The human body; an account of its structure and activities and the conditions of its health working

Front matter

AMERICAN SCIENCE SERIES—ADVANCED COURSE

AN ACCOUNT OF

dqr

Preface To The Ninth (revised) Edition

In making this revision I attempted to preserve, so far as possi¬ ble, the spirit and method of the Author. Numerous changes were necessary, however, as well as the addition of much new material, because of the great advances in Physiological knowledge during recent years.

One important departure was made from the former plan of the book in emphasizing the adaptive character of human activities; a departure which required an entirely different treatment of the Nervous System, as well as minor alterations throughout the book. It involved likewise a rearrangement of the chapters so as to bring together those dealing with Muscles, Nerves, and Sense-organs. The conception of the Body as an Adaptive Mechanism is so help¬ ful to an understanding of its activities, particularly on the part of general students, that I felt justified in introducing it even at the cost of very considerable departures from the original plan.

To make room for the new material without enlarging the vol¬ ume unduly some of the purely anatomical portions were con¬ densed; I hope without impairing their value seriously. Likewise about forty illustrations were omitted, where they could be spared.

For the few new figures introduced, electrotypes were furnished by the publishers of Howell's and Kirke's text-books of Physiology, and of Bailey's and Scymonowicz and MacCallum's text-books of Histology.

E. G. M.

Boston, April, 1910 v

*

N,

Preface To The First Edition

In the following pages I have endeavored to give an account of the structure and activities of the Human Body, which, while in¬ telligible to the general reader, shall be accurate, and sufficiently minute in details to meet the requirements of students who are not making Human Anatomy and Physiology subjects of special ad¬ vanced study. Wherever it seemed to me really profitable, hy¬ gienic topics have also been discussed, though at first glance they may seem less fully treated of than in many School or College Text-books of Physiology. Whoever will take the trouble, how¬ ever, to examine critically what passes for Hygiene in the majority of such cases will, I think, find that, when correct, much of it is platitude or truism: since there is so much that is of importance and interest to be said it seems hardly worth while to occupy space with insisting on the commonplace or obvious.

It is hard to write a book, not designed for specialists, 'without running the risk of being accused of dogmatism, and some readers will, no doubt, be inclined to think that, in several instances, I have treated as established facts matters which are still open to discus¬ sion. General readers and students are, however, only bewildered by the production of an array of observations and arguments on each side of every question, and, in the majority of cases, the chief responsibility under which the author of a text-book lies is to select what seem to him the best supported views, and then to state them simply and concisely: how wise the choice of a side has been in each case can only be determined by the discoveries of the future.

Others will, I am inclined to think, raise the contrary objection that too many disputed matters have been discussed: this was de¬ liberately done as the result of an experience in teaching Physi¬ ology which now extends over more than ten years. It would have been comparatively easy to slip over things still uncertain and subjects as yet uninvestigated, and to represent our knowledge of the workings of the animal body as neatly rounded off at all its contours and complete in all its details— totus, teres, et rotundus.

vii

PREFACE TO THE FIRST EDITION yiii

But by so doing no adequate idea of the present state of physi¬ ological science would have been conveyed; in many directions it is much farther traveled and more completely known than in others; and, as ever, exactly the most interesting points are those which lie on the boundary between what we know and what we hope to know. In Gross Anatomy there are now but few points calling for a suspension of judgment; with respect to Microscopic Anatomy there are more; but a treatise on Physiology which would pass by, unmentioned, all things not known but sought, would convey an utterly unfaithful and untrue idea. Physiology has not finished its course. It is not cut and dried, and ready to be laid aside for reference like a specimen in an Herbarium, but is comparable rather to a living, growing plant, with some stout and useful branches well raised into the light, others but part grown, and many still represented by unfolded buds. To the teacher, more¬ over, no pupil is more discouraging than the one who thinks there is nothing to learn; and the boy who has “finished ” Latin and “ done ” Geometry finds sometimes his counterpart in the lad who has “gone through” Physiology. For this unfortunate state of mind many Text-books are, I believe, much to blame: difficulties are top often ignored, or opening vistas of knowledge resolutely kept out of view: the forbidden regions may be, it is true, too rough for the young student to be guided through, or as yet path¬ less for the pioneers of thought; but the opportunity to arouse the receptive mental attitude apt to be produced by the recognition of the fact that much more still remains to be learned—to excite the exercise of the reasoning faculties upon disputed matters—and, in some of the better minds, to arouse the longing to assist in add¬ ing to knowledge, is an inestimable advantage, not to be lightly thrown aside through the desire to make an elegantly symmetrical book. While I trust, therefore, that this volume contains all the more important facts at present known about the working of our Bodies, I as earnestly hope that it makes plain that very much is yet to be discovered.

