly swell up and their contents escape, leaving the framework
as a nearly invisible disc. This action of water is, however, so
vigorous that it is difficult to follow the series of changes. It
is better to take a solution of sodium chloride of about 0.3 to
0.4 per cent.
Make next solutions of cane-sugar of 15, 10, and 5 per cent.
i/o INTRODUCTION TO GENERAL PHYSIOLOGY
Place a drop of each on a slide, and add a drop of blood, mixing
with a needle. The corpuscles in 10 per cent, solution ( = 0.3
molar) will be practically unaltered in size. Those in 5 per cent,
will be increased, those in 15 per cent, will be decreased. Since
the osmotic pressure of the serum is not always exactly the same
in different cases, some increase or decrease may be produced by
10 per cent, cane-sugar. If the former, try a slightly stronger
solution ; if the latter, a slightly weaker solution.
Take 2 or 3 c.c. of 10 per cent, cane-sugar solution in a test-
tube. Add a drop of blood. The corpuscles will deposit gradually,
leaving the solution colourless. Add water slowly, noting how
much is added. At a certain dilution, the solution will begin to
become coloured with haemoglobin, owing to breaking up of the
corpuscles (Jicei no lysis}. Note that the dilution required is greater
than corresponds to the solution in which the corpuscles maintain
their normal size (isotonic sohitioti}. This is due to the fact that
they are able to swell to a notable degree before bursting.
Similar experiments may be made with 0.3, 0.9, and 2 per cent,
sodium chloride. The isotonic solution will be found to be 0.9 per
cent. Note that a 0.3 molar solution is 1.75 per cent, and that
0.9 per cent, is only 0.154 molar. A 1.75 per cent, solution makes
the corpuscles shrink ; it is hypertonic.
Next take a 0.3 molar solution of urea (=1.8 per cent). It
causes haemolysis as if water. Dissolve the urea in 0.9 per cent,
sodium chloride ; no haemolysis occurs. Hence the effect of the
pure urea solution is not due to a toxic action of the urea, but to
the permeability of the corpuscles for urea.
Saponin or ether, even in 0.9 per cent, sodium chloride, causes
haemolysis, which is due in these cases to a destruction of the
osmotic properties of the cell membrane.
Take defibrinated blood, to be obtained from the slaughter-
house. Or better, cut off the head of a rat, collect the blood, and
stir it with a feather. Put about 10 c.c. into a graduated centrifuge
tube, and centrifuge until the volume of corpuscles, read off on the
scale, no longer changes its value. Pour off the serum, which is
probably slightly coloured with haemoglobin, so as not to disturb
the deposit. This may be done by using a glass rod touching the
lip of the tube. It does not matter if a little serum remains. Add
15 per cent, cane-sugar, shake up the corpuscles with it gently
and centrifuge again. The volume is less. Repeat with 5 per
cent, solution, the volume is increased as compared with the
original one.
For this experiment a simple hand centrifuge, as sold by the
dealers, suffices. The hoematocrite, in which smaller tubes are used
and spun at a greater rate, is used for small quantities of blood,
LABORATORY WORK 171
such as can be obtained from the finger. It is more expeditious,
but rather more difficult The point at which no further change
in volume occurs by centrifuging longer is recognised by the fact
that the corpuscles are so closely packed together that the narrow
column appears transparent red, instead of being opaque.
Plasmolysis. — Make a preparation of the hairs of Tradescantia
virginica, the epidermis of the leaf of T. discolor, or a thin section,
made with a razor, of the root of the red beet. The protoplasm
of the cells in each case is a bag containing coloured cell sap. If
exposed to a solution which has a higher osmotic pressure than the
cell contents, provided that the cell membrane is impermeable to
the solute, water passes out and the protoplasmic sac shrinks,
leaving gaps in places between itself and the cellulose envelope.
Solutions of potassium nitrate may be used. As a rule, 0.15 molar
(=1.5 per cent.) gives no effect, 0.25 molar (2.5 per cent.) has an
obvious effect, 0.2 molar is about isotonic. This is equivalent to
an osmotic pressure of five atmospheres
Turgor. — Take the stalk of a flower, such as the daffodil or
dandelion. It is stiff owing to the tension of the cell walls pro-
duced by the difference of osmotic pressure between the cell
contents and the very dilute watery solution outside. As the
tissue dies it becomes flaccid. The effect can be produced quickly
by exposure to ether vapour in a test-tube, and, if the action has
not been too great, recovery may be brought about by soaking in
tap water.
The cells may also be killed by putting a little water in the
bottom of a test-tube, placing the stalk in the tube and then
boiling the water.
"•" Contractile VacuoJe. — To see the discharge of this vacuole in
the amoeba or other protozoon, add suspension of indian ink to
the liquid in which the organism is. Do not use too much. When
the vacuole contracts, the particles are driven away by the current
from it.
Direct Measurements of Osmotic Pressure. — Such measurements
are not easy, on account of the difficulty of preparing suitable
membranes. To see the fact of the production of pressure, parch-
ment paper may be used with a solution (5 to 10 per cent.) of an
electrolytically dissociated colloid, such as caseinogen or congo-red.
The paper may be clamped in an osmometerof the pattern described
by Moore and Roaf (Biochem. Journ., vol. 2, p. 34), or the simpler
form of Roaf (Quart. Journ. Exper. PhvsioL, vol. 3, p. 79). It is
possible also to fix the membrane on the edge of a small bell glass
open at the top and provided with a flange at the lower edge.
The membrane, in this case, must be glued on with gelatin, and
parchment paper is not readily wetted with the solution. It should
1 72 INTRODUCTION TO GENERAL PHYSIOLOGY
be first well soaked in warm gelatin solution, the edge of the glass
being also immersed in the solution. After removal of both, the
paper is left in position on the glass until the gelatin is set. It
should then be hardened by immersion in 5 per cent, formalin. It
is necessary to support it by a disc of nickel gauze, tied on by
string or wire brought over the top of the glass. A narrow U-tube
containing mercury is fixed in a rubber cork in the upper opening
of the glass.
The caseinogen solution is made by taking the required amount,
rubbing it in a mortar with a little water to which a drop of phenol-
phthalein has been added. Ammonia is dropped in until the colour
has become pink. The volume is then made up in a measuring
flask to the required value.
If possible, an accurate measurement should be made of the
osmotic pressure of the colloids of blood serum, using as the outer
liquid a 0.9 per cent, sodium chloride.
Exact determinations of freezing and boiling points are also
rather difficult. The student should take that of distilled water by
immersing a test-tube, containing a few c.c. of it, in a mixture
of ice and salt ; there should not be a large proportion of salt. A
thermometer is immersed in the water, which must cover the bulb.
Continually stirring by means of a ring of wire fixed in a glass
tube by means of sealing-wax the temperature will fall steadily to
o°, and remain so for a little time. Taking out the tube from the
freezing mixture, particles of ice will be seen floating in it, or a
thin layer of ice may be present on the inside of the tube. The
temperature will be seen to remain at zero until this ice has melted,
provided that the stirring is continued. Repeat with a molar
solution of sodium chloride (5.85 per cent). The temperature will
fall lower before remaining steady, and will remain at this Jower
temperature on removal until the ice has disappeared. Only a
small amount of ice should be allowed to form, because it is pure
ice (frozen water) that separates, leaving a more concentrated salt
solution. The lowering of the freezing point by molar sodium
chloride is about 3°.
In connection with the problem of electrolytic dissociation, it
is well at this stage to compare the freezing points of molar solu-
tions of sodium chloride and of urea. We have already found that
of the former. Make a molar solution of pure urea ( = 6 per cent).
It will be found to have a freezing point of — 1.86° only.
A rough measurement of the boiling point of a solution may
be made by the use of a flask in which it is kept boiling gently,
while the bulb of the thermometer is suspended just above the
level of the liquid. A bit of porous clay, such as the stem of a clay
tobacco pipe, put in the liquid, will assist steady gentle boiling.
LABORATORY WORK
173
The effect of a solute in lowering the vapour pressure of the
solvent may be tested by allowing two vessels, such as flat porcelain
capsules, one containing water, the other 10 per cent, sodium
chloride, to remain for some time side by side under a small bell
glass, closed by resting the rim, greased with vaseline, on a piece
of plate glass. The two vessels are weighed to begin with, and \
again after a few days. Water will be found to have passed from the /
vessel containing it in the pure form to that containing the solution.
Electrolytic Dissociation.— To make measurements of electro-
lytic conductivity with accuracy requires somewhat complicated
B
FIG. 10.
A, dry cell or storage cell.
B, key.
C, high resistance slide wire to obtain a fraction of the E. M. F. of the battery. For
the present purpose, it need not be graduated, but this may be necessary for other
work.
D, sliding contact.
E, galvanometer.
F, small beaker with the platinum electrodes.
apparatus. For our purpose it will suffice to use a galvanometer,
and to observe the deflections produced by the same potential
difference through the various solutions to be compared. The
single-pivot galvanometer (one scale division = one micro-ampere),
made by Paul, will be found to be convenient. The current is sent
through the solution by means of two platinum plates, about I cm.
square, immersed therein. They should have platinum wires welded
to them, and the wires then fused into glass tubes passing through
holes in a flat rod of ebonite, about 2 cm. apart. The circuit is
arranged as in the diagram (Fig. 10). The wires to the electrodes
should be fine copper wires and inserted into mercury in the glass
tubes, in order to make contact with the platinum plates.
174 INTRODUCTION TO GENERAL PHYSIOLOGY
Take first normal hydrochloric acid, which can be bought.
Adjust the slider until on closing the key a deflection is obtained
which does not exceed the limits of the scale of the galvanometer.
Note the maximum reading. This rapidly diminishes if the circuit
is kept closed, on account of polarisation of the electrodes. In
accurate work an alternating current is used to avoid this effect, as
described below, and the electrodes are coated with platinum black
(see Findlay's " Practical PhysicalChemistry," p. 150).
Repeat the experiment, using acetic acid in the same mole-
cular concentration. It is made by taking about 54 c.c. of glacial
FIG. ii.
A is a resistance of 1000 ohms.
B is the cell containing the solution to be measured.
C and D are the slide wire of one metre length, graduated.
E is a small induction coil, such as used for medical purposes, but provided with a
light steel spring as vibrating contact to give a high pitched note. The secondary
winding is connected to the circuit.
F is a telephone.
acetic acid and diluting to a litre with distilled water. It is then
titrated with standard sodium hydroxide, using phenol-phthalein as
indicator. It is best to make it rather too strong and then diluting
to the proper volume. The standard alkali can be bought, but
should be tested against the normal hydrochloric acid. The
deflection obtained with the acetic acid will be less than that with
hydrochloric acid. Hence there are more conducting constituents
in the latter than in the acetic acid. In other words, the strong
acid is more highly dissociated electrolytically than the weaker
acid.
To make more accurate determinations, the use of alternating
currents is necessary. The following arrangement of the Wheat-
LABORATORY WORK 175
stone bridge is used (Fig. 11). When C and D are of such relative
lengths that no sound is heard, the resistances in the arms are such
A C
that A is to B as C is to D, or — =— z>., if C is in centimeters, and
JD L)
A = 1000 ohms, -=— = -^ or B = 1000 ( - ~ j in ohms.
If this apparatus is available, the resistances of molar and deci-
molar solutions of both hydrochloric and acetic acids should be
compared. If dilution had no effect on the acid, the resistance of
the deci-molar acid should be ten times that of the molar. In the
case of the strong acid, this will be found to be very nearly what is
found. In that of the weak acid, the resistance of the deci-molar
solution will not be so much as ten times that of the molar acid,
showing that dilution has caused the formation of a greater pro-
portion of conducting ions than were present in a portion of the
original solution containing the same quantity of acid.
Take next defibrinated blood and obtain some serum by the
use of the centrifuge. Place the serum in the vessel used for the
acids and determine its resistance, best by the telephone method,
but it may be possible with care to see the effects when the first
simple method is used. Replace the serum by an equal volume of
the corpuscular deposit. The resistance will be much greater.
That the lower conductivity of the corpuscles is not due to the
absence of electrolytes from them, but to the fact that they are im-
permeable to these electrolytes and cannot therefore conduct an
electrical current, can be shown by adding a small quantity of
saponin. This, as can be tested, has itself a negligible power of
conduction. But the conductivity of the blood corpuscles is
greatly increased, because saponin destroys the cell membrane.
Indicators. — That it is the hydrogen-ion concentration of a
solution to which these substances react can be made obvious by
taking a solution of crystal-violet of about o.i per cent, and adding
a drop to each of a series of dilutions of hydrochloric acid, begin-
ning with twice molar and going down to one-thousandth molar.
The dye is yellow in the strongest, green in molar acid, greenish-
blue in o.i m., blue in o.oi m., violet in o.ooi m.
That different indicators react to different hydrogen-ion con-
centrations can be seen by taking a known volume of deci-normal
sodium hydroxide (adding phenol-phthalein, a few drops of a o.i
per cent, solution in alcohol), and adding from a burette deci-
normal phosphoric acid until the colour disappears. Note the
amount of acid required. This indicator shows a concentration of
hydrogen-ions of o.oooooooi (io~8) normal as being acid. Add
next to the same solution a few drops of o.i per cent, methyl-
orange. The colour is yellow, that of the dye in alkaline solution.
1 76 INTRODUCTION TO GENERAL PHYSIOLOGY
Add acid until it becomes red, and note that the extra quantity
required is distinct. This indicator does not show a solution to be
acid until its hydrogen-ion concentration has risen to o.oi (io~2)
normal. The colour is orange-red at o.ooi normal. The difference
in the amount of acid required will, of course, be more obvious if
o.oi molar acid is taken instead of the o.i molar.
As exercises in the use of indicators, determine the hydrogen-
ion concentration of urine, which will usually be found to be acid
to methyl-red, just alkaline to methyl-orange. The colour of the
latter dye in it is brownish, not yellow, corresponding to a
hydrogen-ion concentration of io~4(see P., p. 189). Blood serum
after exposure to the air is yellow to neutral-red ( = io~9). Brought
into equilibrium with the last fraction of a deep expiration
( = alveolar air, containing 4.5 per cent, of carbon dioxide) it becomes
orange-red. That is, just on the alkaline side of neutrality.
Neutral-red in pure distilled water is red, not crimson. The
slighest trace of acid turns it crimson ; of alkali, yellow. It is thus
a valuable indicator in the region about the neutral point (io~ri).
The saturation of liquids with gas mixtures, such as alveolar air, is
performed in vessels containing a large volume of the gas in pro-
portion to the liquid. For the above purpose, a stoppered bottle
of about 100 c.c. capacity will serve. Two or three c.c. of the
serum with a drop of o.i per cent, neutral-red are placed in the
bottle and the expired air breathed into the bottle through a glass
tube. The stopper is replaced quickly, and the bottle rotated so
as to make a thin layer of the serum over the surface.
The Colloidal State
Colloidal Gold is readily prepared by Faraday's method as
follows : — Take a solution of gold chloride in pure distilled water,
containing about one part of the salt in 8,000 of water. Put it in
a clean bottle or flask. Add a drop of a solution of phosphorus in
carbon bisulphide. Shake and leave for a few hours. A beautiful
clear red solution is obtained.
This solution is shown to contain solid particles in suspension
by passing a bright beam of light through it (Faraday phenomenon).
The beam from an arc-lamp, brought to a focus by a condenser, is
appropriate. To avoid disturbing reflections at the surfaces of the
glass, the vessel may be immersed in water in a large beaker.
This fact was described by Faraday, and correctly interpreted as
showing the presence of particles of metallic gold. Tyndall pointed
out subsequently that if the track of the beam be looked at through
a Nicol prism, it is found to be polarised, being extinguished in a
particular position of the prism. This shows that the size of the
particles is near that of the wave length of light.
LABORATORY WORK ' 177
As another example of a suspensoid, skake up kaolin (china
clay) with distilled water, and allow the coarser particles to settle.
The finest particles will remain dispersed for some hours, and such
a preparation serves well for experiments.
Emulsoids. — Prepare a solution of gelatin by allowing a sheet of
the dry substance, as sold, to soak in water until softened, and
then dissolving it in hot water. As the solution cools, it sets to the
familiar jelly.
Another emulsoid which does not set to a jelly is white of egg.
This has another property, that of becoming solid when heated, as
well known. In this state its properties change to those of a
suspensoid.
The increased swelling of gelatin in the presence of acid may
be shown thus : Allow a sheet to soak in water. Cut out a number
of discs with a cork borer of about c cm. in diameter. Place some
in distilled water, others in deci-normal hydrochloric acid. After
some hours measure the diameters of a few of each.
Surface Tension and Dispersion. — Soap has a powerful effect in
lowering surface tension. Olive oil usually contains a small
quantity of free oleic acid, and when alkali is added, this forms
soap (see later, page 190). Olive oil shaken with water forms an
emulsion, but the drops of oil quickly coalesce and rise to the
surface. If a very small amount of sodium hydroxide be added
and the mixture again shaken, the soap formed lowers the surface
tension at the contact of the water and oil, so that the drops
have little tendency to unite, and a nearly permanent emulsion is
produced.
Electrical Charge. — The simplest method of determining the
sign of the charge on colloids is to take a U-tube of about a centi-
metre in diameter. Fill with the solution, and place in it at the
upper end of each limb a piece of platinum foil connected with a
source of potential difference of some 200 volts, such as the direct
current house lighting supply. One electrode will be positively
charged ; the other negatively. A lamp should be inserted in the
circuit to diminish risk, should the electrodes be accidentally brought
into contact. The sign of the poles is determined by placing the
two electrodes 2 or 3 cm. apart on a piece of filter paper wetted
with a solution of sodium sulphate to which phenol-phthalein has
been added ("pole-finding paper"). The negative pole produces a
red stain, owing to the alkali formed. After the connection to the
tube has been made for some minutes, the space around and below
one of the poles will become clear, owing to the repulsion by the
electrode of particles of the same sign as itself.
