Human physiology

Front matter

Med

ADVANCED SCIENCE MANUALS.

Written specially to meet the requirements of the Advanced Stage of Science Subjects as laid dawn in the Syllabus of the Directory of the Science and Art Department.

Preface

This work has been mainly prepared for lay students preparing for the Second or Advanced Stage Examination of the Science and Art Department. Such students are supposed to have acquired some preliminary knowledge of the subject, and I have endeavoured in the following pages to furnish them with an opportunity of deepening and extending that knowledge. I have not tried to produce a superficial cram book, but have endeavoured to furnish precise and accurate information on such parts of histology and anatomy as are required, as well as to give a reasoned account of the physiological processes of the human body.

Attendance at a course of lectures and demonstrations given some years ago by Dr. A. Gamgee, F.R.S., first gave me a real interest in, and insight into, the subject, while continued study and experience in teaching have only served to deepen that interest and insight. In writing the book the chief authorities have been consulted. In Anatomy I have made use of Gray, Quain (edited by Schafer and Thane), and Testut. In Physiology I have consulted Beaunis, Foster, Halliburton, Hermann, Langlois and Varigny, Landois and Stirling, and Waller. To the student who wishes to continue his study I would strongly recommend Dr. Waller’s admirable work as his vi Human Physiology

next book, to be followed by Professor Foster’s Text-book or that of Landois and Stirling. Care should be taken to obtain the latest edition of these standard works.

Most of the illustrations have been derived from works published by Messrs. Longmans & Co., and for permission to use these I am very grateful. A large number has been employed, partly to supplement the text, partly to illustrate objects not easily obtained, and partly to take the place of detailed description. The student is strongly advised to give considerable attention to the figures and their description, as his progress will thereby be greatly facilitated.

A set of progressive questions has been added to the book, not merely to let the student see the nature of the examination he may have to undergo, but with a view of calling attention in another way to certain important points connected with his study.

I am indebted to my brother, G. Thornton, M.D., for reading over the proof sheets, and for some valuable suggestions.

JOHN THORNTON.

Bolton : December 1893.

Contents

CHAPTER

I. HISTOLOGY, OR THE MICROSCOPICAL STRUC- TURAL ELEMENTS OF THE BODY .

II. MUSCULAR AND NERVOUS TISSUE . . . .

Chapter I Histology , Or The Microscopical Structural Elements Of The Body

i. The Microscopic Elements of the Body. — A simple examination and dissection of the human body, or that of any other higher animal, soon shows that it is made up of easily separated portions having distinct forms, such as the heart, lungs, stomach, brain, &c. Such parts are called organs, and each organ is found to have its special work or function in the animal economy. A department of study called descriptive anatomy furnishes an account of the several organs, members, and regions of the body, pointing out their form, structure, and mutual connections. Much of this knowledge may be obtained from the dead body, and the student is supposed to have already acquired by this means and from the earlier chapters of an elementary work on animal physiology a rudimentary acquaintance with the build of the human body and with the position, form, and function of its chief organs. Dissection further teaches that the body of any of the higher animals is made up of different kinds of materials, such as muscle, fat, cartilage, nervous matter, &c. These various constituent materials are spoken of as textures or tissues , and while some of the organs are formed mainly of one elementary texture or tissue, other organs or parts of the body are compounded of several tissues The portion of anatomy that treats of the minute structure of the tissues is called Histology (Gk. hist os, a web : logos a discourse). Histology requires the use of the microscope, and by its aid wc quickly learn that the separable structural

B elements of the various tissues are for the most part either minute corpuscles named ‘cells’ or elongated threads named ‘fibres.’ But since many of the fibres, if not all, are either modified cells or are derived directly or indirectly from cells, and since all the tissues contain cellular elements and originate as collections of cells, the cell may be regarded as the histological unit of the body. To the minute anatomy of the tissues we shall devote our first chapter, as a sound knowledge of this is essential to the proper understanding of much that follows. Human physiology, in fact, may, from one point, be regarded as the knowledge of the structure, properties, and functions of cells and their derivatives.

As we shall presently see, the cells of the body differ widely in form, appearance, and structure in the different tissues entering into the composition of the organs, while corresponding to these differences of structure and constitution will be found differences of function or properties. In other words, morphological differentiation (Gk. morphe , form) is accompanied by physiological differentiation. Thus the cells of muscle, or motor cells, possess the power of contraction, and do the mechanical work of the body ; the cells of the alimentary canal either secrete fluids from the blood and pour it out on the food to digest it and render it soluble, or they are specially adapted for absorbing into the blood-stream the food so prepared. The liver cells, among other functions, further elaborate the food stuffs absorbed into the blood so that they may serve the more easily as nutriment for the cells of the various tissues. For every cell needs to assimilate food in order to live, and every cell must be supplied with oxygen for the oxidation of the matter by which its energy is produced. The cells must also be able to get rid of the waste products of their activity. The lungs are the organs of the body where a thin layer of cells in the cavities allows the passage of oxygen into the blood, to be carried to all the tissues, and where also a discharge of carbon dioxide, C02, one of the products of cell activity, takes place ; while the cells of the kidneys perform the special duty of freeing the blood from the nitrogenous waste passed into it. There is, therefore, a physiological division of labour in the

.rru human body, and the parts work together harmoniously because the organs and tissues are co-ordinated and regulated by a complex system of cells and fibres known as the Nervous System.