A work of the scope of the present volume is, of course, not the proper medium for the publication of novel facts; but, while the “ Human Body,” accordingly, professes to be merely a compilation, the introduction of constant references to authorities would have been out of place. I trust, however, that it will be found through-

PREFACE TO THE FIRST EDITION

IX out imbued with the influence of my beloved master, Michael Foster; and on various hygienic topics I have to acknowledge a special indebtedness to the excellent series entitled Health Primers.

The majority of the anatomical illustrations are from Henle’s Anatomie des Menschen, and a few from Arendt's Schulatlas, the publishers of each furnishing electrotypes. A considerable num¬ ber, mainly histological, are from Quain’s Anatomy , and a few figures are after Bernstein, Carpenter, Frey, Haeckel, Helmholtz, Huxley, McKendrick, and Wundt. About thirty, chiefly diagram¬ matic, were drawn specially for the work.

Quantities are throughout expressed first on the metric system, their approximate equivalents in American weights and measures being added in brackets.

H. Newell Martin.

Baltimore, October, 1880.

Chapter I The General Structure And Composition Of The Human Body

Definitions. Tissues and organs. Histology. Zoological position of man. The vertebrate plan of structure. The mammalia. Chem- —ical composition of the Body.

Chapter II

The properties of the living Body. Physiological properties. Cells. Cell division. Indirect, karyokinetic or mitotic cell division. As¬ similation; reproduction. Contractility. Irritability. Conductiv¬ ity. Spontaneity. Protoplasm. The fundamental physiological properties.

Chapter III

Development. The physiological division of labor. Classification of the tissues. Undifferentiated tissues. Supporting tissues. Nutritive tissues. Storage tissues. Irritable tissues. Conductive tissues. Motor tissues. Protective tissues. Reproductive tissues. Organs. Physiological systems. The relation of man to his environment. Adaptive mechanisms. Maintenance mechanisms.

Chapter IV

Connective tissue. Cartilage. Bone.

Chapter V

Exoskeleton and endoskeleton. The bony skeleton. Peculiarities of the human skeleton. Hygiene of the bony skeleton. Articulations. Joints. Hygiene of tjie joints.

PAGE.

xii

CHAPTER YI

Motion in animals and plants. Amoeboid cells. Ciliated cells. The muscles. Histology of striated muscle. Structure of unstriated muscular tissue. Cardiac muscular tissue. The chemistry of mus¬ cular tissue. Beef-tea. Rigor mortis.

PAGE

Chapter XI Structure, Nerve Connections, And Functions Of The Cerebrum

Cerebrum dependent on the receptor system. Tracing nerve paths.

Paths of the various senses. Structure of the cerebrum. Cortical localization. Sensory and motor areas. Cortical reflexes. Memory.

xiii

PAGE

Association. Volition. Inhibition. Habit formation. Language. Consciousness. Emotions. Cerebral functions compared in man and animals. Nourishment of the brain.142

Chapter XII

THE CEREBELLUM. THE MEDULLA AND MIDBRAIN. THE SYMPA¬ THETIC SYSTEM

The cerebellum. The medulla and midbrain. The sympathetic system.