Two typical suspensoid colloids are arsenious sulphide and
ferric hydroxide. The former is negative ; the latter, positive.
12
i;8 INTRODUCTION TO GENERAL PHYSIOLOGY
Arsenious sulphide is made by passing a current of hydrogen
sulphide through a saturated solution of arsenious acid in distilled
water and allowing the coarse particles to deposit. It may, with
advantage, be dialysed to remove the dissolved gas. The process
is described below.
Ferric hydroxide is made by taking a strong solution of ferric
chloride (or better, ferric acetate, as used by Graham, on account
of the greater hydrolytic dissociation of the salt of the weak acid).
Place in a dialyser made by tying a piece of wet parchment paper
over the wide end of a bell glass, such as used for osmotic ex-
periments previously, but larger. Repeated changes of distilled
water on the outside remove most of the hydrochloric or acetic
acid formed by the hydrolysis of the salt. Colloidal ferric hydrox-
ide may also be made by dissolving precipitated ferric hydroxide
in ferric chloride and then dialysing to remove excess of electro-
lytes. The " solution of dialysed iron " of the " Pharmacopoeia "
may serve also, but it is better to prepare it.
Action of Electrolytes. — Make solutions of:
Potassium sulphate — o.i molar in K' (0.88 per cent.).
Calcium sulphate-o.oi molar in Ca" (0.173 Per cent- of gypsum).
Lanthanum sulphate-o.ooi molar in La-" (0.0364 per cent, of the
cryst. salt).
Add equal volumes of each to three samples of arsenious sulphide.
The precipitating power is about the same. Potassium sulphate
in o.ooi molar solution has no effect. Since the lanthanum solution
is much more effective than the potassium, although the SO/ ion
is in only T)\th the concentration, it is clear that it is the cation
(positive) that is the active one, and the greater power of the
bivalent and trivalent ions is obvious. Thus, a negative colloid
is precipitated by a positive ion.
A similar series of experiments may be made by precipitating
ferric hydroxide with the following solutions : —
Sodium chloride -o.i molar in Cl' (0.585 per cent.).
Sodium sulphate -o.oi molar in SO4"(o.33 per cent, of the cryst. salt).
Sodium phosphate -o.oo i molar in PO4"' (0.0138 per cent of the acid
phosphate).
The phosphate solution is made neutral to neutral red by adding
sodium hydroxide. These three solutions will be about equal
in precipitating power. Thus the electro-positive colloid is aggre-
gated by an ions.
The sign of the charge on kaolin in suspension may be deter-
mined by testing with the two series of salts. It will be found
to be electro-negative.
LABORATORY WORK 179
Mutual Precipitation of Oppositely Charged Colloids
Take a series of test-tubes containing equal quantities of
arsenious sulphide, add gradually increasing quantities of ferric
hydroxide. At a certain relative proportion, dependent on the
concentration of the colloids, there will be complete precipitation
of both. At other proportions, a compound colloid will be pro-
duced with excess of one or the other sign, and will remain more
or less completely in suspension. As a rule, the ferric hydroxide
solutions used will be found to be more concentrated than the
arsenious sulphide.
Take also a 5 per cent, solution of egg-white and filter it.
Make a part acid to neutral red by the addition of acetic acid, and
another part alkaline with sodium hydroxide. The former will
be precipitated by arsenious sulphide, not by ferric hydroxide ;
the latter, the converse. Thus we have produced an electro-
positive colloid by excess of H-ions, and an electro- negative one
by excess of OH-ions.
Excess of Electrolyte. — If we add a solution of a precipitating ion
in excess, it may happen that, instead of obtaining precipitation, the
particles have conferred upon them a charge of the sign opposite to
their original one, so that the concentration of the requisite precipi-
tating ion is insufficient. They remain, in such a case, suspended.
The experiment is rather difficult, on account of the fact that the
actual amount required can only be found by trials. It varies with
the dimensions of the particles. It may be tried with a suspension
of gamboge obtained by pouring a small amount of an alcoholic
solution of the gum-resin into a large amount of distilled water.
Adding 0.0016 molar cerium chloride to an equal volume will
precipitate it, whereas 0.16 molar will probably not do so, or not
so rapidly. If no difference is found, try intermediate concentra-
tions. The distinction is seen best by shaking again after the
first deposition and noting the second effect.
Staining and Electrical Adsorption. — Take some circles of filter
paper of 9 or 10 cm. in diameter. For good results, the paper
should be the purest analytical preparation, and the dyes should
be free from mineral salts. Congo-red may be dialysed, since it
usually contains sodium sulphate and chloride. Immerse a circle
of paper in a weak solution of crystal-violet and another in a weak
solution of congo-red. The former rapidly takes on a deep colour,
the latter very little. Add 0.5 per cent, sodium chloride to each
of two fresh samples. The staining will be much increased in the
case of congo-red, decreased in that of crystal-violet.
Rate of Chemical Reaction between Colloids. — The best way to
observe the comparative slowness of this is to prepare a colloidal
i8o INTRODUCTION TO GENERAL PHYSIOLOGY
solution of the free acid of congo-red and add to it a suspension of
aluminium hydroxide, which has been well washed. Metallic zinc
in powder (zinc dust) may be used, but the experiment is not so
striking. The colour of the mixture is at first that of the free acid,
deep blue. Allowing it to stand, chemical combination slowly
occurs, with the production of the aluminium or zinc salt. Com-
bination can be hastened by warming. The solution of the acid
is made by adding hydrochloric acid to a solution of congo-red.
The deposit is suspended in water, and dialysed until free from acid.
Contrast the rate of this chemical reaction in heterogeneous systems
with the immediate deposition of saponin in the bubble made in
the experiment on p. 168.
CHAPTER II
Chemical Composition of Organisms
Carbon. — Heat a little yeast or any organic tissue in a dry test-
tube. It will char, owing to the production of carbon.
Hydrogen. — If the tissue had been previously dried in the above
experiment, the deposition of water on the upper part of the test-
tube will show the presence of hydrogen.
Oxygen. — That the oxygen contained in the water of the above
experiment comes from the tissue, and not from the air, can be
shown by performing the experiment in an atmosphere of coal
gas. The test-tube is closed by a cork through which pass two
glass tubes, one to the lower end, the other ending just below the
cork. The former is connected to the gas supply by means of a
rubber tube, the latter to a Bunsen burner. Gas is passed through
the tube until the air is displaced. The Bunsen burner is then
lit and the tube heated as before.
Nitrogen. — Mix the yeast, dried, with some dry soda-lime.
Heat in the test-tube. Ammonia is given off, detected by its
smell, and its turning moist red litmus paper blue. The fumes
given off when hydrochloric acid on a glass rod is held at the
mouth of the test-tube are also characteristic of ammonia.
Sulphur and Phosphorus. — Boil with a little strong nitric
acid to oxidise the sulphur to sulphate and the phosphorus to
phosphate. Dilute with water. Filter if necessary. Test a part
with barium chloride, precipitate shows sulphate. Add to another
part a few drops of a strong solution of ammonium molybdate and
heat. A yellow precipitate indicates phosphate.
The Polarimeter
A simple form of this instrument will serve to ascertain the
fact of optical activity. In fact, if a strong solution of sugar or
of egg-white be used in a vessel with flat glass sides, two Nicol
prisms may suffice. A beam of light is sent through one of these,
clamped in a retort stand, in order to polarise it. The beam then
passes through the solution and, lastly, through the second Nicol,
181
1 82 INTRODUCTION TO GENERAL PHYSIOLOGY
which must be capable of rotation. In the absence of the solution,
adjust the position of the second prism so that the field is as dark
as possible. Interpose the solution. If optically active, the field
will become light. In the case of cane-sugar, the second prism
(analyser) must be rotated in the direction of the movement of the
hands of the clock in order to restore darkness ; in that of egg-
albumin, in the opposite direction. The actual amount of rotation
must not exceed 180° in this experiment, otherwise confusion may
result. If in doubt, dilute the solution until the fact of rotation can
be only just made out.
A more accurate and sensitive polarimeter is necessary for
certain important experiments with enzymes to be described later.
Waste Products
To prove the production of carbon dioxide in the course of
vital reactions, all that is needed is to breath out through a tube
FIG. 12. — Urea Apparatus.
A, large, test-tube, immersed in warm water.
B, bottle containing acid, connected to a filter pump.
into recently filtered lime water, or solution of barium hydroxide.
The precipitate is shown to be carbonate by its solution in dilute
acetic acid.
The chief waste-product of nitrogen metabolism is urea, which
is excreted in the urine. To prove its presence, the best method is
to convert it into ammonium carbonate by the agency of the enzyme
urease, found in Soy beans and elsewhere. A few beans are ground
in a coffee mill, the powder sifted through a sieve, and a portion of
what passes through is added to some fresh urine in a closed bottle.
Allow to stand for an hour or two in a warm place. On opening
LABORATORY WORK 183
the bottle, ammonia may be detected by its smell and the other
tests described above. If not, add dry sodium carbonate, draw a
current of air through the warmed mixture, and then through water
made acid (blue) to congo red. This colour will turn red quickly,
and more acid may be added from time to time. Fig. 12 shows the
arrangement.
Carbon Cycle
Water Culture.— The solution to be used may consist of the
following salts :
Calcium nitrate 4 gm>
Potassium nitrate -
Magnesium sulphate
Acid potassium phosphate
Potassium chloride
i
i
i
0.5
Tap water 3 iitres
A drop or two of dilute ferrous sulphate solution
A large glass jar is provided with a wooden lid having a hole in the
middle. The seedling of a Windsor bean, selected from a number
which have been allowed to germinate between wet filter paper, is
gently supported in the hole by means of bits of cork, so that the
root dips into the solution. The wooden cover and the corks
should have been soaked in melted paraffin wax. The weight of
the bean, dried in air, is noted before germination. After the plant
has grown to a foot or more in height, it is removed, allowed to dry
in the air, and weighed again.
For success, growth must take place in a good light, the root
being kept dark by a covering of brown paper on the jar. If
exposed to the sun, the jar should be in a box filled with sawdust
to prevent the solution becoming hot. The solution is changed at
a few days' interval, and air blown through it occasionally to supply
oxygen to the roots.
The experiment, of course, requires some weeks for completion.
Action of the Green Plant on Carbon Dioxide
Make first an analysis of atmospheric air. This may be
done with the Hempel burette and two Hempel pipettes, one
for caustic soda, the other for alkaline pyrogallol. For our pur-
pose, it will be simpler to use a nitrometer tube, connected with
a reservoir of mercury, and to perform the analysis in the tube
itself. The arrangement is represented in Fig. 13. The tube A
is graduated into o.i c.c. and can be connected either with the
funnel H or the tube C, or closed altogether, by means of the
3- way stopcock. A thick-walled rubber tube is attached to the
1 84 INTRODUCTION TO GENERAL PHYSIOLOGY
B
lower end and also to a reservoir of mercury. The measuring
tube A is first filled with air by lowering the reservoir while the
tap is open. The tap is then closed and the mercury made level in
the reservoir and the tube. The volume is read off. Two or 3
c.c. of strong (40 per cent.) caustic soda are placed in the funnel,
the reservoir lowered somewhat, and the reagent allowed to run in
slowly by opening the tap. Close the tap and raise and lower the
reservoir a few times to spread the
soda over the sides of the tube.
Any carbon dioxide present is
absorbed, but its concentration in
air is so small that probably no
diminution in volume will be
noticed when the level of the
mercury is again adjusted. We
know, however, that it is present
because lime-water left exposed
becomes covered with a film of
calcium carbonate, and the amount
of it can be determined with a more
sensitive method of analysis.
Place next in the funnel 2 or
3 c.c. of a strong solution of pyro-
gallol, and allow it to run into
the measuring tube and mix with
the caustic soda already present
there. Alkaline pyrogallol is a
powerful absorbent of oxygen,
and there will be found to be a
marked decrease in the volume
of the gas. Repeat the process of
raising and lowering the reservoir
until no further reduction in
volume occurs. The difference
between this reading and that
obtained after the addition of soda
gives the volume of oxygen in the mixture, and its percentage
in the air can be calculated. The remainder is nitrogen
and inert gases. After use, the tube is washed out repeatedly
with water, by running in through the funnel and expelling
by the tube C. Finally 5 per cent, sulphuric acid should be run
through.
Breathe backwards and forwards from a rubber football bladder
until the asphyxial effect is too great to continue. A tube has
been tied into the bladder for the purpose, and should have a piece
FlG. 13. — Apparatus for Analysis
of Gas Mixtures.
LABORATORY WORK 185
of rubber tubing on the outer end in order to place a spring clip on
it after the last expired air has entered.
Fill the tube C with mercury by raising the reservoir and open-
ino- the tap slowly. Any mercury that escapes is allowed to fall
into a cup and replaced in the reservoir. Holding the bladder
under the arm, press it gently while the clip is open and slip the
rubber tube on the tube C., the three-way tap being closed. Open
the latter and draw a sample of the gas into the measuring tube as
before Read its volume, absorb the carbon dioxide and the oxygen
in turn It will be found that there is an increase in carbon dioxide
a decrease in oxygen. Thus the combustion processes in the animal
body have consumed oxygen and replaced it by carbon dioxide
We require now to allow this expired air to be subjected to the
action of a green plant in sunlight. We may take a sma 1 plant of
mint in a small flower-pot, (Mint was used by Priestley 1 in his
classical experiment) Place this pot in the middle of a shallow
earthenware tray and cover it with a bell jar which has an opening
at the top closed7 with a rubber stopper through which a short glass
tube passes A piece of rubber tube with a pmchcock is. fitted on
he glass tube. Suck up water from the dish until the plant is
mmlrsed and the jar filled, replacing the water in the : tray as it
p-oes into the jar. The flower-pot may have muslin tied over the top
to prevent the soil being washed out. Close the rubber tube by a
clip Attach the football bladder ^*^ «H»y^"«. °Pe"
Jclip and fill the jar with the air, pressing the bladder so as to
drive some of the contents out at the bottom of the jar.
is allowed to run into the sink. Finally pour mercury into
S° ^ It accuracy6 a°sample of the gas should be taken out of the
jar after it has stood in the dark for an ^***^^^
If a narrow rubber tube be attached to the tube oi the jar, t
tl: wiU bJcome filled with the gas by the pressure £ £c mercury
when the clip is opened for a moment, and^a P^sample can
1 86 INTRODUCTION TO GENERAL PHYSIOLOGY
dioxide decreased, or nearly disappeared. Thus the green plant
in sunlight restores the normal composition to air which has been
vitiated by respiratory processes.
The experiment may be repeated in the dark. There will be
no increase in oxygen. Probably a decrease will be noted, together
with an increase in carbon dioxide.
Chlorophyll.— 'To see the absorption bands in the spectrum of
chlorophyll, take grass cut in short pieces, and rub in a mortar with
methylated spirit. Filter and place in a test-tube in front of the
slit of a pocket spectroscope. If the dark band in the red is not
clearly seen, dilute with spirit.
The chloroplasts are most easily seen under the microscope in a
thin leaf, such as that of a moss. Mount in water.
Formation of Starch. — Take a plant, a bean plant does well,
growing in a pot. Keep it in the dark for two days. The starch
will be transported from the leaves to the stem and root. Take one
of the leaves, dissolve out the chlorophyll by warming in methylated
spirit, and place the colourless leaf, after washing with water, in a
dilute solution of iodine in potassium iodide. It will not turn blue.
Allow the plant to be exposed to sunlight for a day or two, and test
the leaf for starch again. It will turn blue. The iodine test should
also be made with a solution of starch, made by rubbing some dry
starch powder in water and pouring into boiling water.
The Nitrogen Cycle
Bacteria — Allow hay or grass to putrefy in water. Various
forms of bacteria will be found on examination under the micro-
scope. Use the highest magnifying power available.
Their forms and movements can be made more obvious by the
addition of indian ink, or better, collargol, to the preparation, or
by the use of dark-ground illumination.
Formation of Nitrates in the Soi7.—Add half a gram of garden
soil to 50 c.c. of the following culture fluid : —
Ammonium sulphate - - 0.5 gm.
Potassium acid phosphate i „
Water - r iitre.
Half a gramme of magnesium carbonate to the 50 c.c. is also
required to preserve neutrality. After about four weeks or so, the
ammonia will have disappeared, and nitrate have taken its place.
The presence of nitrate may be shown as follows : — Filter the
liquid. Evaporate to dryness in a porcelain basin on the water-
bath. Add, with a glass rod, a drop of 0.5 per cent, solution of
diphenylamine in pure, nitrate free, strong sulphuric acid. The
presence of nitrate is shown by the production of aniline blue.
LABORATORY WORK
187
Root Nodules.— A fully-grown lupin plant is dug up and the
roots washed in water. Numbers of tubercles of various sizes
will be seen.
Salts
The importance of calcium for physiological processes may be
shown with the frog's heart. The canula of Symes is the most
convenient (Fig. 14).
The apex of the ventricle (see anatomy of the heart, p. 1 88 below)
has a tiny bent pin passed through it; a light clip is better. A
FiG.14-
A frog heart tied on the end of the glass canula B, which has a side t
rubber tubing to a siphon C, which dips into the solution D in a
E, a light straw lever pivoted at F.
G, enlarged view of the end of the canula.
thread is attached to this clip, and to a straw lever, which magnifies
the beats. A tracing may be obtained by making a paper point,
fixed on the end of the lever, to write on a glazed paper gummed
around a cylinder and then smoked. The cylinder ,s slowly
rotated by clock-work or electric motor. Such apparatus i
supplied by makers of physiological apparatus : paper is
smoked by a gas flame, fed with coal gas which has passed over
cotton-wool on which benzene has been dropped During the
smoWnTthe drum is rotated quickly by hand. To fix the tracing,
'it is removed from the drum by a vertical cut in an appropriate
place and passed through a dilute spirit varnish, or a solution of
paraffin wax in petrol. It is then hung up to dry.
i88 INTRODUCTION TO GENERAL PHYSIOLOGY
The heart is prepared thus : A frog, whose central nervous
system has been destroyed, is laid on its back. The heart is
exposed by removing the sternum (the bone in front between the
fore-legs), cutting the bones which unite it to the legs with strong
scissors. The beating heart will be seen in a
transparent bag (the pericardium). Open this
carefully and expose the heart, which has the
appearance of Fig. 15. By aid of a large needle,
fixed into a wooden handle by its point, a thread
is passed between B and A, taking care not to
injure the auricles. The needle will be more
handy if bent after heating in a flame, and then
carefully smoothed by emery paper (Fig. 16). pJG z-
The heart is now turned up forwards, after cutting v? is the ventriciei
through a little fibrous band which connects it to A', the two auricles.
the pericardium. The thread is tied by a loose B> the commence-
i j,i -i • i • j.i ment of the main
knot around the auricles, a snip made in them as arteries carrying
far from the ventricle as possible, the end of the blood, expelled
canula is inserted, and the thread tied around it. by the contraction
By cutting through the tissues behind the heart t°0f
with scissors, the heart is removed tied on the end general.
of the canula. The siphon is now connected up,
and, by sucking gently at the top of the canula, the solution flows
into the heart and out by the cut ends of the main arteries. -The level
of the solution in the canula should be 2 or 3 cm. above the heart.
Use first a solution containing 0.65 per cent, of sodium chloride
P'iG. 16. — Simple Aneurism Needle.
and 0.014 per cent, of potassium chloride, in distilled water. The
sodium chloride should be free from calcium, if marked results are
to be obtained. The beats will be small and may in time dis-
appear. Take next a solution containing calcium ions in addition
to the sodium and potassium ions, namely, 0.65 per cent, sodium
chloride, 0.014 per cent, potassium chloride, and 0.012 per cent.
LABORATORY WORK 189
calcium chloride. Large and vigorous beats will be obtained,
lasting for several hours.
Sources of Energy
Connect an inverted funnel, held in a clamp, to a wash-bottle
containing lime-water. Bring under the funnel a piece of sugar or
a bit of fat which has been set burning on iron gauze by aid of a
Bunsen burner. At the same time suck the products of combus-
tion through the wash-bottle. The lime-water becomes milky.
Note that by burning food materials in air we obtain carbon
dioxide and energy (heat) just as when burned (oxidised) in a
more gentle way in the living organism.
Alimentary Canal of Frog and Rabbit
Opening a pithed or chloroformed frog along the middle ventral
aspect, notice —
The gullet (cesophagus), leading from the back of the mouth
to the stomach.
The stomach.
The narrow, small intestine, passing from the stomach and
forming a few coils before opening into the broader.
Large intestine.
The large brown liver, with its gall bladder containing bile.
The pancreas, a small pale yellow organ near the beginning
of the small intestine.
In a rabbit, killed by chloroform, the same organs will be
better seen. The pancreas is more diffuse. The beginning of
the large intestine has a voluminous sac, full of food material,
attached to it ; this is the caecum, which is exceptionally large
in herbivorous animals. Careful dissection will show the duct
from the pancreas, opening into the small intestine about two
or three inches from the stomach.
Enzymes and Digestion
Rates of Reactions. — i. Add a small quantity of silver nitrate
solution to a beaker of dilute sodium chloride solution. A pre-
cipitate of silver chloride falls instantly.
2. Add i or 2 c.c. of methyl acetate to a beaker of water, con-
taining a few drops of an alcoholic solution of phenol-phthalein.
Add dilute sodium hydroxide, drop by drop, until the colour
turns red. We have now an excess of alkali present. This is
capable of decomposing the ester into acetic acid and methyl
190 INTRODUCTION TO GENERAL PHYSIOLOGY
alcohol. The former combines with the sodium hydroxide, form-
ing a neutral salt, so that when sufficient has been produced
the red colour of the phenol-phthalein disappears. This dis-
appearance will be seen to take place slowly.
The experiment may be made more accurately by taking
methyl acetate and water in molecular proportions, that is, 74.1
parts by weight of the former to 18 parts by weight of the latter.
Approximately, 10.5 c.c. of the ester to TO c.c. of water. Take
out a sample, say 2 c.c., by means of a pipette every day or two,
and determine, after dilution, the amount of acid present, by
titration with standard caustic soda. The rate of change will
ultimately become very slow and finally cease. By calculation
of the amount of acid formed, it will be found that there is
still a notable quantity of the ester left unhydrolysed, so that
the reaction has come to a balanced position. Take now a sample
of the mixture in equilibrium, dilute it with water to twenty times
its volume. Allow it to stand for a day or two and titrate again.
More acid will be present ; that is, further hydrolysis has taken
place.
The experiment may be performed with ethyl acetate, but
the time taken will be much longer.
Hydrolysis by Enzymes. — The enzyme lipase may be used for
the first experiments on this question. This catalyst acts on
esters in general, and we will take the glycerol esters known as
fats Lipase is found in the pancreatic juice, in the liver, and in
various fatty seeds. The most convenient source is the seed of
the castor-oil plant ; but it is important that fresh seeds, capable
of germination, be used, otherwise there may be no enzyme
present. The seeds should, in fact, be obtained from a seedsman,
not from a druggist.
It will suffice to rub some of the seeds with a little weak
acetic acid in a mortar, after removing the outer shells by a blow
with the pestle. The acid is required to form the active enzyme
from a preliminary stage in the resting seed.
Add some of the paste thus obtained to an emulsion of a small
quantity of olive oil in water. After vigorous shaking for a
moment, take out a sample of 5 c.c. and titrate with sodium
hydroxide. It will contain a small amount of acid. It is best to
add the sample to 25 c.c. of methylated spirit to dissolve the oil,
and then to dilute with 25 c.c. of water and add phenol-phthalein
before titration.
Allow the rest to stand in a warm place, shaking at intervals,
for a day or two, and then titrate again. The fat will have been
partially hydrolysed to oleic acid and glycerol. Under these condi-
tions, with excess of water, the ester is hydrolysed by the enzyme.
LABORATORY WORK 191
To observe the opposite process of synthesis, add the paste
containing lipase to a mixture of oleic acid and glycerol, to which
a very small quantity of water has been added. Titrate as above
at once, and again at intervals of a few days, in a warm place. The
sample is best weighed in a flask, taking 2 or 3 gm. and working
out the result for 10 gm. in each case.
The difficulty in this experiment is that the constituents of
the mixture separate from each other, so that the action is very
slow, unless continual shaking is practised. But if the mixture
be well shaken at intervals, an obvious synthesis, shown by decrease
in the oleic acid present, should be detected.
If a polarimeter is available, an experiment showing the same
facts may be made with the enzyme emulsin made from almonds.
An active preparation can be bought. This enzyme acts on
glucosides, in particular that kind called the /3-glucosides, which are
laevo-rotatory. The synthesis of the /3-glucoside of glycerol can
be shown thus : Take 9 gm. of glucose, dissolve by aid of heat
in 6 gm. of water. Cool. Add 20 gm. of glycerol, and rub in
a mortar with I or 2 gm. of emulsin. The exact quantities are
not essential ; the proportions given ensure the most rapid result.
Take a sample of the mixture at once, say 2 gm. weighed as in
the olein experiment. Add 2 or 3 c.c. of a solution of mercuric
nitrate to precipitate the proteins introduced with the enzyme,
make up to 50 c.c., and filter. Determine the degree of rotation.
It is well to allow the solution to stand for some time after addition
of the mercuric nitrate, to allow aggregation of the particles.
Place the remainder in a warm place, preferably at 45° C. Take
samples every day for the first four days, then every second day.
The dextro-rotation due to the glucose will become less and less,
owing to the formation of the glucoside with the opposite rota-
tion. The rotation ultimately passes to the opposite side of zero.
Finally, take a sample, dilute about twenty times with water, add
a little fresh emulsin, and warm for a day. Treat as before, and
it will be found that the original dextro-rotation returns. Thus
the same enzyme hydrolyses in dilute solution.
Ethyl-glucoside (ft) in crystals can be obtained by the aid of
emulsin thus (Bourquelot) : Add powdered glucose in excess to
100 c.c. of 90 per cent, alcohol, so that part remains undissolved.
Then add about a gram of emulsin and leave in a warm place,
shaking at intervals, for two or three weeks. Filter. Evaporate
to dryness on a water bath. Extract with a small quantity of
pure, cold, dry acetone. This dissolves the glucoside, leaving
glucose. Crystals separate on standing. Filter off and dry over
sulphuric acid in a desiccator. The crystals will be found to
form a laevo-rotatory solution and not to reduce alkaline copper
192 INTRODUCTION TO GENERAL PHYSIOLOGY
sulphate. If a dilute solution in water be acted on by emulsin,
glucose and ethyl-alcohol are formed.
Enzymes Act at their Surfaces. — Take a quantity of ground
Soy beans containing urease. Add 75 per cent, alcohol, shake
together, and filter off a part. Add a few crystals of urea to this
filtrate and also to the suspension of the powder in alcohol. Keep
in a warm place for a day, and test for the production of ammonia
by aeration as described above (p. 182). A mere trace will be found
in the sample in which the filtered extract was used, arising from
the slight spontaneous change of urea. In that in which the solid
was suspended a large amount will be found. Thus urease acts
in a liquid in which it is insoluble.
Catalytic Action. — Faraday's platinum effect maybe obtained in
a modified way thus : Take a little spiral of fine platinum wire. If
held in forceps over the tip of a Bunsen burner and the gas turned on,
it is probable that no effect will be obtained owing to the surface
not being clean. Light the gas and allow the platinum to be heated
to a red heat, then put out the flame by pinching the tube, and as
soon as the wire has ceased to glow let the gas on again. The
platinum will gradually get red and the gas be ignited. If allowed
to become quite cold again it rapidly loses its activity.
The oxidation of methyl alcohol by platinum may be seen
thus: Place a few c.c. of methyl alcohol in the test-tube of
Fig. 12 and in the horizontal part of the exit tube, which should be
fairly wide, some platinised asbestos, that is, asbestos coated with
finely-divided platinum. The bottle, empty, should preferably be
immersed in ice, but cold water will serve. Immerse the test-tube
in warm water and draw air through it over the platinum. The
reaction will probably not commence until the latter is warmed, but
will then continue of itself when the external heat is removed. If
the reaction has not been too violent, formaldehyde may be detected
by its smell in the condensing bottle.
Add a few c.c. of methyl acetate to water as in the experi-
ment on p. 189. Add paste of castor-oil seeds with dilute acid,
and make just alkaline to phenol-phthalein. The red colour will
disappear much more rapidly than it did in the former experiment.
When this has taken place, make red again and observe the
renewed disappearance, and so on. The lipase acts as a catalyst
in accelerating the hydrolysis of the ester.
Model. — Some instructive experiments can be made with a
schema. Take a piece of polished plate glass about 3 ft. long and
6 in. broad, such as is sold for shelves in shops. Raise one end
on an adjustable support some 5 in. high. Carefully polish with
chamois leather so as to remove dust. Polish also the bottom of a
brass kilogram weight. Place the weight at the top of the sloping
LABORATORY WORK 193
surface. By adjusting the height, a position can be found at which
the weight will slide slowly down. This represents the course of a
reaction proceeding spontaneously at a slow rate. Apply next a
few drops of oil to the bottom of the weight and repeat. It slides
down rapidly. The oil represents a catalyst. Note that the energy
set free in the process, being given by the height from which the
weight falls, is not altered by the catalyst. The form of the energy
may, however, be changed. It will be noted that the weight arrives
at the bottom with more kinetic energy in the presence of the catalyst
than when it slides slowly down. In the latter case, there is more
heat produced by friction. Another fact to be taken note of is that
there is some loss of the oil " catalyst " by sticking to the glass.
This represents the disappearance of a catalyst by subsidiary
reactions, which often occurs.
Various Digestive Enzymes
Amylase. — Add a little saliva to some starch paste. It is quickly
liquefied, and sugar will be found by boiling with alkaline copper
sulphate. The blue colour with iodine will disappear.
Invertase. — Add a little yeast to a solution of cane-sugar to
which a drop of chloroform has been added. This addition prevents
alcoholic fermentation. Cane-sugar does not reduce alkaline copper
sulphate, but the glucose and fructose resulting from its hydrolysis
by invertase do so.
A similar experiment may be made with scrapings from the
inside of the small intestine of a mammal.
Pepsin. — Take scrapings from the inner lining (mucous mem-
brane) of the stomach. Add 0.5 per cent, hydrochloric acid and
filter. Add two or three little cubes of hard-boiled white of egg
and keep in a warm place. The egg white will be dissolved:
Trypsin. — Make a similar experiment with an extract of the
pancreas, made by rubbing in a mortar with sand and 0.2 per cent,
sodium bicarbonate. Add a scraping of the mucous membrane of
the small intestine (containing enterokinase) to activate the tryp-
sinogen into trypsin. Filter.
Absorption
Histological preparations of the mucous membrane of the various
parts of the alimentary canal can be bought. If made in the
laboratory the following method is employed :—
In order that thin sections may be cut, all tissues require to be
" fixed " or hardened by some means. There are many solutions
used for the purpose, and the appearance of the cells is not the same
194 INTRODUCTION TO GENERAL PHYSIOLOGY
in all cases. The fact shows that it is not to be assumed that the
minute structure of the protoplasm corresponds to that of the living
state. If, however, there are things to be seen in one kind of cell
that are not visible in another, we are justified in holding that some-
thing was present during life in one and not in the other.
The most generally useful fixing solution appears to be that of
Bouin : —
Saturated solution of picric acid in water 60 c.c.
Commercial formalin (40 per cent, formaldehyde) 18 „
Glacial acetic acid 2 „
The pieces of tissue should not be large, and may remain in the
mixture for one or two hours. Wash repeatedly in 70 per cent,
alcohol until no more yellow colour comes out. Transfer to
methylated spirit for forty-eight hours and then to chloroform.
The material must be supported by being impregnated with and
embedded in paraffin, which should have a melting point of 50°.
Since paraffin is soluble in chloroform, the material may be trans-
ferred directly to melted paraffin, which should not be at a tempera-
ture higher than sufficient to keep it melted. The tissue remains
in this for one or more hours, according to size, and may be changed
to fresh paraffin, since the chloroform must be got rid off. In
the case of delicate tissues, it is better to pass through a solution
of paraffin in chloroform before placing in the pure paraffin. If
the piece of tissue is a rather thick one, it should be passed through
oil of cedar-wood between the spirit and paraffin, since chloroform
does not penetrate very well.
A mould is made by wrapping paper around a wooden rod,
projecting beyond the end of the rod. The piece of tissue is taken
out of the paraffin, by means of warmed forceps, and placed in
position in the mould, so that the sections made transversely across
will be in the desired plane. The mould is filled with melted
paraffin and cooled as quickly as possible. The paper is taken off
and the cylinder separated from the wood. Paraffin may be sliced
off so as to leave the tissue at the apex of a pyramid.
Sections are cut by fixing in a microtome. This is an instru-
ment by which the embedded tissue is advanced by fractions of a
millimetre at a time and slices cut off by a razor. The rocking
microtome of the Cambridge Instrument Co. is convenient.
Sections should be io/* in thickness. They must next be mounted
on slides, and are usually stained in order to render their con-
stituents more easily visible. As they leave the microtome, they are
generally more or less folded or creased. To flatten them, pick up
carefully with forceps and lay on warm water (not above 40°).
When flat, float them on to a microscope slide by bringing the
LABORATORY WORK 195
slide under them, lifting out and draining off the water. Press into
contact with the glass by means of filter paper, and lay aside in a
warm place until completely dry. Subsequent treatment will not
then wash them away, except in rare cases.
Next dissolve away the paraffin by warming until it melts and
pouring over some solvent ; xylol is generally used. Wash away
the xylol with acetone — this by alcohol, first strong and then dilute,
and finally water. Various stains can then be applied. We may
use : I per cent, eosin for ten minutes, rinse with water ; then I per
cent, toluidine blue for twenty minutes. Remove excess of stain
with absolute alcohol. Drop on solution of dried Canada balsam in
acetone and apply cover-glass. The balsam will harden in a few
hours. The use of other stains will be found in histological text-
books, such as Schafer's " Essentials of Histology."
To see the globules oifat in the intestinal epithelium, the follow-
ing method is adopted : Feed a rat with butter and kill it with
chloroform four hours later. Place a piece of the upper part of the
small intestine into a mixture of equal parts of I per cent, osmic
acid and 3 per cent, potassium bichromate and leave for ten days.
Unsaturated fats reduce osmic acid and become stained black.
Solvents of fat cannot be used, so that the tissue must be soaked in
strong gum, and sections cut by freezing with ether spray on a
simple microtome arranged for the purpose. The sections are then
mounted in glycerine.
The synthesised fat can also be seen by placing a small bit of
the mucous membrane in 0.5 per cent, osmic acid for forty-eight
hours and then in water for a few days. A shred is placed in
glycerine on the slide, a cover-glass over it, and broken up into cells
by tapping the cover-glass.