Relation of the sympathetic system to emotional states . . . . 161

Chapter XIII

THE RECEPTOR SYSTEM. INTERNAL AND CUTANEOUS SENSATIONS

The receptor system. Differences between sensations. Psychophysical law. Classification of receptors. Internal senses. Muscle sense. Hunger and thirst. Fatigue. Cutaneous senses. Pain. Touch. Temperature sense. Peripheral reference of sensations. Percep¬ tions. Sensory illusions. 169

Chapter XIV

THE EAR. HEARING AND EQUILIBRATION. TASTE AND SMELL

The external ear. The middle ear. Auditory ossicles. Internal ear.

Bony labyrinth. Membranous labyrinth. Organ of Corti. Loud¬ ness, pitch, and timbre of sounds. Sympathetic resonance. Func¬ tions of tympanic membrane. Functions of auditory ossicles. Function of the cochlea. Auditory perceptions. Nerve endings in semicircular canals and vestibule. Equilibrium sense. Smell.

Taste.187

Chapter XV

The essential structure of an eye. The appendages of the eye. The lach¬ rymal apparatus. The muscles of the eye. Anatomy of the eyeball.

Optic nerves, chiasma, and tracts. The retina. Refracting media of the eye. The ciliary muscle. Properties of light. Refraction of light. Accommodation. Short sight and long sight. Optical de¬ fects of the eye. Hygiene of the eyes.207

Chapter XVI

The excitation of the visual apparatus. Intensity of visual sensations. Function of the rods. Visual purple. Duration of luminous sensa-

XIV

PAGB

tions. Localizing power of retina. Color vision. Function of the cones. Distribution of color sense over the retina. Color blindness. After-images. Contrasts. Theories of color vision. Visual percep¬ tions. Single vision with two eyes. Perception of solidity . . . 231

Chapter XVII The Structure And Functions Of Blood And Lymph

The external medium. The internal medium. The blood. The lymph. Filtration, osmosis, and dialysis. Renewal of the lymph. Lym¬ phatic vessels. Lacteals. Composition of blood. Red corpuscles. Hemoglobin. Leucocytes. Blood-plates. Blood-plasma. His¬ tology and chemistry of lymph.254

Chapter XVIII

THE HORMONE-CARRYING AND DISEASE-RESISTING FUNCTIONS OF THE BLOOD. BLOOD-CLOTTING

Hormones. Ductless glands. Thyroid. Parathyroids. Thymus. Pi¬ tuitary. Adrenals. Infection. Resistance to infection. Recovery from infection. Opsonins, immune bodies, and agglutinins. An¬ titoxin. Immunity. Inoculation. Coagulation of blood. Blood transfusion.270

Chapter XIX The Anatomy Of The Heart And Blood-vessels

General statement. Position of heart. Membranes of heart. Anatomy of heart. Valves of heart. The arterial system. Aorta and its branches. The capillaries. The veins. The pulmonary circulation.

The portal circulation. Arterial and venous blood. Structure of the arteries, of the capillaries, and of the veins.288

Chapter XX

THE ACTION OF THE HEART. THE REGULATION OF THE HEART-BEAT

The beat of the heart. Cardiac impulse. A cardiac cycle. Use of papil¬ lary muscles. Sounds of the heart. Action of the heart valves. Function of the auricles. Work done by the heart. Relation of nerve and muscle elements within heart. Peculiarities of heart. Passage of beat over heart. Neurogenic and myogenic theories of heart-beat. Nature of automatic rhythmicity. Extrinsic nerves of heart. Inhibitory and augmentor centers.304 xv

Chapter XXI

THE CIRCULATION OF THE BLOOD. BLOOD PRESSURE AND BLOOD VELOCITY. THE PULSE

Circulation seen in frog’s web. Resistance to blood-flow. Conversion of intermittent into continuous flow. Arterial pressure. Weber’s schema. The pulse. Blood-pressure in man. The rate of the blood-flow. Secondary factors affecting the circulation. Aspiration of the thorax. Proofs of the circulation of the blood.. 318