Voluntary and Involuntary Muscle
The microscopic appearance of the former is best seen in the
muscles of an insect, such as a wasp or beetle. Cut off the head,
and divide the trunk with scissors lengthwise. Notice muscles
attached both to the legs and to the wings. Take a shred of the
former, and tease out with needles on a slide into separate fibres, if
possible. Add a drop of the insect's blood, cover and examine
with a high power. The cross striation will be seen, and, by care-
ful focussing, the longitudinal fibrils (" sarcostyles ") embedded in
the " sarcoplasm."
The cells of the involuntary muscle may be seen thus : Allow a
small piece of intestine to macerate in \ per cent, potassium
bichromate for two days. Hold it in water on a microscope slide
with forceps and fray out with a needle. The cells separated in
196 INTRODUCTION TO GENERAL PHYSIOLOGY
this way may be examined with a high power, after covering the
preparation.
The urinary bladder of the frog exhibits the arrangement of
smooth muscle well. Distend it with alcohol by means of a pipette.
When hardened, cut out a piece of a few millimetres square. Stain.
Mount in glycerine.
Contractions of the Frog's Stomach
Cut with scissors a ring from the frog's stomach by two parallel
transverse cuts. Pass a bent pin through the ring and hang it up
on a rod over the lever used above for the heart (p. 187). Pass
through the ring another hook and connect it to the lever by a
thread. The muscle will usually at first be in tonic contraction.
Note that running warm (25° to 30° C.) 0.7 per cent, sodium chloride
over it causes relaxation. If the muscle has already relaxed some-
what, the first effect of the warm saline may be to excite a con-
traction, but this is followed by a marked relaxation. A series of
rhythmic contractions sometimes follows.
The tonic contraction will slowly give way without warming,
especially if a small weight, I or 2 gm., be attached to the lever
so as to stretch the preparation slightly. After a time there may
be slow rhythmic contractions and relaxations.
Apply induction shocks by twisting the end of a fine copper
wire, attached to one terminal of the secondary coil, around the
upper pin and a wire from the other terminal to the lower pin.
Note the slow contraction.
Secretion
Examine under the microscope a thin bit of the rabbit's
pancreas. Note the granules in the cells.
Make sections of the salivary gland as described above for
intestinal mucous membrane. Study the general arrangement.
Vertical sections through the frog's skin show typical simple
glands.
The disappearance of granules in the act of secretion may be
seen in the living stomach of the newt, as described by Langley
and Sewall (Journal of Physiology, vol. ii., p. 286). Feed a newt
with small earthworms or by introducing with a pipette some
diluted white of egg into the stomach. In twenty-four hours the
digestive process is over, the glands have assumed the resting
appearance and are ready for renewed secretion, being full of
granules. Take another newt three hours after feeding. The
granules in some of the cells have nearly disappeared ; in others
LABORATORY WORK 197
there are still some remaining, forming an inner granular zone
around the lumen.
The observations are made by pithing the animal, opening
the body cavity, dividing the stomach along the greater curvature
and pinning one half of it over an opening in a plate of cork or
thin wood, with the muscular coat uppermost. It may be con-
venient to pin the newt on its side. The muscular coat is
sufficiently transparent to allow the deeper ends of the glands
to be examined under a fairly high power. If care be taken to
avoid loss of blood, it is possible to see the circulation around
the glands. The muscular coat may be snipped off by a fine
pair of scissors at some spot, if difficulty be found in seeing the
glands.
Flow of Water. — One of the forms of osmometer described on
p. 171 above is filled with a strong solution of congo-red, a fine
glass tube bent over at the top is inserted and the osmometer
immersed in distilled water. After a time, drops of dye solution
will issue from the upper end of the tube and may be collected.
Electrical Change. — Although this may be regarded as a some-
what difficult experiment for the student, it has much importance
in the general theory, and, at all events, it should be shown as
a demonstration.
A sensitive high resistance galvanometer is required, such as
the Broca pattern made by the Cambridge Instrument Co. It
should be made as sensitive as possible by very careful adjustment
of the controlling magnet, so as to produce a long period of
oscillation. A spot of light from a lamp is reflected by the
mirror attached to the moving magnet and received on a divided
scale. If an arc lamp is used, the spot will be bright enough
to be visible at some distance.
The tissue to be investigated must be led off by non-polarisable
(and equipotential) electrodes. The most convenient pattern is
that in which mercury and calomel are used. Take a small wide-
mouthed bottle, fitted with a paraffined cork through which three
glass tubes pass, two of these ending a short distance below the
cork, the other is longer. One of the two short ones is bent at
a right angle outside the bottle. The other short one is fitted
with a piece of rubber tube closed by a clip and is used for
filling. The third tube has a short piece of platinum wire fused
into the lower end, which is then sealed and contact made with
the platinum by pouring in a little mercury and dipping into
it a fine copper wire. This tube passes down into a layer of
mercury of a few millimetres depth at the bottom of the bottle
and makes contact with it by the platinum wire. Some calomel,
together with a little mercury, is rubbed in a mortar to a paste
198 INTRODUCTION TO GENERAL PHYSIOLOGY
with 0.7 per cent, sodium chloride if the electrodes are to be
used on frog's tissue, or 0.9 per cent, if for mammals. This .paste
is shaken up with a larger quantity of the solution and poured
into the bottle so as nearly to fill it. A piece of cotton spirit
lamp wick has previously been pushed into the bent tube, and the
solution is run in until it escapes from the end of this tube. The
clip is then closed. The wick may be cut to any size or shape,
especially if a little kaolin is put on it, according to the organ or
tissue to be investigated. Two of these electrodes will be needed.
It will be found that when any two points on a living tissue are
FIG. 17. — Circuit for Experiments on Electrical Changes.
A B, slide-wire.
C, battery.
D, key.
E, electrodes.
F, tissue.
G, galvanometer.
connected to the galvanometer there is usually a deflection, which
may be great enough to send the spot of light off the scale. This
is partly due to differences of potential in the tissue itself, partly
to unavoidable inequalities in the electrodes. To diminish the
latter as far as possible, it is well to keep the electrodes connected
together when hot in use. This is done by placing a wire with
its ends one in each of the mercury tubes and connecting the bent
tubes with a piece of india-rubber tubing filled with salt solution.
But in any case a means of balancing the electro-motive force
present in the tissue is necessary. An equal and opposite electro-
motive force is put into the circuit by an adjustable contact on a wire
through which a current is flowing. The slide-wire used previously
will serve. The whole circuit is arranged as in Fig. 17.
It is convenient to have an adjustable resistance connected across
LABORATORY WORK
199
a -
the terminals of the galvanometer so as to be able to adjust the
sensibility for various purposes. If the direction of the current in
the slide-wire is not such as to oppose the E. M. F. of the tissue,
the connections of the battery must be reversed, or a commutator
may be interposed in the circuit.
The skin of the frog is the simplest secretory structure in which
the electrical change of activity can be seen. The electrodes are
placed one on each leg between the knee and the ankle. The
sciatic nerve is prepared on one side, a ligature tied around it, and
the nerve cut on the central side of the ligature. It is laid on two
wires, best of platinum, connected with the secondary coil of the
induction apparatus and the current
thrown in when the galvanometer
has been brought to zero. The
nerve should be raised out of the
wound and laid on the stimulating
electrodes held in some support. A
pillar of plasticine is the simplest.
Since the skin of one leg only is
stimulated, that of the other acts
merely as a conductor. If both
nerves were stimulated, the changes
in one would neutralise those in the
other, owing to the opposite direc-
tions in which they are connected
to the galvanometer. The actual
electrical change seen is sometimes
of a complicated nature, consisting of
more than one phase. The explana-
tion of the whole is not altogether
clear, but we need only observe the
fact of a change in the gland cells.
Since the stimulation of the sciatic nerve causes the muscles to
contract, and this would interfere with observation of the glandular
effect, we must first give the frog an injection of the arrow poison,
curare by means of a hypodermic syringe inserted under the skin
of the back. About 3 drops of a 0.05 per cent, solution will usually
be sufficient, but different samples of curare vary in strength,
solution should be made fresh. In small doses it prevents stimula-
tion applied to the nerve from reaching the muscle, but does not
affect the glands. When the frog is completely paralysed
pithed or beheaded.
The sciatic nerve is found by making an incision through the
skin between the end bone of the spinal column, urostyle, and the
pelvic arch on one side at the extremity of the former (as in tig. 18),
FlG. 1 8. — Position of the
Sciatic Nerve.
a, place in which the incision
is made.
200 INTRODUCTION TO GENERAL PHYSIOLOGY
the frog lying on its lower side. Cutting carefully through the
tissues under the skin with fine pointed scissors, the nerve will be
found. Pass a thread under all the nerve trunks seen and, raising
them by the thread, not yet tied, follow up with the scissors to
the place where they leave the spinal column. Tie here and cut
between the ligature and the bone. Dur-
ing the experiment keep the skin moist
C f^\ with 0.7 per cent, sodium chloride.
Respiration
Trachece of Insect. — Any small piece of
tissue cut from the interior of an insect
and spread out on a microscope slide in
07 per cent, saline will show the branch-
ing system of tubes containing air.
Hemoglobin. — The carriage of oxygen
by the red corpuscles of the blood can be
shown by a simplified vacuum pump
made by a glass-blower. Fig. 19 shows
the pattern, which will be found useful for
many purposes.
With the stopcock B open to the cup
C, and D making communication between
A and E, raise the mercury reservoir ¥
until a little mercury has entered C. Close
B and lower the reservoir until the mercury
leaves E, that is 760 mm. below E. There
is now a Torricellian vacuum in A and E.
Take about 10 to 15 c.c. of blood,
which should be fresh, and either defibrin-
ated by stirring with a feather, or pre-
vented from clotting by the addition of a
small amount of powdered potassium oxa-
late. Sufficient will be otained from a rat
killed by cutting its throat. If defibrinated,
it will need straining through muslin to
remove bits of fibrin.
Allow a known volume of this blood to run into the vacuum by
putting it in the cup C and turning the stopcock slowly. It will be
seen to froth and to become more crimson in colour. Raise the
reservoir, after closing the stopcock, until the blood, neglecting the
froth, just fills the vessel E. Then turn the stopcock D so that the
blood is driven out. Collect it in a small bottle. Bring the mercury
reservoir into connection with A again, and in such a position that
FIG. IQ. — Vacuum Pump
for Blood Gas Experi-
ments.
A, graduated tube.
B, stopcock.
C, cup.
D, three-way stopcock.
E, reservoir.
F, mercury vessel.
LABORATORY WORK 201
the level of the mercury is the same in both. Read the volume of
the gas.
Add I or 2 c.c. of strong sodium hydroxide through the cup
after lowering the reservoir. The volume of the gas will diminish,
owing to absorption of carbon dioxide. Next add in the same way
I or 2 c.c. of pyrogallol. Nearly the whole of the rest of the gas
will be absorbed showing that it is oxygen. The small residue is
nitrogen.
We now want to see whether the blood which has lost oxygen
can take it up again and give it off to a vacuum. We must first
wash out the pump by running in water and expelling it from the
side by the stopcock D, until it comes away colourless, finally
rinsing out with 0.9 per cent, sodium chloride.
Now rotate the bottle containing the dark blood so that a thin
film is formed over the interior. The blood becomes bright red
again. Repeat the process of removing the gas by the pump, noting
how much blood is used. We obtain practically the same volume
of oxygen as before, taking account of the respective quantities of
blood used.
To be satisfied that it is the corpuscles and not the plasma that
has this function, the experiment may be repeated with serum or
plasma. A little oxygen may be given off if haemolysis has occurred
and the serum is red. Colourless serum can be obtained by allow-
ing blood to clot and to stand until the serum has exuded from the
clot and can be collected in a pipette.
Absorption Spectrum. — After exposure to oxygen there are two
bands in the yellow of the spectrum, best seen in very dilute
solution of haemoglobin in water. Addition of a drop of blood
from the finger to a test-tube full of water will serve. Add more
water if too concentrated. The addition of a few drops of a
reducing agent, such as ammonium sulphide, and warming, changes
this spectrum to one of a single band. Shaking with air brings
back the original two bands for a sho# time.
It may be thought more convincing to remove the oxygen by
the pump. Take a dilute oxy-haemoglobin such as shows the two-
banded spectrum in a tube of the same diameter as that of the
pump. Fill the pump with mercury, and then run in gently 2 or
3 c.c. of the solution. Observe with the spectroscope whether the
two bands can be seen. If so, close the stopcock and lower the
mercury vessel. Drive out the gas given off through the cup C.
It will not be entirely reabsorbed during the operation. Repeat until
the spectroscope shows the single band. A small relative amount of
the oxy-haemoglobin is sufficient to show the double band.
Carnage of Carbon Dioxide. — To show that haemoglobin also
carries carbon dioxide, it is necessary to remove the serum from the
202 INTRODUCTION TO GENERAL PHYSIOLOGY
blood by washing the corpuscles with 0.9 per cent, sodium chloride.
Centrifuge some defibrinated blood. Pour off the serum. Shake
up the deposit with saline and centrifuge again. After repetition
for two or three times there will be practically no serum left. The
reason for this procedure is that the sodium bicarbonate in the serum
gives off carbon dioxide to a vacuum, although it does not to the
tension of this gas in the air-cells of the lungs.
The final suspension of corpuscles in saline is first put in the
pump and the gas removed. Then the process to which blood
was subjected in the former experiment is repeated, except that
the corpuscles are subjected to an atmosphere of carbon dioxide
in the bottle instead of to air. It will be found that much more
carbon dioxide is obtained in the pump afterwards than could be
dissolved in the water present.
The carbon dioxide may be made in a Kipp generator, and
should be passed through sodium bicarbonate solution in order to
stop spray containing hydrochloric acid.
Stimulation of Respiration by Carbon Dioxide
Breathe from a football bladder a gas mixture containing about
10 per cent, of carbon dioxide, together with more oxygen than
serves to make up that displaced by the carbon dioxide. The
respiration will be found to be quickened and deepened, while the
feeling of "want of breath," as after running upstairs, will be
experienced. It may be found that less carbon dioxide will give
the result better.
The oxygen is most conveniently obtained from a cylinder of
the compressed gas, but it may be made by the usual process of
heating potassium chlorate with manganese dioxide. It should be
washed by passing through caustic soda. A sample of the mixture
as breathed should be analysed in the apparatus used for the green
plant experiment (p. 184).
Oxidation
Autoxidation. — Expose some benzaldehyde and also linseed oil in
shallow dishes to the air. Note that crystals of benzoic acid appear
in the former, and that the latter becomes hardened as in varnish.
No effect is to be seen with sugar or lactic acid exposed to the
air. That they are not oxidised to carbon dioxide and water can
be shown by leaving a small quantity in a large closed bottle for
some days and determining the carbon dioxide content of the gas
in the bottle by analysis. Or, more simply, insert a perforated
rubber stopper with two tubes, and draw air, freed from carbon
dioxide by first passing through a wash-bottle of caustic soda,
through the bottle and then through lime-water.
LABORATORY WORK 203
Peroxides. — Hydrogen peroxide oxidises lead sulphide to
sulphate, which the oxygen of the air does not. But hydrogen
peroxide alone does not oxidise lactic acid. The addition of a
catalyst, such as iron, results in its oxidation, as shown thus :
Using the apparatus of Fig. 12 (p. 182), put a dilute solution of
lactic acid into A and lime-water into B. The air entering should
preferably have passed through caustic soda. No carbon dioxide
is formed. Add hydrogen peroxide (the commercial 2O-volume
solution will serve). Again suck air through. There is still no
formation of carbon dioxide. Add I or 2 c.c. of a dilute solution
of ferrous sulphate ; carbon dioxide is produced.
Peroxidase. — Instead of ferrous sulphate as above, take a fresh
mixture of lactic acid and hydrogen peroxide and add grated
horse-radish root. Carbon dioxide is evolved.
Guaiacum Reaction. — Take an excract of horse-radish in water,
add to some of it in a test-tube a drop of freshly made solution of
guaiacum resin in alcohol (guaiaconic acid is better). It is precipi-
tated by the watery solution, so that any change of colour is
difficult to see. Accordingly, add alcohol to dissolve the deposit.
It is not blue. In another test-tube, after the addition of guaiacum,
add a small quantity of hydrogen peroxide. Alcohol then added
will dissolve the blue oxidation product of the guaiacum.
Solution of guaiacum dropped on the cut surface of a potato is
blued at once, so that the peroxide is already there.
The cut surfaces of potatoes, apples, and other fruits turn brown
on exposure to the air. This is because there is a compound in
them which turns brown on oxidation.
The peroxide in these cases is only formed when free oxygen
is present. Place a potato with a cut surface uppermost in a wide-
mouthed glass bottle, through whose cork three glass tubes pass, one
to the bottom of the bottle, another leads from the top to a Bunsen
burner, while the end of the third opens just above the potato and
is, at its outer end, connected by a rubber tube with a clip on it to
a little funnel containing guaiacum solution. Pass coal gas through
the bottle and light it at the burner. The peroxide previously
present is soon used up. Allow the guaiacum to drop on the
potato. No blue colour will be seen. Turn off the gas and empty
out the potato into the air. It rapidly turns blue.
Reduction.— Add sufficient solution of methylene blue to quite
fresh milk to give a distinctly blue colour. Then a small quantity
of formalin. The blue colour disappears more or less rapidly.
The reaction does not take place if the milk has been boiled, so
that it is due to an enzyme. Milk that has undergone bacterial
change reduces methylene blue without the presence of an aldehyde,
since some of the bacterial products serve the same purpose.
CHAPTER III
THE articulated arm of a skeleton (the arm can be bought
separately) should be examined to realise how movements are
brought about by muscles. Articulated bones, various useful
models of organs and so on, can be obtained from Deyrolle of
Paris.
Attach a cord in the place, say, of the biceps muscle, at one end
to one of the bones of the fore-arm near the elbow, at the other
end to the bone of the scapula (shoulder-blade) just above the
shoulder joint By looping up the cord so as to shorten it the
elbow is flexed. If the cord is attached at one end to the ulna
where it forms the prominence of the elbow, at the other to the
bone of the upper arm, it extends the elbow joint or straightens
out the arm, since it is attached on the opposite side of the fulcrum
to that of the biceps. It represents the triceps.
The "Contraction" of Muscle. — Take a pithed or beheaded
frog, cut across above the pelvic girdle with strong scissors.
Remove the remains of the viscera in the posterior part. Seize
the skin at the cut edge with forceps and the pelvic girdle with the
left thumb and forefinger. The skin can now be pulled off, turning
inside out in the process. The muscles of the legs are exposed,
and, by the application of electrodes connected to an induction
apparatus to each in turn, its action can be observed.
Tension in Muscle. — The string or thread used for the experi-
ment described in the text must not be too strong,otherwise sufficient
weight required to break it may not be available.
Spring. — Suitable springs may be bought at the ironmonger's.
Those of about i in. diameter and 4 or 5 in. long do well. But
the exact size is immaterial.
Nerve-Muscle Preparation.— Skin the legs of a pithed frog as
above. Prepare the sciatic nerve as on p. 199, but dissecting it free
down to the knee-joint. Pass a thread under the tendon of the calf
muscle where it is attached to the heel. Tie and divide the tendon
below the knot. Separate the muscles from the leg-bone up to the
knee. Cut across the thigh-bone at the middle, and remove the
muscles attached to the lower part, taking care not to injure the
nerve. Finally cut across the leg-bone just below the knee.
204
LABORATORY WORK 205
The preparation may be most conveniently mounted on the
cork plate of a " myograph," which is a flat board with a bell-
crank lever attached. Put a pin through the end of the femur
into the cork. Tie the thread on the tendon to the upright of
the lever. Lay the nerve on electrodes, which may be held in
place by means of a lump of plasticine. A weight of 10 or 20 gms.
is hung on the lever near its axis.
For stimulating electrically, some form of induction coil is used.
The secondary coil is wound on a bobbin separate from the primary
coil, so that it can be placed at different distances for adjusting the
strength of the stimulation. There is an automatic interrupter to
give shocks in continuous series. Such coils are sold by dealers in
physiological or electro-therapeutical apparatus.
Electrical stimuli of moderate intensity do no damage to the
nerve, whereas it is difficult to avoid killing it by other forms.
Hence these latter can only be applied once at a particular spot.
The end of the nerve furthest away from the muscle must be used
first. A gentle tap with the back of a scalpel serves as a mechanical
stimulus, a crystal of salt as a chemical one, a heated wire as a
thermal one.
Notice that the excised muscle does work in raising a weight.
It may be said that the weight falls again, so that no actual external
work is done. But when it is raised, a support may be slipped
under it, and, after the muscle has relaxed on cessation of stimula-
tion, the thread to which the weight is attached, which will now be
lax, may be shortened, and then the next stimulation raises the
weight still further. With patience the process may be repeated
many times until the muscle becomes fatigued. An automatic
" work-collector " on the principle of the ratchet wheel is constructed
to perform the operations described above.
Formation of Acid
Cut across a muscle which has not been stimulated, and press
a piece of neutral litmus paper on the cut surface. It will become
blue. Thus the reaction is alkaline. Stimulate a muscle until it
ceases to respond and repeat the test. It will be acid, turning the
paper red. A more elegant form of the experiment is to inject
under the skin of the back of a frog a few drops of a strong solution
of the dye, acid-fuchsin. This is colourless in alkaline solution, so
that when examined next day the muscles have their usual yellow-
brown colour, j Excise one and stimulate it until it is fatigued. On
cutting across, it will be found to have become red, the change
being most obvious when compared with a similar muscle which
has remained at rest.
206 INTRODUCTION TO GENERAL PHYSIOLOGY
Effect of Length of Fibres
The simplest way to see the fact that more work is done when
the fibres are longer is to load the muscle with increasing weights
and to determine the height of the contraction produced by a single
induction shock in each case. The product of the weight by the
height gives the work. If the actual value in gram -centimetres is
wanted, it is of course necessary to measure the relative distances,
from the axis of rotation of the lever, of the attachment of the
muscle, of the weight and of the tracing point of the lever. For
the present purpose we only require to compare the different values
of work with each other. The point may write on a vertical smoked
paper. Move the surface so as to produce a short horizontal line
after each weight has been added and then stimulate the nerve.
An arc of a circle is drawn. The addition of each weight, say
5 gm. at a time, stretches the muscle a little more, so that the curve
starts from a lower level. The lever may be brought to the horizontal
position again by moving it in the slot of the myograph. The
tracing should be fixed, as described above (p. 187), and when dry
can be measured with compasses. It will be found that the work
done increases with the length of the muscle up to a certain point,
and then begins to decrease as the muscle becomes abnormally
stretched.
The Structure of Voluntary Muscle
This has been studied previously (p. 195), but should be brought
into relation \vith the fact of the preceding experiment. If only
insect muscle has been examined, it would be well to make a
preparation of the voluntary muscle of the rat or mouse. Mount
it in a little serum from the animal itself. Observe under the
highest power available, focussing the surface carefully.
Effect of Temperature
This has also been observed on the tonic contraction of the
frog's stomach. A further case is described under the heart (p. 225).
To study the action on the voluntary muscle requires special
methods, and is not easy to observe correctly. Note its significance
in relation to changes of surface tension between sarcostyle and
sarcoplasm.
Production of Tetanus
On a nerve-muscle preparation observe the height of the con-
traction produced by a single shock, and that in which the shocks
are given as quickly as possible after one another by opening and
LABORATORY WORK 20;
closing a key in the primary circuit, and also by using the automatic
interrupter, which has a much more rapid rate of vibration.
Heart Muscle
There are certain facts which require delicate and exact
apparatus to demonstrate on voluntary muscle, but can more
easily be observed on the slowly contracting heart muscle of
the frog.
It is necessary to be able to make use of a frog heart which
does not beat spontaneously. This is attained by the application
of a ligature in such a way as to cut off the sinus from the rest of
the heart (Stannius' ligature). Pass a thread between the aortas
and the auricles as described above (p. 188). Turn the heart
forwards. Bring the ends of the thread around the auricles and
tie so that the knot presses on the place where the auricles join
the sinus, marked by a whitish line. In addition to cutting off
the impulses from the sinus (see below, p. 225), it is probable that
the vagus nerves are stimulated also. The stoppage of the heart
does not last long as a rule, so that the experiments on it must
be done as quickly as possible. Have everything ready before the
application of the ligature. Cut out the heart and fix it to the
cork plate by a pin through' the aortic bulb. Pass a tiny bent pin
through the apex of the ventricle and connect it by a thin thread
to the lever, which is fixed above the heart. If the same lever as
that used in previous experiments with the heart be used, there
must be a prolongation to the opposite side of the axis, since the
contraction of the heart pulls downwards in the present case.
The " Staircase" — This requires that the muscle should have
rested for some time, so that it should be the first experiment
made.
Make electrodes of fine wire, the two wires being attached by
sealing wax at about 2 mm. apart, leaving about a centimetre free
at the end. Arrange these so that they touch the ventricle gently,
supporting them on" a lump of plasticine. Arrange the stimulating
coil so that single shocks can be given by a key in the primary
circuit. Do not use shocks stronger than can be comfortably felt
on the tongue. Allow the tracing point of the lever to rest lightly
in contact with a thinly smoked surface, which is at rest. Give a
series of stimuli, each following the other just after the effect of
the previous one is over, and moving the surface by hand a few
millimetres between each. The height of the contractions will
increase for the first few beats and then remain stationary.
" All-or-Nothing"— When the steady stage has been reached,
vary the strength of the stimulus. It will be found that the beats
208 INTRODUCTION TO GENERAL PHYSIOLOGY
will be all of the same height if the stimulus is strong enough to
stimulate at all. This is because the whole of the heart muscle
contracts at each beat, all its cells being in functional connection
with each other, contrary to the case of the voluntary muscle, where
a varying number of fibres can be set into activity.
The Refractory Period. — Using the weakest stimulus found in
the last experiment to be effective, first produce a beat, and at
various stages in its course, judged by the eye, apply a second
stimulus. This will be found to have no effect until a certain stage
in the relaxation period has been reached.
If the laboratory is warm, the contraction may be so rapid that
the experiment is difficult. If so, lumps of ice may be placed on
the cork around the heart. The water from the melting of the
ice must be prevented from reaching the heart by a little wall of
plasticine.
In case the heart has recommenced beating by the time this
last experiment is arrived at, the first stimulation of the preceding
method is omitted, using instead of the artificial contraction, the
natural one, and applying the second stimulus at various points of
the natural beat, which may be slowed by ice as before.
CHAPTER IV
Spinal Frog
DESTROY the brain of a frog by inserting a pointed bit of wood
into the skull cavity from the gap between the back of the skull
and the spinal column, which can be felt by taking the frog in a
cloth and bending the head downwards. A large pin may be
found more convenient to destroy the brain, since it penetrates the
skin more easily ; but the bit of wood should afterwards be inserted
to stop bleeding.
After a few minutes' time, reflex movements can be elicit 5d by
pinching the toes, dropping dilute acid on the skin, applying a hot
wire or an electrical stimulus. All of these are effected without
conscious sensation, since the brain is absent.
Central Nervous System
The general arrangement of the nerve centres in the frog should
be observed at this stage. Take a frog which has lam in methy-
lated spirit for a day or two. Remove the bony covering of the
skull by inserting one point of a strong finely pointed pair c
scissors through the membrane between the skull and the spinal
column Also remove the arches of the vertebra posteriorly from
the same place. Note the nerves connecting the nerve centres
to all parts of the body ; especially the optic nerves conveying
impulses from the eyes and the fact that the sciatic nerve which
we have seen to cause muscular movement, is given off
spinal cord.
Nerve
Structure.— Tease a piece of fresh sciatic nerve of the frog in
07 per cent, saline, obtaining the fibres as long and straight as
possible. It is not very easy to separate individual fibres uninjured
Note the double contour of each fibre, due to the highly refracting
medullary sheath, and the interruptions of this sheath at intervals
(Nodes of Ranvier). The material of the medullary sheath may
210 INTRODUCTION TO GENERAL PHYSIOLOGY
be seen forming highly refracting masses of curious shapes where
it has escaped from the cut ends of the fibres. To see the axis
cylinder, the contents of the medullary sheath may be dissolved
away by the action of chloroform.
Electrical Change in Nerve, — The arrangement of apparatus is
the same as that for the skin glands (Fig. on p. 198 above). Owing
to the high resistance of nerves, the two sciatic nerves of a frog may
be laid side by side on a glass slide. One electrode is placed on
the surface of the preparation, near the middle, the other one on
the cut end, which has been killed for a length of a few mm, by
touching with a hot wire. Electrodes for stimulating are placed
near the opposite end. The leading-off electrodes are first made
to touch each other. If there is any potential difference between
them, it is compensated by the slide-wire. It will be small in any
case. There will be found to be a fairly high potential difference
between the injured end of the nerve and the normal surface, such
that the longitudinal surface is positive to the end. This should
be absent when two symmetrical places on the longitudinal surface
are led off, provided that neither of them is injured. This may be
ensured by allowing the nerve to lie in saline solution in the cold
for a day before examining it ; all the injured fibres will then be
dead. To find which way the spot of light moves when either
electrode is positive, attach a bit of zinc to a copper wire from one
terminal of the galvanometer and a plain copper wire to the other.
On placing the zinc and copper in dilute salt solution, the copper
becomes positive to the zinc. The galvanometer must be short-
circuited by a low resistance in order to avoid too violent a
deflection.
Having compensated the so-called "current of injury," or
" resting current," stimulate by rapid induction shocks with the
automatic interrupter of the coil. A deflection will be obtained
in a direction opposite to that of the current of injury.
Nerve Unexcited by Light or Sound Waves. — The nerve-muscle
preparation is used. No contraction is obtained if a beam .of light
be allowed to fall on the nerve by placing it in front of the window,
screened by a piece of black cardboard, and suddenly removing the
screen. Neither will the sound of a bell excite the nerve trunk.
Receptors of the Skin
Histological preparations of human skin, showing various forms,
may be bought. If fresh skin can be obtained, vertical sections
may be made by the method described above (p. 193). Receptors
are looked for in the projecting papillae underneath the outer
" epidermis."
LABORATORY WORK 211
Taste- Buds
These will be found in sections across the " papilla foliata " of
the rabbit's tongue. These are two small oval areas, one on each
side of the back of the tongue.
Olfactory Cells
Take a small piece of the mucous membrane of the upper part
of the nasal cavity of the frog, beneath the olfactory lobes pro-
jecting from the front of the brain. Place in i per cent, osmic acid
for a few hours. Soak in water for two or three days. Tease in
dilute glycerine, or break up by tapping on the cover-glass. Look
for narrow cells with brush-like outer ends. These are the smell
receptors.
Heat and Cold Spots
Explore the skin of the back of your hand for these, using a
simple instrument made thus : — Draw out a piece of half-inch glass
tubing to about Jth in. Cut it at the narrow part. Cement with
sealing wax into the end a short piece of thick copper wire filed to
a small rounded point. Wrap flannel around the tube so that the
temperature may remain fairly constant for some minutes. For heat
spots, put water at 40° C. into the tube and allow it to rest gently
on various places of the skin. When one is found which is sensitive
to heat, mark it with red ink for future identification. For cold
spots, put finely broken ice into the tube and proceed in a similar
way, marking the spots black. They will be found to have different
situations from the heat spots.
Stimulate both kinds with induction shocks by placing on them
finely pointed electrodes. The sensation from each will be its own
appropriate one, so that, if stimulated at all, the sensation is always
the same. A temperature which feels distinctly warm to the heat
spots does not affect the cold spots, and vice versa. But a tempera-
ture sufficiently high to stimulate the nerve fibres themselves will
produce a sensation of cold from a cold spot. Thus, while 15° pro-
duces a sensation of cold from a cold spot and none from a heat
spot, one of 45° produces a sensation of heat from a heat spot and
cold from a cold spot. This paradox is due, of course, to the
operation of the law of specific sensation, as applied to the fact
that the receptors for temperatures above that of the skin and below
it are separate organs.
212 INTRODUCTION TO GENERAL PHYSIOLOGY
Hearing
Preparations of the cochlea are difficult to make. They may
sometimes be bought. That of the guinea-pig is the easiest to
prepare on account of its size. The method used is as follows :—
The part of the skull containing the petrous bone is cut out with
bone forceps. A hole is filed at the top of the cochlea, which is
seen as a conical eminence. The preparation is then placed in
fixing fluid, and the bone afterwards decalcified by immersion in a
solution made by dissolving I gm. of phloroglucin in nitric acid
with the aid of heat and diluting to 100 c.c. with water. The object
of the phloroglucin is to counteract the effect of the acid in causing
the tissues to swell. Wash well and transfer to alcohol, gradually
increasing in strength. Then through cedar oil and paraffin as
usual. The transition from one liquid to the other must be
gradual, by using mixtures with the preceding one, since the organ of
Corti is very readily broken up. Sections are cut through the axis
of the spiral, so that it is not possible to obtain many from one
preparation.
The resonance of a stretched membrane may be seen by taking
a triangular piece of sheet india-rubber, and attaching two of its
sides to strips of wood by screwing down on to each of the strips
a second one of the same size. One side is firmly clamped to the
edge of the bench, the other held in the hand and used to stretch
the membrane. Sand is dusted on to the membrane, and a pitch-
pipe or other similar source of a musical note sounded near. By
adjustment of the pitch of the note, and by different degrees of
tension on the membrane, a particular place may be found which
vibrates in sympathy with the note, as shown by scattering of the
sand. Suppose that this place is near the wider end when a low
note is sounded, then by raising the pitch a narrower region will
resonate.
Photo- Receptors. The Eye
Visual Purple. — Keep a frog in the dark for a day. Kill it by
pithing in a room lit only by a sodium flame or a dim photographic
red lamp. Excise the eye-balls. Cut each one into a front and a
back half by means of a razor. Put the latter half into a dish of
0.7 per cent, sodium chloride. Seize the outer coat at its cut edge
with forceps and shake the retina loose. If it does not come free
from the place where the optic nerve enters, a pointed scalpel or fine
scissors may be used to cut it free. Taking the dish into ordinary
light, the beautiful crimson colour of the retina will be seen. It
will become bleached more or less rapidly according to the intensity
of the light.
LABORATORY WORK 213
Anatomy of the Eye.— Take the fresh eye of an ox. Note—
The transparent cornea in front, continuous with the opaque
sclerotic.
The entrance of the optic nerve, opposite the cornea.
The coloured iris, with the aperture (pupil) in the middle.
Divide the eye into front and back halves by a razor. Note
In posterior half —
Gelatinous vitreous humor in posterior chamber.
Thin transparent retina, covering the
Black choroid coat.
Entrance of the optic nerve and its continuity with the
retina.
In anterior half —
The coating membrane of the vitreous humor attached to
the front of the choroid and holding the lens in a
capsule behind the pupil.
Remove the lens, noting its elastic nature.
Note the aqueous humor, filling the anterior chamber between
the cornea and the lens.
Take another eye and freeze it in a mixture of ice and salt.
When solid, divide it by a section at right angles to the preceding
one, namely, perpendicular to the surface of the cornea and passing
through the entrance of the optic nerve.