Chapter XXII

THE VASOMOTOR MECHANISM. SLEEP. THE LYMPHATIC SYSTEM

Distribution of blood in different parts of the body. Nerves of the blood¬ vessels. The vasoconstrictor nerves. The vasoconstrictor center. Depressor nerve. Blushing. Taking cold. Adrenalin. Vasodi¬ lator nerves. The vasodilator center. Relation of vasomotor tone to cerebral activity. Sleep. Lymphatics. Thoracic duct. Lymph nodes. Movement of lymph.336

Chapter XXIII

RESPIRATION. THE MECHANISM OF BREATHING. THE REGULATION ~

OF BREATHING

Definitions. Respiratory organs. Air-passages and lungs. Trachea and bronchi. Structure of lungs. Pleura. Respiratory move¬ ments. Anatomy of thorax. Expiration. Forced respiration. Respiratory sounds. Capacity of lungs. Hygiene of respiration. Aspiration of thorax. Influence of respiratory movements upon the blood circulation. Influence of respiratory movements on lymph-flow. Respiratory center. Excitation of respiratory center. Eupnea, Dyspnea, Apnea. Asphyxia. Artificial respiration. Modified respiratory movements.348

Chapter XXIV

Nature of the problems. Changes produced in air by being once breathed. Ventilation. Changes undergone by the blood in the lungs. The blood gases. The laws governing the absorption of gases by a liquid. The absorption of oxygen by the blood. The oxygen inter¬ changes in the blood. The carbon dioxid of the blood. Hormone action of carbon dioxid. Tissue respiration ........ 370

Chapter I The General Structure And Composition Of The Human Body

Definitions. The living Human Body may be considered from either of two aspects. Its structure may be especially examined, and the forms, connections and mode of growth of its parts be studied, as also the resemblances or differences in such respects which appear when it is compared with other animal bodies. Or the living Body may be more especially studied as an organism presenting definite properties and performing certain actions; and then its parts will be investigated with a view to discovering what duty, if any, each fulfils. The former group of studies constitutes the science of Anatomy, and in so far as it deals with the Human Body alone, of Human Anatomy; while the latter, the science con¬ cerned with the uses—or in technical language the functions —of each part is known as Physiology. Closely connected with physi¬ ology is the science of Hygiene , which is concerned with the con¬ ditions which are favorable to the healthy action of the various parts of the Body; while the activities and structure of the diseased body form the subject-matter of the science of Pathology.

Tissues and Organs. Histology. Examined merely from the outside our Bodies present a considerable complexity of structure. We easily recognize distinct parts as head, neck, trunk and limbs; and in these again smaller constituent parts, as eyes, nose, ears, mouth; arm, forearm, hand; thigh, leg and foot. We can, with such an external examination, go even farther and recognize dif¬ ferent materials as entering into the formation of the larger parts. Skin, hair, nails and teeth are obviously different substances; simple examination by pressure proves that internally there are harder and softer solid parts; while the blood that flows from a cut finger shows that liquid constituents also exist in the Body. The conception of complexity which may be thus arrived at from ex¬ ternal observation of the living, is greatly extended by dissection of the dead Body, which makes manifest that it consists of a great number of diverse parts or organs, which in turn are built up of a limited number of materials; the same material often entering into the composition of many different organs. These primary build¬ ing materials are known as the tissues, and that branch of anatomy which deals with the characters of the tissues and their arrange¬ ment in various organs is known as Histology; or, since it is mainly carried on with the aid of the microscope, as Microscopic Anatomy. If, with the poet, we compare the Body to a house, we may go on to liken the tissues to the bricks, stone, mortar, wood, iron, glass and so on, used in building; and then walls and floors, stairs and win¬ dows, formed by the combination of these, would answer to ana¬ tomical organs.