Study the relative positions of the parts as they become visible
on thawing. Note especially the way the capsule of the lens is
attached to thickened ridges of the choroid close behind where the
cornea and sclerotic meet ; and that the iris is also attached to
these "ciliary processes." The fibres of the ciliary muscle in them
are firmly attached at the junction of the cornea and sclerotic, and
when they contract they pull forward and relax the suspension of
the lens, so that the latter takes up more or less its natural more
spherical form. Thus the focal length is altered to accommodate
for near objects.
Image on the Retina. — Expose the back of the eye in a dead
rabbit by cutting away the bone and other tissues. Place an
incandescent lamp two or three yards in front of the eye, and
examine the back of the eye with a lens. If the pigment of the
choroid is not too dense, a minute inverted image of the filament
is seen on the retina. It is easily seen if an albino rabbit, which
has no pigment in the choroid, be taken. It is possible, with care,
in an ordinary rabbit to cut out a little window in the sclerotic,
and to brush away the pigment with a camel hair brush wetted
with 0.9 per cent, saline.
When the image is seen, place a microscope slide in front of the
214 INTRODUCTION TO GENERAL PHYSIOLOGY
cornea, so as just to avoid touching it. Drop 0.9 per cent, saline
into the gap between the surfaces. The image will disappear, because
the curved refracting surface is no longer present. Remove the
glass slide, the image reappears.
Structure of the Retina. — The posterior half of the eye of a
rabbit, after removal of the vitreous humor, is fixed, and sections
cut of a strip of the retina and sclerotic together.
Place a piece of retina in I per cent, osmic acid for a few hours,
and then in dilute glycerine for a day or two. Take a small
fragment and tease it with needles in dilute glycerine. Further
separation of the constituents is brought about by tapping the
cover-glass. Many of the points in the structure of the retina can
be made out by aid of the figures in the text-books of histology.
Receptors for Position
The small transparent fresh-water mollusc, Cyclas, has a statocyst
attached to the ganglion in the foot, and can be examined in the
living state under a low power of the microscope.
Expose the cartilaginous skull of a skate or dog-fish.
Notice the position of the semi-circular canals, one set on each
side, after removal of the roof of the skull and the brain. They are
contained within a mass of cartilage behind the eyes. Carefully
slice away the cartilage until the three canals become easily visible.
Note their position in the three dimensions of space.
CHAPTER V
The Central Nervous System
ALLOW a frog to lie for a day or two in methylated spirit. Remove
the muscles from the surfaces of the arches of the spinal column.
Open the neural canal by dividing the membrane between the skull
and the vertebrae. Cut away, bit by bit, the bony roof of the skull
by means of a narrow-bladed but strong pair of scissors. Remove
the arches of the vertebrae in the same way. The spinal cord is
usually concealed more or less by soft material, which can be gently
removed with a blunt pair of forceps.
Notice the brain in the skull, the spinal cord in the vertebral
column, continuous with one another.
The Brain. — In front, two elongated masses forming half of the
brain. Each has an anterior small part and a large posterior part.
The former is the centre for smell (olfactory lobes'] ; the latter, the
cerebral hemispheres ; on their surface the cortex.
Between the posterior ends of the cerebral hemispheres the
thalami appear, and from them an optic lobe (the mid-brain),
forming a rounded eminence, projects on each side.
A narrow band, the cerebellum, lies transversely just behind the
optic lobes. It is very small in the frog.
The rest of the brain forms the bulb, and contains very important
nerve centres. Its dorsal aspect presents a triangular hollow, the
fourth ventricle.
Raise the brain carefully, beginning anteriorly and cutting
through the nerves passing from it through the skull. Notice the
ventral aspect of the structures mentioned above.
In the spinal cord note the number of nerves given off, and that
each arises by two roots. The dorsal root contains the afferent
or sensory fibres, the ventral root the efferent or motor fibres.
Sympathetic System. — Turning the frog on its back, remove
the viscera and find the sympathetic trunk on each side of the main
arterial trunk, the aorta. It is a slender cord with ganglia (enlarge-
ments containing nerve cells) at intervals. Note the delicate nerves
uniting these ganglia to the spinal nerves, most easily seen in the
case of the long nerves to the hind-legs.
215
216 INTRODUCTION TO GENERAL PHYSIOLOGY
Make a similar preparation of a mammal, such as a rat. Note
that the cerebral hemispheres and the cerebellum have grown so
much as to cover over the other parts, which can only be seen by
raising or removing the former. The optic lobes, on the other
hand, are relatively small, and are represented in the mammal by
the corpora quadrigemina, four little eminences.
Examine in a museum the remains of various extinct verte-
brates, such as the Plesiosaurus. Note the small size of the skull
in proportion to the rest of the body. Compare the state of affairs
in the cat or man, and realise that the law of progress is that the
prizes go to the wise, and that the main factor in evolution is the
development of the nervous system.
The cell-bodies of the spinal neurones are well seen by taking a
piece of the fresh spinal cord of the ox. With the point of a
scalpel, take out a small piece of the central grey matter from the
more ventral broader part Tease in 0.9 per cent, saline and put
on a cover-glass, with a bit of hair to protect the cells from pressure.
Look for large branching cells.
Another way by which fixed cells are obtained is to allow a
small piece of the grey matter to macerate for a day or two in very
dilute chromic acid in a small stoppered bottle. Shake. Allow
to deposit, and mount some of the deposit as above. This prepara-
may be stained and preserved by mounting in dilute glycerine.
Similar preparations should be made of the cortex of the
cerebral hemispheres and of the cerebellum of the rat.
Spinal Reflexes
Some observations were made above (p. 209) on the spinal frog.
A more detailed study is to be made here.
Having made the preparation as before, hang up the frog to
the edge of the table by a pin through its jaw.
Gently pinch one of the toes. Note the movement.
Apply a very small bit of filter-paper dipped in acetic acid to
the skin of one flank or thigh. The leg will wipe it away.
Stimulate the toes of one side by a stimulus that can be adjusted
in strength, such as the induced currents from the induction coil.
Note that the number of muscles engaged in the reflex increases
with the strength of the stimulus. Also that the time the move-
ments last after the stimulus has ceased is longer with strong
stimuli.
Dip the foot in weak sulphuric acid of progressively increasing
strength (o.i, 0.2, 0.3, 04, and 0.5 per cent), dipping the foot in
water as soon as the reflex has been observed, Note that there
LABORATORY WORK 217
is a pause before the foot is withdrawn, and that this is longer, the
weaker the acid.
Find a strength of acid that invariably results in withdrawal
of the foot after a moderate time. Then pinch firmly the opposite
foot at the same time as the acid is applied. The reflex is stopped
or takes much longer to show itself (inhibition}.
Inhibition
That an action in progress can be stopped by nervous influences
is most clearly shown by stimulating the vagus nerve, and thus
stopping the beats of the heart. It is somewhat difficult to dissect
out this nerve in the frog, but it may be stimulated at its origin
from the bulb in the following way : —
Kill a frog by cutting off the brain with scissors just behind
the eyes. Then cut through the spinal column between the
shoulder blades and destroy the posterior part with a pin. The
bulb is thus isolated, with the vagus nerves passing from it to the
heart.
Expose the heart and connect the apex of the ventricle to the
lever as on p. 207 above. Put electrodes into the exposed surface
of the bulb where the front part of the head has been cut off,
fixing them to the cork by means of plasticine. Stimulate at first
with weak currents, increasing the strength until the heart stops.
The fact may also be shown by stimulating the vagus nerves
in their course from the sinus to the auricles. The electrodes are
placed on the white crescentic line marking the junction of these
chambers. Only a weak stimulus must be used, otherwise the
muscle itself is stimulated to rapid contractions.
CHAPTER VI
Model of the Circulation. — Prepare with glass and rubber tubing, etc.,
an arrangement such as that indicated in Fig. 20. A is the bulb
of a Higginson syringe, to be obtained from the druggist, the valves
being removed. At one end a valve B is connected. This consists
of a glass tube, represented separately at M, closed at one end, and
having a hole blown in the side about 2 cm. from the closed end.
The edges of this hole are smoothed in the flame. This is pushed
through a rubber cork fitting the end of a wider tube, which is
drawn out at the other end to fit f^- in. soft rubber tubing. A piece
of sheet rubber is tied over the hole, with its free edges just meet-
ing opposite the hole, the threads being in the situations represented.
When water is pressed through this valve it escapes between the
rubber and the cork. Any pressure in the opposite direction closes
the hole by forcing the rubber into it. The bulb with its two
valves represents the heart. D is a mercury manometer connected
by a T-tube in the course of the current. This T-tube is continued
by a 4-ft. length of y\ in. rubber tubing (c), of moderately thin
walls, representing the arterial system. E is a screw pinchcock to
represent peripheral resistance. F is a wide glass tube, fitted with
rubber corks through which short glass tubes pass, and filled with
small pieces of sponge, loosely packed. This represents the
capillaries. A short rubber tube connects this with a 6-in. length
of the inner tube of a bicycle tyre (G), which has rubber corks
similar to those of F. There is another mercury manometer (H) to
indicate the pressure in the veins, which are represented by that
part of the model between F and K. K is a valve similar to B, and
represents the mitral valve of the heart, whereas B represents the
aortic valves. K is, of course, connected in such a way that water
can pass it from the vein to the bulb and not the opposite way.
G and K are connected by a short piece of rubber tubing, so that
they can be disconnected for convenience of filling the system with
water. L is a T-tube joined to a funnel, for the purpose of running
in more water. It is closed by a pinchcock.
With this schema a large number of instructive experiments can
be performed. The following are some of these : —
Disconnect the tube C from F, and G from K. Insert a glass
218
LABORATORY WORK
219
^Si .3
I > §6.9-3
fi-3S3"2
3'22>Sli
2 s^ a a -a
220 INTRODUCTION TO GENERAL PHYSIOLOGY
jet into the end of C. Immerse the end of K in a bowl of water.
With the clamp E widely open, compress A gently with the hand
at regular intervals of about one or two seconds. Air will first be
driven out and then water, which will escape from the end of C in
spurts. Gradually close E, continuing the regular squeezing of the
bulb as before. The spurts will be converted into a more or less
constant stream.
Remove the jet and connect up again to F. Open the screw
clamp and pump water through until all air is driven out, and then
slip on, under water, the connection between G and K.
Periodic compression of A will be seen to cause the valve B to
open, K to close. On releasing the pressure the suction of the bulb
draws water through K, while B closes. The mercury in both
manometers oscillates greatly, while the mean pressure in D is not
much higher than in H (vaso-dilatation). Close next the screw
clamp gradually. Note the rise in D and the nearly complete
cessation of oscillations in H (vaso-constriction). If G is only
partially distended at rest, owing to the amount of water in the
system being insufficient to fill it, it will be noticed that when
the compressions are started, particularly with a high resistance
at E, G becomes less full. The arterial tube is stretched by the high
pressure.
After running in more water under pressure through L, it can
be shown that a higher arterial pressure is maintained with a similar
degree of compression of A.
Take away the manometer H with its T-tube and connect up
the tubes at the gap. Insert the T-tube between E and F. Pump-
ing as before, note that the pressure beyond E is low when the screw
clip is narrowed. Next insert the manometer D between F and L,
and notice that there is not much difference of pressure between
the two ends of the capillary region, but that if E is widened both
pressures rise, thus indicating a rise in the capillary pressure.
Restore the manometers to their original positions. Arrange
that G is well filled, pump regularly and then compress G with the
hand, continuing the pumping as before. Decrease of the capacity
of the system raises the arterial pressure.
It will facilitate the more accurate comparison of pressures if
the compression of A is arranged to be done by a piece of wood,
attached at one end by a hinge to the base of the board on which
the system is fitted. This piece of wood passes over A, resting
upon it. The free end, when pressed down, meets a stop of such
a height as to give an appropriate degree of compression to A. This
stop may be a long screw passing through the wooden base.
The rate of the compression is kept constant by the beat of a
metronome.
LABORATORY WORK 221
If this regularity of compression is ensured, the effect of viscosity
may be investigated. Notice the height of the arterial pressure
when water is used. Then fill the system with 7 per cent, gum
arabic, without altering any of the adjustments. Repeating the
experiment, the pressure will be much higher. An instructive way
of doing the experiment is to disconnect between F and L, and put
the funnel into the end of the tube connected with K. Run in
gum solution through the funnel while the bulb A is being com-
pressed regularly. As the water is displaced the pressure in D will
steadily rise.
Seven per cent, gum arabic has a viscosity about equal to that
of blood. It is easily made by placing the necessary amount in
water the day before it is wanted. The solution will need straining
through fine muslin or glass wool To find out how much solution
is wanted, the system is filled with Vvater and then emptied into a
graduated cylinder.
The whole apparatus should be taken to pieces after use and
the parts kept separated. The sponges should be taken out of
their tube and dried.
The Circulation in the Frog's Web
Take a piece of thin wood, such as the lid of a cigar box, about
6 in. by 2 or 3 in. Near one end make a hole about half an inch
in diameter.
Anaesthetise a frog by an injection of urethane. One minim of
a 5 per cent, solution for a medium-sized frog (25 gm.), under the
skin of the back, will be about the correct dose. When motionless,
lay the frog, belly downwards, on the board. Tie threads to two
of the toes, place a glass slide over the hole, and draw the web over
it by fixing the threads into notches in the end of the wood.
Do not stretch it tightly. Keep the frog moist by wet filter paper
on its back, and the web by occasionally dropping water on it.
Examine with low magnification, and note the network of blood
vessels below the layer of pigment cells.
The arteries are recognised by the rapid current of blood in
them ; they run mainly towards the free edge of the web. The
direction of the flow is from larger to smaller vessels.
The capillaries, in which the arterial branches end, form a close
network in which the current is slow.
The veins are formed by union of capillaries. The blood-flow
is more rapid than in the latter, but not so fast as in the arteries.
The direction of flow is from smaller to larger vessels, and mainly
away from the free edge of the web.
Next take a triangular bit of cover-glass, place it on the web
222 INTRODUCTION TO GENERAL PHYSIOLOGY
and examine with a higher power. Note the difference in thick-
ness of the walls of the arteries, veins, and capillaries. The walls
of the latter appear merely as thin lines. Also note the rapid
stream in the middle of the arteries, the slower one near the walls ;
this latter may be seen to contain many leucocytes. The flow in
the capillaries is much slower ; the red corpuscles are often distorted
in shape, but recover again. They are thus elastic.
The Heart of the Sheep
Obtain a sheep's heart from the butcher, if possible with the
lungs attached. This is often called the " pull."
The anterior or ventral surface is recognised by the groove filled
with fat, which marks the boundary between the two ventricles;
Note that the right ventricle is softer than the left. The pulmonary
artery comes from the former nearly in the middle line. The aorta
is behind it and will be cut.
The two great veins from the body, superior and inferior venae
cavae, have been cut. Note that they enter the right auricle, so
that the blood passes into the right side of the heart and from the
ventricle into the lungs. Find the veins returning from the lungs
to the left auricle, their blood thence flowing to the left ventricle
and to the aorta.
Open the right auricle and cut away most of it. Four water
into the orifice leading to the right ventricle. Note the flaps of the
tricuspid valve floating up.
With scissors, one blade between two of the flaps of the valve,
cut through the wall of the ventricle towards the apex. Then
upwards again along the septum, but not quite into the pulmonary
artery. Cut across the pulmonary artery, and tie into it a glass
tube a few inches long and as large as will fit in. Pour water into
this tube, and observe from below the closed semilunar valves.
Note also the fine cords attaching the flaps of the tricuspid valve
to the walls of the ventricle.
Open up the left auricle and ventricle in a similar way, noting
especially the openings of the arteries feeding the heart (coronary
arteries) behind the aortic valves.
Look for the band of special conducting tissue passing from
auricles to ventricles. Branches will be found on both sides of the
septum between the ventricles. They stain deeply with iodine on
account of the large amount of glycogen they contain.
Blood
Mount quickly a small drop of blood from the finger. Examine
at once with a high power. Note :—
LABORATORY WORK 223
The red corpuscles, mostly collected in rouleaux (strings of
corpuscles stuck together by their flat sides). They appear, when
seen singly, to be of a faint yellow colour.
The colourless corpuscles or leucocytes. If the cover-glass be
touched with a needle, they tend to stick to the glass, while the red
corpuscles are driven about by the current.
Look also for fibrin filaments in the clear spaces.
If the preparation is to be observed for any length of time, the
cover-glasj should have been painted around the edge with vaseline,
as described above (p. 166).
An interesting preparation showing the fibrin produced in the
process of clotting may be made thus : — Make a preparation as
above, and leave for ten minutes. Remove the cover-glass, run a
"few drops of distilled water over the slide to dissolve the corpuscles.
Drop on a moderately dilute solution of Spiller's purple. Leave
for a few minutes. Wash with distilled water. Allow to dry and
mount in balsam. A network of fibrin will be seen, having lumps
of precipitated colloid at the junctions of the filaments (so-called
" platelets "). (Schafer.)
The Kidney
Sections of the kidney of the mouse may be prepared in the
ordinary way. Notice the tufts of blood vessels in capsules in the
cortical part. These tufts can be made out better in sections of a
kidney of which the blood vessels have been injected with a coloured
material. Such sections can be bought.
The Salts of the Blood
Pass a 0.7 per cent, solution of pure sodium chloride in distilled
water through a frog's heart preparation, as described above
(p. 187). The beats will become weak or cease. Take 100 c.c. of
the solution and add 3 c.c. of o.i molar (=1.1 per cent.) calcium
chloride. Pass through the heart. The beats increase in size, but
the heart tends to become tonically contracted, shown by incom-
plete relaxation between the beats. Take another 100 c.c. of the
sodium chloride solution, add 3 c.c. of the calcium chloride solution
and also 6 c.c. of o.i molar ( = 0.75 per cent.) solution of potassium
chloride. Pass through the heart. The beats become vigorous
with normal relaxation. Pass again the pure sodium chloride.