Zoological Position of Man. External examination of the Hu¬ man Body shows also that it presents certain resemblances to the bodies of many other animals: head and neck, trunk and limbs, and various minor parts entering into them, are not at all peculiar to it. Closer study and the investigation of internal structure demonstrates further that these resemblances are in many cases not superficial only, but that our Bodies may be regarded as built upon a plan common to them and the bodies of many other creatures: and it soon becomes further apparent that this resemblance is greater between the Human Body and the bodies of ordinary four-footed beasts, than between it and the bodies of birds, reptiles or fishes. \Hence, from a zoological point of view, man’s Body marks him out as belonging to the group of Mammalia (see Zoology), which includes all animals in which the female suckles the young; and among mammals the anatomical resemblances are closer and the differences less between man and certain apes than between man and the other mammals; so that zoologists still, with Lin¬ naeus, include man with the monkeys and apes-in one subdivision of the Mammalia, known as the Primates. That civilized man is mentally far superior to any other animal is no valid objection to such a classification, for zoological groups are defined by ana¬ tomical and not by physiological characters; and mental traits, since we know that their manifestation depends upon the struc¬ tural integrity of certain organs, are especially phenomena of function and therefore not available for purposes of zoological arrangement.

As man walks erect with head upward, while the great majority of Mammals go on all fours with the head forward and the back upward, and various apes adopt intermediate positions, confusion is apt to arise in considering corresponding parts in man and other animals unless a precise meaning be given to such terms as “ an¬ terior ” and “posterior.” Anatomists, therefore, give those words definite arbitrary significations. The head end is always anterior whatever the natural position of the animal, and the opposite end posterior; the belly side is spoken of as ventral, and the opposite side as dorsal; right and left of course present no difficulty: the terms cephalic and caudal as equivalent, respectively, to anterior and posterior, are sometimes used. Moreover, that end of a limb nearer the trunk is spoken of as proximal with reference to the other or distal end. The words upper and lower may be con¬ veniently used for the relative position of parts in the natural standing position of the animal.

The Vertebrate Plan of Structure. Neglecting such merely apparent differences as arise from the differences of normal posture above pointed out, we find that man’s own zoological class, the Mammals, differs very widely in its broad structural plan from the groups including sea-anemones, insects or oysters, but agrees in many points with the groups of fishes, amphibians, reptiles and birds. These four are therefore placed with man and all other Mammals in one great division of the animal kingdom known as the Vertebrata. The main anatomical character of all vertebrate animals is the presence in the trunk of the body of two cavities, a dorsal and a ventral, separated by a solid partition; in the adults of nearly all vertebrate animals, a hard axis, the vertebral column (i backbone or spine), develops in this partition and forms a central support for the rest of the Body (Fig. 2, ee) . The dorsal cavity is continued through the neck, when there is one, into the head, and there widens out. Within it are inclosed the chief organs of the nervous system. The bony axis is also continued through the neck and extends into the head in a modified form. The ventral cavity, on the other hand, is confined to the trunk. It contains the main organs connected with the blood-flow together with those of digestion and respiration.

Upon the ventral side of the head is the mouth-opening leading into a tube, the alimentary canal, /, which passes back through the neck and trunk and opens again on the outside at the posterior part of the latter. In its passage through the trunk-region this canal lies in the ventral cavity.

The Mammalia. In many vertebrate animals the ventral cavity

Fig. 1 . —The Body opened from the front to show the contents of i^s ventral cavity, lu, lungs; h, heart, partly covered by other things; le, le', right and left liver-lobes respectively; via, stomach; ne, the great, omentum, a membrane con¬ taining fat which hangs down from the posterior border of the stomach and covers the intestines.

is not subdivided, but in the Mammalia it is; a membranous trans¬ verse partition, the diaphragm (Fig. 1, 2 ), separating it into an anterior chest or thoracic cavity , and a posterior, or abdominal cavity. The alimentary canal and whatever else passes from one of these cavities to the other must there¬ fore perforate the diaphragm.