When the beats have become small, take 100 c.c. of the solution
and add 6 c.c. of the potassium chloride solution. No improvement
results. Hence all these three cations— sodium potassium, and
224 INTRODUCTION TO GENERAL PHYSIOLOGY
calcium — are required to maintain normal beats. This is Ringer's
solution. (See P, p. 209).
Make the Ringer's solution just acid to methyl-red (io~4 N in
H-ion) and perfuse. On the other side, make it alkaline to phenol-
phthalein (icr9) and perfuse.
Hydrogen-ion Regulation. — Make a solution of sodium bicarbonate
of 0.25 per cent., which is about the usual concentration in the blood.
Put 5 c.c. of this into a stoppered bottle, add a drop of o.i per cent,
neutral red. Fill the bottle with the first air expired after a deep
inspiration. Shake together. The colour is orange-red. Fill the
bottle with alveolar air (p. 176). The colour becomes red. Run in
carbon dioxide from a Kipp generator. The colour is crimson.
Replace by repeated changes of atmospheric air. The colour goes
back through red to orange and yellow. Bicarbonate solutions
lose carbon dioxide to air, becoming more and more alkaline.
Thus the hydrogen-ion concentration is regulated by the proportion
of carbon dioxide to bicarbonate.
Vaso-Motor Effects
Prepare a frog for observation of the circulation in the web
(p. 221). Carefully dissect out the sciatic nerve in the thigh, avoid-
ing injury to the blood vessels. Pass a thread under the nerve
Select an arteriole for observation, and measure its diameter by the
ocular micrometer. Lift up the sciatic nerve and cut it. Note the
widening of the arteriole. Pick up the peripheral end with forceps ;
tie a thread just beyond the points of the forceps. After waiting
a minute or two, measure the arteriole again and then stimulate
the nerve with induction currents of moderate strength. The
arteriole narrows. It may be necessary to tie down the leg or
pin it to the cork, owing to the muscular contractions produced by
the stimulation. But the better way would be to curarize the frog,
as described above (p. 199).
Apply a drop of dilute adrenaline solution (i in 10,000) to the
web. The arteriole becomes almost obliterated.
Take the web of the opposite side and allow a drop of a
saturated solution of chloroform to fall upon it. The arterioles
dilate, and a more copious flow through the capillaries is seen.
Wash away the chloroform, and when the circulation has become
normal again, apply a drop of a solution of histamine (o. I per cent.).
If the dose is correct the capillaries may dilate, but not the
arterioles.
Adrenaline is sold in convenient tabloids by Burroughs &
Wellcome, under the name of " hemisine." Histamine tabloids are
sold as " ergamine."
LABORATORY WORK 225
Action of Drugs on the Heart
Arrange a frog heart for perfusion (p. 187). Add a small amount
of adrenaline to the Ringer's solution used for perfusion (one part
in several thousands is active). The heart beat is increased in
rate and in strength owing to stimulation of the accelerator
(sympathetic) nerve-endings in it.
Add a very small quantity of acetyl-choline to another part of
the Ringer's solution (one part in a million or less). The heart is
stopped.
Prepare another frog for recording the effect on the heart of
stimulation of the vagus (p. 217). Having obtained slowing or
complete inhibition, drop i per cent, solution of atropine sulphate
in Ringer's solution over the heart. Stimulate the vagus again.
No effect will be seen, nor even if the junction of the sinus with
the auricles be stimulated. The endings of the vagus nerves are
paralysed.
The Beat of the Heart
Expose the heart of a frog as previously described.
Note that each beat consists of a cycle, beginning in the sinus,
followed by the auricles, ventricle, and bulbus in turn.
To show that the beat is initiated by the sinus, which thus
controls the rate, Gaskell's method of local warming may be
used. Take two pieces of copper wire of about fV in. diameter.
Solder flexible electric light cable on to each, so that they may
be joined up to a storage cell, placing a key in the circuit. Wrap
thread around one of the wires up to half an inch of the free end,
and then tie the wires together. Bend each wire outwards at the
free end, so that there is a gap of about a quarter of an inch. Solder
across this gap a bit of fine German silver wire. When the circuit
is closed, the fine wire is heated. Take care that it does not get
hot enough to melt the solder. It will be better to have an adjust-
able resistance in the circuit.
Having prepared a frog with the heart joined to the lever as
on p. 217, fix the heating wire on a pillar of plasticine so that it is
near to the sinus, but not touching it. On closing the circuit, the
rate of the heart beats increases markedly. Now move the heating
wire to lie just above the apex of the ventricle ; the fine wire
may be bent concave to escape the suspension thread. When the
heating current is put on there is no change in the rate if the sinus
is protected. But the ventricular beats decrease in amplitude.
This last fact is another aspect of the temperature coefficient of
surface energy which we saw exemplified in the effect of heat on
the tonus of smooth muscle (p. 196).
CHAPTER VII
Dividing Nuclei
To see the phenomenon as it actually takes place, the blood
corpuscles of the newt are favourable objects.
Newts are kept without food for three months. They are then
fed with small worms, cautiously at first. After about ten days
there are a large number of young red corpuscles in the blood and
in the process of division. These cells are almost devoid of haemo-
globin, round and with large nuclei. The complete process takes
about half an hour. For further details, see the paper by Jolly
(Archives cT Anatomie Microscopique, Tome vi. (1904), p. 455).
The Development of the Frog should be watched. Collect
some of the spawn in the spring, placing it in water in shallow
dishes. Green weeds should be supplied. Notice the changes,
first to the tadpole, then the appearance of legs, the disappearance
of the tail, and the attainment of the form of the adult frog. Many
details will be noticed and sketches may be made. The very early
stages are best observed under a simple lens.
Ova and Spermatozoa
The ovary of the frog is most easily made out just after the
breeding season, after the full-grown ova have been shed.
Remove one of the ovaries and place it in 0.7 per cent, saline.
Make an incision into it. It contains a cavity. Upon the walls
of this cavity are round eminences of various sizes. These are ova
in various stages in development.
Tease out a bit of ovary in 0.7 per cent, saline. Examine with
a low power. Note that there are many ova much smaller than
those seen with the naked eye. They are granular spherical cells,
with a clear central part.
The spermatozoa can be seen by opening one of the testes and
pressing out some of its contents upon a slide, mounting in v/ater
and examining with a high power. They appear as long motile
filaments. More typical spermatozoa can be obtained from the
rat, and consist of a small head and a long motile tail. The
226
LABORATORY WORK 227
contents of the testis may be pressed out into 0.9 per cent, sodium
chloride.
The Structure of a Flower
Examine a buttercup or other large regular flower. Note
the four sets of organs : —
1. The outer green calyx, which formed the covering of the
flower in bud.
2. The corolla, consisting of petals, the most conspicuous
part of the flower, usually coloured.
3. The stamens or male elements. Each consisting of a
stalk we filament, ending in a knob, the anther. Tease
an anther in water and examine with the microscope.
Note the pollen grains.
4. The/w/*/, consisting of separate carpels in the buttercup.
At their upper ends they have a sticky surface, the
stigma ; in the interior of the lower part, the ovary,
will be found the ovules.
Process of Fertilisation in the Plant
This is difficult to follow in all its stages in one species of plant.
^^penetration of the pollen tube into the stigma and style can
be seen in the evening primrose.
Hold the stigma and style between the finger and thumb of the
left hand. Moisten with a drop of water and cut successive
sections with a razor, wetted with water. Spread them out in
water with a needle and examine with a low power. The
triangular pollen grains send out a tube from one angle into the
tissue of the stigma.
The entrance of the pollen tube into the ovule can be made out
in yeronica serpyllifolia, the speedwell, common in meadows.
Take a flower from which the corolla has just dropped. Dissect
out the small ovary, and open one of its two chambers with needles
in a drop of water. The use of a lens may be necessary. A mass
of ovules is removed from the interior and teased apart. Put on a
cover-glass and search for an ovule showing the entrance of the
pollen tube. The further progress of the pollen tube can be seen
better if dilute glycerine be run under the cover-glass and allowed
to soak into the ovule and make it transparent.
INDEX
ABSOLUTE zero, 12
Absorption from stomach, 7 1
— of amino-acids, 75
- of fats, 74
— spectrum of haemoglobin, 201
Accelerator nerves, 149
Accessory factors of food, 61
Accommodation in eye, 115
Acetic acid, 45
Acid of stomach, 75
- production in muscle, 205
Acid-fuchsin, 205
Acidic dyes, 37
Acidity, 26
Acidosis, 89, 143
Acids, 25
— strength of, 26
Activation of oxygen, 90
Active mass, 68
Adaptation, 158, 159
Addison's disease, 136
Adjustment to outside changes, 120
Adrenaline, 131, 147, 150, 224, 225
Adrenals, 131
Adsorption, 15
Adsorption by charcoal, 168
- by enzymes, 70
- compounds, 36
- in catalysis, 69
- of basic and acidic dyes, 37, 39
- of gases by charcoal, 86, 87
- of ions by colloids, 34, 36
Advantage of raised temperature, 101
Afferent nerves, 121
After-action in cortex, 130
Air-sacs, 89
Alanine, 48
Alcohol and guinea-pigs, 157
Alcohols, 43, 64
Aldehydes, 43
-- in reduction processes, 92
Aldoses, 43
Alimentary canal, 66, 189
- movements of, 78
Alkalinity, 27
"All-or-nothing," 97, 105
- in heart muscle, 207
— in nerve, 130
Amino-acids, 45
— as food, 46
Ammonia, 45
- as source of nitrogen, 57
- test for, 181
— to neutralise acid in blood, 144
Amoeba, sources of, 164
Amoeboid movement, 6
Amphoteric substances, 46
Amplitude of vibrations, 108
Amylase, 70
- of saliva, 193
Anabolism and catabolism, 155
Anaesthesia of nerve fibres, 131
Anaphylactic shock, 141
Anion, 24
Anode, 24
Antherozooids, 153
Anti-bodies, 141
Anti-scorbutic factor, 61
Antiseptic method, 57
Aorta, 133
Appetite, 80, 82
Arc-lamp, 163
Arsenious sulphide, 178
Arteries, 133
Arterioles, 133
Articulated bones, 204
Aseptic method, 57
Association neurones, 122
Asymmetric carbon atom, 48
Atoms, 4, 6, 24
Atropine 150, 225
Auditory nerve, number of fibres in,
no
Auricle, 132
Auriculo-ventricular bundle, 150
Autoxidation, 90, 202
Axon, 122
BACTERIA, 56, 151, 158, 186
Bacterial changes in large intestine,
78
Balance, 167
- of excitation and inhibition, 125
Balanced salts, 142
Basal metabolism, 63
229
230
INDEX
Bases, strength of, 26
Basic dyes, 37
Basilar membrane, no
Beat of the heart, 225
Beri-beri, 61
Bicarbonate of blood, 224
Biceps muscle, 93
Bile, 74
- pigments, 135
Bladder, 99
Blood, 85, 135, 222
- circulation of, 132
- supply of kidney, 140
- vessels, permeability of, 137, 140
Blushing, 146
" Bonds," 44
Bones and joints, 93
Bouin's fixing fluid, 194
Bound energy, 13
Boundaries, 8
Boyle's law, 22
Brain, blood supply to, 146
- of frog, 215
Breathing, 88
Brownian movement, 3, 6, 166
Bundle of His-Kent, 150
Butter, 30
CALCIUM, 59
- and clotting, 142
- function of, 142
- on frog heart, 187
Calomel electrodes, 197
Calorie, 63
Cambrian period, 60
Camera and eye compared, 114
Cane-sugar, 44, 72
Capacity factor, 14
Capillaries, 133
Carbohydrate cycle, 73
Carbohydrates, 42
- digestion of, 70
Carbon atom, properties of, 42
- compounds, 41,42
- test for, 181
- dioxide absorption, 88
- action of, on respiration, 202
- and haemoglobin, 88
- in expired air, 182
- source of, 41
Carboxyl, 43
Carriage of carbon dioxide, 201
- of oxygen by hemoglobin, 200
Catalase, 55, 91
Catalysis model, 192
Catalysts, 66
Catalytic action of platinum, 192
Cathode, 24 %
Cation, 24
Cell, 3
— membrane, 9, 15, 65
- sap, 20
Central nervous system of frog, 209,
215
- of mammal, 216
Cerebral cortex, 127
- hemispheres, 103, 120
Changes in permeability, 29, 98
Charcoal, adsorption by, 16
Chemical action on blood vessels, 147
— affinity, 14
- combination, 36, 44
— nature of, 15
— energy, 11
— form u la?, 44
- inertness of colloids, 35
- messengers, 136
- potential, 14, 90
- raised by adsorption, 70
- senses, 107
- stimulation of glands, 82
Chlorophyll, 51, 55
- absorption spectrum of, 186
Chloroplasts, 54
Ciliary muscle, 115, 213
Circulation in frog's web, 221
- model of, 218
- of blood, 132
Clotting of blood, 141
Cochlea, 108
- preparations of, 212
Cohesion, 5
Cold, 104, 106
Colloidal complexes, 36
- gold, 176
- ion, 33
— state, 30, 32
Colour perception, 117
Combustion, 11
- in air, 189
— in animals, 1 1
- mechanism of, 72
- of sugars, 72
Concentration of reagents, 68
Conditioned reflexes, 127
Cones of retina, 116
Conjugation, 152
Consciousness, 103, 105, 120, 124
Constituents of diet, 65
INDEX
Continuity of germ plasm, 153
Continuous phase, 30
Contractile vacuole, 171
Contraction of muscle, 93, 204
- by stretching, 80
Contractures, 100
Co-ordination of eye and hand, 119
Cornea, 115
Coronary arteries, 222
Corpus luteum, 1 56
Corpuscles of blood, 135, 223
Cortex inhibited, 129
Cortical points, stimulation of, 129
Cream, 30
Cross-fertilisation, 154
Curare, 199
Current of injury, 210
DARK-GROUND illumination, 4, 166
Deamination, 76
Deficiency diseases, 61
Degeneration of nerves, 121
Degradation of energy, 12
Denitrifying organisms, 58
1 )epression of freezing point, 22
Depressor reflex, 146
Desensitisation, 141
Development of the frog, 226
Dextrin, 70
Diabetes, 136
Diagrams, viii.
Diaphragm, 88
Diet, constituents of, 65
Digestion, 66
Direct adaptation, 158
Dispersed phase, 30
Dissipation of energy, 13
Dissociation of oxy-h^emoglobm, b6
- tension, 87
Distance receptors, 107, 120
Distribution of water between phases,
31
Dividing nuclei, 226
Dogmatic presentation, vi.
Dominant, 156
Dorsal root ganglia, 124
Drugs, action of, on the heart, 225
Drum of ear, 108
EARTHWORM, nervous system of, 123
Economical use of energy, 13
Effectors, 121
Efferent fibres, 124
Egg-cells, 153
Electric charge on colloids, 33
- waves, 113
Electrical adsorption, 36, 38, 179
- change in nerve, 130, 210
— changes in secretion, 83, 197
— charge on colloids, 177
- resistance of living cells, 28
- stimulation of amoeba, 165
Electrodes, 24
- for heart, 207
Electrolysis, 24
Electrolytes, 25
- action of, on colloids, 35
- precipitation of colloids by, 178
Electrolytic colloids, 33
- conductivity, 173, 174
- dissociation, 23
Electrolytically dissociated colloids,
, 33,34
Electrons, 6
Emulsions, 30
Emulsoids, 30, 177
Energetics, 10
- laws of, 10, ii
Energy, 10
— of muscular work, 93
- requirements of man, 63
Enterokinase, 82, 193
Entropy, 13
Enzymes, 66, 189
Equation of state, 22
Equilibrium and death, 14
- in ester and water, 68
in photo-chemical reaction, 117
— under enzyme action, 67
Equimolecular solutions, 19
Erepsin, 75
Esters, 64, 65
— hydrolysis of, 67
Ethyl-glucoside, synthesis of, by emul-
sin, 191
Evolution of nervous system, 122
Examinations, viii.
Excitation, 125
Excitatory nerves, 79, 149
Exhaustion, 126
Experiments, value of, vi.
Expired air, analysis of, 185
Explanation, meaning of, 7
Extent of surface, 31
Extero-ceptors, 119
Eye, H2
— anatomy of, 213
232
INDEX
FARADAY phenomenon, 176
Faraday's platinum experiments, 69
Fat from carbohydrates, 72
- in intestinal epithelium, 195
Fatigue in nerve, 130
- of muscle, 94, 126
- of nerve centres, 126
Fats, 64, 65, 73
"FaFsoluble A-factor, 61
Feeding on amino-acids, 46
Female gamete, 153
Fenton's reaction, 90
Ferric hydroxide, 178
Fertilisation, 153
- in plant, 227
Fever, 101
Fibrillas of muscle, 96
Fibrin, 141
- preparation, 223
Filtration in glomeruli, 138
Final common path, 123, 124
First law of energetics, 11
Flagella, 56
" Flesh-formers," 64
Flow of water by osmosis, 197
Flower, structure of, 227
Food in hot weather, 101
- of muscle, 95
- use of, 10, 40, 41
Form, perception of, 119
Formaldehyde, 43, 54
Free energy, 1 2
Freezing point, 172
— of solutions, 22
Frog heart perfusion, 187
Fructose, 44, 72
Function of viscosity of blood, 145
GALACTOSE, 72
Galvanometer, 173, 197
Gamboge, 4
Gametes, 152
Ganglion, 123, 131
Gas analysis, 183
- phase, 9
Gastric juice, 71
General principles, value of, v.