In the chest, besides part of the ali¬ mentary canal, lie important organs, the heart, h, and lungs, lu; the heart being on the ventral side of the alimentary canal. The abdominal cavity is mainly occupied by the alimentary canal and organs connected with it and concerned in the digestion of food, as the stomach , ma, the liver, le, the pancreas, and the in¬ testines. Among the other more promi¬ nent organs in it are the kidneys and the spleen.

In the dorsal or neural cavity lie the brain and spinal cord, the former occu¬ pying its anterior enlargement in the head. Brain and spinal cord together form the cerebrospinal nervous center commonly called the central nervous system; in addition to this there are found in the ventral cavity a number of small nerve-centers united to each other and to the cerebrospinal center by connecting cords, and with their off-

Fig. 2.—Diagrammatic longi¬ tudinal section of the Body, a, the neural tube, with its upper . , 7 . enlargement in the skull-cavity

shoots forming the sympathetic nervous. a t a'; N, the spinal cord; N',

, the brain; ee, vertebrae form-

system. ing the solid partition between

The walls of the three main cavities the dorsal and ventral cayi- . ties; o, the pleural, and c, the

are lined by smooth, moist serous mem¬ branes. That lining the dorsal cavity is

the arachnoid; that lining the chest the

pleura; that lining the abdomen

peritoneum; the abdominal cavity is in

consequence often called the peritoneal

cavity. Externally the walls of these f^J^TA'rough ^

cavities are covered by the skin, which abdominal cavity to the pos- . „ , , i terior opening of the alimen-

consists of two layers.' an outer horny tary canal.

abdominal division of the ven¬ tral cavity, separated from one another by the diaphragm, d; i, the nasal, and o, the mouth chamber, opening behind into the the pharynx, from which one tube leads to the lungs, l, and another to the stomach, f; h, the heart; k, a kidney; s, the sympathetic nervous chain.

layer called the epidermis, which is constantly being shed on the surface and renewed from below; and a deeper layer, called the dermis and containing blood, which the epidermis does not. Be¬ tween the skin and the lining serous membranes are bones, muscles (the lean of meat), and a great number of other structures which we shall have to consider hereafter. All cavities inside the Body, as the alimentary canal and the air-passages, which open directly or indirectly on the surface are lined by soft and moist prolonga¬ tions of the skin known as mucous membranes. In these two layers are found as in the skin, but the superficial bloodless one is called epithelium and the deeper vascular one corium.

Diagrammatically we may represent the Human Body in lon¬ gitudinal section as in Fig. 2, where aa' is the dorsal or neural cavity, and b and c, respectively, the thoracic and abdominal sub¬ divisions of the ventral cavity; d represents the diaphragm separat¬ ing them; ee is the vertebral column with its modified prolongation into the head beneath the anterior enlargement of the dorsal cavity; / is the alimentary canal opening in front through the nose, i, and mouth, o; h is the heart, l, a lung, s the sympathetic nervous system, and k a kidney.

A transverse section through the chest is represented by the diagram Fig. 3, where x is the neural canal containing the spinal

cord. In the thoracic cavity are seen the heart, h, the lungs, ll, part of the alimentary canal, a, and the sympathetic nerve-centers, sy; the dotted line on each side covering the inside of the chest-wall and the outside of the lung represents the pleura.

Sections through correspond¬ ing parts of any other Mammal would agree in all essential points with those represented in Figs. 2 and 3.

The Limbs. The limbs present no such arrangement of cavities on each side of a bony axis as is seen in the trunk. They have an axis formed at different parts of one or more bones (as seen at U

Fig. 3.—A diagrammatic section across the Body in the chest region. x, the dorsal tube, which contains the spinal cord; the black mass surround¬ ing it is a vertebra; a, the gullet, a part of the alimentary canal; h, the heart; sy, sympathetic nervous system; ll, lungs; the dotted lines around them are the pleurae; rr, ribs; st, the breast¬ bone.

and R in Fig. 4, which represents a cross-section of the forearm near the elbow-joint), but around this are closely-packed soft parts, chiefly muscles, and the whole is enveloped in skin. The only cavities in the limbs are branching tubes which are filled with liquids during life, either blood or a watery-looking fluid known as lymph. These tubes, the blood and lymph-vessels respectively, are not, however, characteristic of the limbs, for they are present in abundance in the dorsal and ventral cavities and in their walls.