Germ plasm, 153
Gizzard, 66
Glands, 78 ^
Glomeruli of kidney, 138
Glycerol, 64, 65
Glycine, 45
Glycogen, 71
Gradation of contraction, 97
Granules in gland cells, 81, 196
Gravity, response of plants to, 137
Green plant, 50
- action of, on carbon dioxide, 1 83
Growth, 40, 151
Guaiacum, 91
- reaction, 203
Gum solutions for intravenous injec-
tion, 140
HABIT, 129
Haematocrite, 170
Haemoglobin, 55, 85
- carriage of oxygen by, 200
Haemolysis, 17, 170
Haemorrhage, arrest of, 142
Hairs and touch, 106
Harmonics, 112
Harvey, 134
Hearing, 107
Heart, 132, 134
- of sheep, 222
Heat, 104, 1 06
- and cold spots, 211
- centre, 101
- energy, 11, 12
- relaxation of muscle by. 97
" Heat-givers," 64
Helmholtz double layer, 34, 83
Heredity, 156
Heterogeneous catalysis, 69
— systems, 8
Hexoses, 42
Hibernation, 101
Histamine, action of, on capillaries,
147
Histological preparations, 193
— staining, 37
Hormones, 84, 136
Hunger, 80
Hydration of ions, 25
Hydrogen electrode, 27
- test for, 181
Hydrogen-ion, 25
-- concentration, 27
- and breathing, 89
- regulation of blood, 224
Hydrolysis by enzymes, 190
Hydrolytic dissociation, 38
IMAGE on the retina, 164, 213
Imbibition, 31, 97
" Immortality " of protozoa, 152
Immunity, 141
INDEX
233
Increase of chemical potential by ad-
sorption, 70
Indicators, 27, 175
Induction coil, 205
Inheritance of acquired characters, 157
Inhibiting substances in plants, 137
Inhibition, 124, 126, 127, 128, 129
- from cortex, 130
- of inhibition, 129
- of reflexes, 217
- of the heart, 217
Inhibitory nerves, 79, 149
- to heart, 149
Insolubility, 33
- of products, 72
Instability of cortical points, 129
Integration in organisms, 103
Intensity factor, 14
Interfaces within cells, 17
Intermediate compounds, 69
Internal phase, 30
- pressure of liquids, 5, 8
- secretions, 84, 136
Intero-ceptors, 119
Interstitial cells, 136, 155
Invertase, 72, 193
Involuntary muscle, contraction of, 196
- structure of, 195
Ions, 24
— velocity of, 25
Isotonic solutions, 22
Iris, 115
Iron in cells, 54
- in chloroplasts, 54
- in haemoglobin, 86
Isomers, optical, 48
KEITH-FLACK node, 150
Ketone, 44
Ketonic acids, 76
Ketoses, 44
Kidneys, 136, 137
Kinetic energy, 1 1
— theory, 4
LABYRINTH, 118
Lactic acid, 73
— in blood, 143
in fatigue, 126
- oxidation of, by Fenton's re-
action, 203
produced by alkalinity, 144
production in muscle, 95
Lactone, 44
Large intestine, 77
Law of progress, 216
- of the heart, 148
- of the intestine, 80
Laws of energetics, 10, 1 1
Lecithin, 65
Length of fibres and energy of muscle,
96, 149, 206
Lens, 114, 163
- of eye, 114
Leucocytes, 135, 165, 223
Life as transfer of energy, 90
- characteristics of, i, 10, 14
Light, 51, 104, 107, 113
Linseed, 72
Lipase, 74, 190
Lip:nes, 65, 90
Liver, 71, 74, 75, 77
- blood supply of, 135
Local sign, 106
Loven reflex, 146
Lowering of surface energy, 15
Lungs, 87, 88
Lymph, 140
Lymphatic. glands, 141
Lymphatics, 74, 140
Lysine in proteins, 143
MAGNESIUM in chlorophyll, 55
Maintenance, 40, 151
- of temperature, 101
Male gamete, 153
Maltase, 71
Maltose, 71, 72
Mammary glands, 155
Manometer, 21
Mass action, 68
Measurement of microscopic objects,
169
Mechanism of combustion, 72
- of secretion, 82
Memory, 129
Mendelism, 156
Mental development, 120
Metabolism, 63
Metals in peroxidase, 91
Methyl acetate, hydrolysis of, 189
Micro-organisms, 56
Microscope, 163
- invention of, 134
Microtome, 194
Milk, 155
— sugar, 72
234
INDEX
Mitochondria, 39
Model of catalysis, 192
— of circulation, 218
Mol, 19
Molar concentration of proteins, 143
- solutions, 19
Molecular concentration, 19
Molecules, 4
Motor areas, 129
- centre, 120
Movements, 93
- of alimentary canal, 78
Muscle system, nature of, 95
Muscles, 79
Muscular sense, 119
- work and loss of structure, 40, 62
Mutual aid, 58, 159
- precipitation of colloids, 179
Myograph, 205
NATURAL selection, 158
Nature of muscle system, 95
- of nerve impulse, 130, 131
Negative after-image, 116
- temperature coefficient of muscle,
96
Nerve, electrical change in, 210
- impulse, 130
- structure of, 209
- unexcited by sound or light, 210
Nerve-cell, 121
Nerve-muscle preparation, 204
Nerves, 104
Neurone, 121
- structure of, 216
Neutral red, 27
Neutrality of blood, 142
- regulation by kidney, 144
Newt's gastric glands, 197
Nitrates as plant food, 57
- formed in soil, 186
Nitrogen, source of, 45
- test for, 181
Nitrogen-cycle, 56, 77
Nitrogen-fixing organisms, 58
Nocuous stimuli, 106
Non-polarisable electrodes, 197
Nuclear division, 152
Nucleus, 3, 39, 42, 121
OCEAN, composition of, 60
Oedema, 140
Oesophagus, 71
Oils, 65
Olein, 65
Olfactory cells, 2H
Olive oil, 65
Optical activity, 48
- isomers, 48, 49
- sensitizer, 52
Organ of Corti, 108, 109
Organic compounds, 41
Origin of heart beat, 150
Osmometer, 171
Osmosis, 17
Osmotic pressure, 21, 169
Ova, 153, 226
Ovary, 153
Oxidase, 91
- of potato, 203
Oxidation, mechanism of, 89
Oxidising enzymes, 76
Oxygen consumption and work, 100
- test for, 181
Oxy-haemoglobin, 87
PACE-MAKER of the heart, 225
Pain, 104, 1 06
Pancreas, granules in, 196
Pancreatic juice, 71, 74, 75
Paralysis, 80
Parthenogenesis, 153
Partial pressure, 85
Penis, 154
Pentoses, 42
Pepsin, 75, 193
Peptide linkage, 47
Peptones, 75
Perception of gravity, 1 1 7
Perfusion fluids, 59, 142
- of frog heart, 223
Peripheral resistance in arterioles, 145
Peristaltic contraction, 79
Permeability change in secreting cells,
82,84
- of blood vessels, 137, 140
- of cell membranes, 9, 10, 16
- of cells, 1 68
- of membranes to solids, 10
Peroxidase, 91
— nature of, 91
- of horse-radish, 203
Peroxides, 55, 90, 203
Phagocytosis, 13$
Phase, 8
— difference in compound sounds, 1 12
Phloroglucin, 212
INDEX
235
Phosphates, excretion of, 144
Phosphorus food, 58
- test for, 181
Photo-assimilation, 54
Photo-chemical reactions, 113, 116
Physical properties and chemical con-
stitution, 35
Physiology, i, 7
Pigment of eye, 116
Pilocarpine, 82
Pitch of notes, 107
Placenta, 154
Plasma, 135, 136
Plasma-membrane, 9, 15
Plasmolysis, 20, 171
Platelets, 223
Poison gases, 86
Polari meter, 49, 181
Polarised light, 49, 113
Pole-finding paper, 177
Pollen grains, 153
Pollen-tube, 227
Polypeptides, 47
Position in space, 104
- receptors, 117
Posture phenomena, 99
Potassium as radio-active element, 60
- function of, 142
Potential energy, 1 1
Practical value of science, 2
Precipitation by electrolytes, 178
- of colloids by electrolytes, 35
Proprio-ceptors, 119
- and tonus, 100
Proteins, 47, 75
- of the plasma, 141
Protoplasm, 2, 7
- chemical composition of, 41
Protoplasmic movements, 166
Protozoa, 56
Pseudopodia, 3, 7, 9
Psychology, i
Pupil of eye, 115
Putrefaction, 56
Pyrrol, 55
Pyruvic aldehyde, 76
QUALITY in sensation, 106
— of musical notes, 108, 1 1 1
RADIANT energy, 1 1
Radio activity of potassium, 142
Rate of conduction of nerve impulse,
130
- reaction between colloids, 179
Rates of chemical reactions, 67
- of reactions, 189
Reabsorption in kidney, 139
Receptors, 103
Recessive, 156
Reciprocal innervation, 125, 146
Red beet root, experiments with, 168
Red corpuscles, 85
destruction of, 135
formation of, 135
Red marrow, 135
Reduction, 92
- by milk, 203
Reflex action, 80, 103, 120, 124, 209,
216
- arc, I2i
Refractory phase of nerve, 130
- period, 98, 208
Regulation of blood supply, 145
— of heart beat, 148
- of reaction of blood, 143 •
- of temperature, 101
Rejuvenescence, 152
Removal of products of reactions, 72
Reproduction, 151
Resistance of blood corpuscles, 175
Resonance, 52, no
- of membrane, 212
Respiration, 84
Respiratory quotient, 95
Restitution phase in muscle, 101
Re-suspension of colloids, 36
Reticular membrane, no
Retina, 114, 116
- structure of, 214
Ringer's solution, 224
Ring form of amino-acids, 47
Rods of retina, 116
Roman farming, 58
Root nodules, 58, 187
- pressure, 82
"Rust" in wheat, 157
SALIVA, 66
- and thirst, 84
Salivary glands, nerves to, Si
— reflex, 128
Salts in food, 59
— of the blood, 142, 223
- required for growth, 60
Saponin, 15
236
INDEX
Saponin, action of, on enzymes, 70
- bubble, 1 68
Sarcoplasm, 96
— and tonus, 100
Sciatic nerve, position of, 199
Scurvy, 61
Sea anemone, nervous system of, 123
- water, 60
Second law of energetics, 12
Secondary sexual characters, 155
Secretin, 82, 137
Secretion, 78, 81
- disappearance of granules in, 196
Segregation of gametes, 157
Semi-circular canals, 118
- of skate, 214
Semi-permeable membranes, 19
Sense-organs, 103
Serum, 142
Sex, 152
Shivering, 101
Shock after wounds, 147
Silver chloride, action of light on, 116
Sine curve, 1 12
Size, perception of, 119
Skin receptors, 106, 210
Small intestine, 71
Smell, 104, 107
Smooth muscle, properties of, 79
Soap bubbles, 167
- film, 6, 10
- solution, 167
Sodium bicarbonate in blood, 143
Solids, permeability to, 10
Solution of gases, 86
Sound, 104, 107
Sources of carbon, 41
Specialised receptors, 107
Specific sense energies, 105
Spectroscope, 52
Speech and writing, 120
Spermatozoa, 153, 226
Spinal cord, 120
- frog, 103, 124, 209, 216
Splanchnic nerve, 80
Sponges, 122
Spores, 56, 151
Spring for muscle model, 204
Staining, 179
- of histological preparations, 37
" Staircase," 98, 207
Stannius' ligature, 207
Starch, 53
- formation of, 186
Statocyst of Cyclas, 214
Statocysts, 117
Stearin, 65
Sterilisation, 57
Stimulus, 94
Stomach, 71
- of frog, contractions of, 196
Storage of fat, 74
Strength of acids, 175
— of acids and bases, 26
Structure of muscle, 96
Struggle for existence, 13, 56, 159
Sugar, test for, 169
Sugars, 41
Sulphur food, 58
- test for, 181
Supra-renals, 131, 136
Surface action by enzymes, 192
- energy, 14
- tension, 6, 8, 9, 167
— and dispersion, 177
- in muscle, 96
- of colloids, 32
Suspensoids, 30
Sweat glands, 84
Sweating, 101
Swelling of colloids by acid, 97
— of gelatin, 177
Symbiosis, 58
Sympathetic system, 131, 146
- of frog, 2 1 5
Synapse, 121, 122
Synaptic system, 123
Synthesis and hydrolysis by catalysts
68
- by enzymes, 191
TASTE, 104, 107
Taste-buds, 211
Tectorial membrane, no
Teeth, use of, 66
Telephone exchange, 127
Temperature, effect of, on muscle,
206, 225
- regulation, 101
Temporary association, 127, 128
Tension of active muscle, 94
- of gas, 85
Testis, 153
Tetanic contraction, 98
Tetanus of muscle, 206
Thermodynamics, 10
Thirst, 84
Thoracic duct, 140
Thought and memory, 124
INDEX
237
Three constituents of diet, 65
Thyroid, 136
Titration, 28
Tonic contraction, 97
Touch, 104, 106
Toxins, 57
Tracheae of insect, 85, 200
Tradescantia hair, 166
Transmission of heart beat, 150
Transport of materials, 132
Trigger-action, 97, 105
Trophic influence, 122
Trypsin, 75, 193
Tryptophane, 46
Tubules of kidney, 138
Turgor, 22, 171
Tyndall phenomenon, 176
ULTRA-MICROSCOPE, 4, 32
Unconditioned reflexes, 127
Unsaturated carbon atom, 65
- fats, 90
Urea, 50, 77, 137
- test for, 182
Urease, 77, 182
Ureter, 138
Urethane for frog, 221
Uric acid, 137
Urinary bladder, 99
Utricle and saccule, 118
VACUOLE, 20
Vagus nerve, 79, 149
- of frog, 217
Value of experimental work, vi.
— of general principles, v.
Van der Waals' equation, 22
Vaso-constriction, 220
Vaso-constrictor nerves, 146
Vaso-dilatation, 220
Vaso-dilator nerves, 146
Vaso-motor centres, 146
— reflexes, 146
- stimulation, 224
Vapour pressure, 23, 173
Variation, 152, 157
Veins, 134
Velocity of ions, 25
- of reactions, 68
Vena cava, 134
Venous blood, 87
Ventricle, 132
Visceral nervous system, 131
— sensory nerves, 131
Viscosity, 144
- of blood, 144, 221
Visual purple, 114, 117, 212
VitP.mines, 61
Voluntary contraction, 98
— muscle, structure of, 195, 206
WAR, 159
Warm stage for microscope, 166
Warming sinus, 225
Waste products, 137
Water culture, 183
— effect of, on equilibrium position,
68
- loss of, from body, 84
Water-soluble B -factor, 61
Wheatstone bridge, 175
Whiskers of cat, 106 -
Wireless telegraph, 113
Work, 10
'k Work-collector," 205
X-RAYS, 113
use of, in investigating movements
of intestine, 78
238 SOURCES OF SUPPLIES
SOURCES OF SUPPLIES
Arc Lamp.— G. CUSSONS LTD., The Technical Works, Manchester.
Articulated Bones and Models.— Messrs DEYROLLE, Paris.
Balance.— F. E. BECKER & Co., Nivoc House, Hatton Wall, E.C.i.
L. OERTLING, Turnmill St., E.G.
Castor Oil Seeds.— J AMES CARTER, Seedsmen, High Holborn, W.C.i.
Centrifuges.— CHAS. HEARSON & Co., 235 Regent St., W.i.
Chemical Apparatus.— BAIRD & TATLOCK, 14 Cross St., Hatton Garden, E.C.i,
Chemicals.— HOPKIN & WILLIAMS, 16 Cross St., Hatton Garden, E.C.i.
Drugs.— W. MARTINDALE, 10 New Cavendish St., W.i.
Electrical Apparatus in General.— R. W. PAUL, Electrical Laboratory, Fortis
Green Road, Finchley, N.2
W. G^. PYE & Co., Granta Works, Cambridge.
Galvanometers.— THE INSTRUMENT Co., Cambridge.
Glass Blowers.— JOHN ORME & Co., 148 High Holborn, W.C.i
Histological Preparations.— WATSON & SONS, 313 High Holborn, W.C.i.
Lenses.— PYE & Co., Granta Works, Cambridge.
Microscopes and Fittings.— SWIFT & SON, 81 Tottenham Court Road, W.i.
BAUSCH & LOME, 37 Hatton Garden, E.C.i.
SPENCER Co., Agents— H. F. ANGUS & Co., 83
Wigmore St., W.i.
Microtomes. — THE INSTRUMENT Co., Cambridge.
Osmometer.— C. F. PALMER, 55 Effra Road, Brixton, S.W.2.
Physiological Apparatus (Drums, Myographs, Heart Apparatus, Coils, etc.). —
C. F. PALMER, 55 Effra Road, Brixton, S.W.2.
Plasticine.— HARBUTT, 56 Ludgate Hill, E.C.4.
Plate Glass Shelves.— MAPLE & Co., Tottenham Court Road, W.i.
Platinum.— JOHNSON & MATTHEY, Hatton Garden, E.C.i.
Polarimeter. — ADAM HILGER, LTD., 75A Camden Road, N.W.i.
Skate and Marine Animals. — Marine Biological Laboratory, Citadel Hill,
Plymouth.
Soy Beans. — Messrs SHEARNS, 231 Tottenham Court Road, W.i.
Surgical Instruments.— HAWKSLEY & SON, 357 Oxford St., W.i.
J. WEISS & SON, 287 Oxford St., W.i.
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