Chemical Composition of the Body. In addition to the study of the Body as com¬ posed of tissues and organs which are opticaily recognizable, we may consider it as the forearm a short distance composed of a number of different chemi- JSdV'its Iwo^uppoVting

cal substances. This branch of knowl- bon , es > the radius and ulna;

....... . e, the epidermis, and d, the

edge, which IS still very incomplete, really dermis of the skin; the latter

presents two classes of problems. On the LnTs n o?ToMec b tiTO W tisOTt

one hand, we may limit ourselves to the which penetrate between ; J . and invest the muscles,

examination of the chemical substances which are indicated by num- ... . , . . i • i /» bers; n, n, nerves and vessels.

which exist m or may be derived from

the dead Body, or, if such a thing were possible, from the living Body entirely at rest; such a study is essentially one of structure and may be called Chemical Anatomy * But as long as the Body is alive it is the seat of constant chemical transformations in its material, and these are inseparably connected with its functions, the great majority of which are in the long run dependent upon chemical changes. From this point of view, then, the chemical study of the Body presents physiological problems, and might be called Chemical Physiology. At present it is customary to include under the term Biological Chemistry the study of the chemical structure of living matter and of the chemical changes occurring in it. At this point we may confine ourselves to the more im¬ portant substances derived from or known to exist in the Body leaving questions concerning the chemical changes taking place within it for consideration along with those functions which are performed in connection with them.

Elements Composing the Body. Of the elements known to chemists only seventeen have been found to take part in the formation of the Human Body. These are carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, chlorin, fluorin, iodin, silicon, sodium, potassium, lithium, calcium, magnesium, iron, and manganese. Copper and lead have sometimes been found in small quantities, but are probably accidental and occasional.

Uncombined Elements. Only a very small number of the above elements exist in the Body uncombined. Oxygen is found in small quantity dissolved in the blood; but even there most of it is in a state of loose chemical combination. It is also found in the cavities of the lungs and alimentary canal, being derived from the inspired air or swallowed with food and saliva; but while con¬ tained in these spaces it can hardly be said to form a part of the Body. Nitrogen also exists uncombined in the lungs and alimen¬ tary canal, and in small quantity in solution in the blood. Free hydrogen has also been found in the alimentary canal, being there evolved by the fermentation of certain foods.

Chemical Compounds. The number of these which may be obtained from the Body is very great; but with regard to very many of them we do not know that the form in which we extract them is really that in which the elements they contain were united while in the living Body; since the methods of chemical analysis are such as always break down the more complex forms of living matter and leave us only its debris for examination. We know in fact, tolerably accurately, what compounds enter the Body as food and what finally leave it as waste; but the intermediate con¬ ditions of the elements contained in these compounds during their sojourn inside the Body we know very little about; more especially their state of combination during that part of their stay when they do not exist dissolved in the bodily liquids, but form part of a solid living tissue.

For present purposes the chemical compounds existing in or de¬ rived from the Body may be classified as organic and inorganic, and the former be subdivided into those which contain nitrogen and those which do not.

Nitrogenous Organic Compounds. These fall into several main groups: proteins *—subdivided into simple proteins, conjugated pro -

* The classification of proteins here given is that recommended by the joint committee on protein nomenclature of the American Physiological Society and the American Society of Biological Chemists, 1907.

teins, and derived 'proteins—nitrogenous extractives, and pigments. The interesting substances known as enzyms probably form like¬ wise a group under this head.

Simple Proteins. Under this head are grouped those proteins whose molecules contain only protein material; in contradistinc¬ tion to the conjugated proteins whose molecules contain protein in combination with a non-protein substance.