Although the modern conception of a cell differs from that which prevailed some few years ago, the doctrine that the bodies of all animals and plants consist either of a single cell or of a number of cells and their products, and that all cells proceed from pre-existing cells ( omnis cellula e cellula ), is now regarded as the basis of biological science. It is often spoken of as the cell theory , and may be regarded as ‘ the greatest discovery in the natural sciences in modern times.’ To the study of the animal cell we now therefore proceed.

2. The Animal Cell. — An animal cell is a minute corpuscle of living substance, or protoplasm, those of the human body seldom exceeding T|’)0th of an inch in diameter, while many are but one-tenth this size. It consists of two distinct parts, the main substance of the cell, called protoplasm , and a tiny body embedded in the protoplasm, called the nucleus. Seen under a low power of the microscope, the protoplasm appears either to be a homogeneous substance or to contain granular matter and vacuoles (globular cavities containing a watery fluid) ; but viewed with very high powers, part of die protoplasm is seen to be fibrillated and to form a fine network.

1 his network -is known as the reticulum or spongioplasm , and the rest of the protoplasm occupying the meshes of the network is a clear substance termed hyaloplasm. The arrangement of the network and the size of the meshes vary in different cells the nodes or junctions of the fibrils probably giving rise to some of the granular appearances before mentioned. It must, however, be remembered that the protoplasm often includes actual granules of nutritive material or of matter stored up for. nutrition

Fig. i.— Diagram ofa Cell, [lie Protoplasm of which is composed of Spongioplasm and Hyaloplasm Highly magnified.

/, protoplasm; «, nucleus; nucleolus.

and that the vacuoles may contain glycogen or other substances in solution.

The external layer of protoplasm in some cells becomes altered so as to form a cell-membrane or cell-wall, but this is more frequently found in vegetable than in animal cells.

Within the protoplasm, and generally near the middle of the cell, is the minute body of somewhat firmer consistence called the nucleus. It is probably a portion of the cell-protoplasm somewhat altered in chemical nature, and its chief office is to preside over the nutritive activity and reproduction 'or division of the cell. It is believed by many biologists that the nucleus is also the conveyer of hereditary properties. The p1G 2_ — A dividing Cell. Formation of Chromatin Network in Daughter-nuclei. (Rabl. The upper daughter-nucleus is at an earlier stage than the lower one. The cell-protoplasm is now completely divided.

nucleus is spherical or elongated in shape, often bounded by a membrane, and consists of a clear substance known as the nuclear matrix or nucleoplasm , pervaded by a network of fibres differing in nature from those of the cell-substance. The intranuclear network may show enlargements called nucleoli (fig. i). Nuclei are often recognised by treating a cell with acetic acid, which destroys the cell-protoplasm and thus renders the nucleus more visible. The intranuclear network stains with certain dyes, and is hence called chromoplasm (Gk. chroma , colour), while the matrix or nucleoplasm is achromatic. Histologists distinguish between a resting or non-dividing nucleus clear areaj cirawicL nucleus and a dividing nucleus. When cells are about to divide, the nucleus undergoes certain definite changes, the protoplasm outside the nucleus taking part in the process by means of the so-called attraction spheres. These consist of a wheel-like arrangement of fine fibrils starting from a central particle called the attraction particle , and the twin spheres are connected by the delicate system of fibrils denominated an achromatic spindle. The fibrillar network of the nucleus becomes arranged into definite patterns (skeins, stars, rosettes, &c.), finally separating into two groups of chromatic fibres and giving origin to daughter-nuclei, around which the protoplasm of the cell ultimately arranges itself and divides to form two separate cells. This indirect process of cell-division is spoken of as karyokinesis (Gk. karuon, kernel, nucleus; and kinesis, movement).

A cell may therefore be described as a minute mass of protoplasm containing a specially formed constituent known as the nucleus, the protoplasm consisting of two substances, a network called spongioplasm and a clear fluid called hyaloplasm, and the nucleus being formed of a fluid called nucleoplasm, and a system of fibres called chromoplasm. Protoplasm is a living substance, and forms the ‘ physical basis of life.’

3. Composition and Properties of Protoplasm. — Protoplasm is a semi-fluid transparent substance that swells up in but does not mix with water. It usually contains granular matter and vacuoles ; but these are not essential to its nature. It consists of 80 to 85 per cent, of water with 15 to 20 per cent, of solids, chiefly proteids, the chief proteid being plastem Proteids are substances containing carbon, hydrogen, nitrogen, oxygen, and sulphur; and they are distinguished by certain

Fig. 3. — An Amoeboid Pale Corpuscle of the Newt, showing a Double Nucleus with Reticulum of Chromoplasm, and the Protoplasm showing Spongioplasm and Hyaloplasm.

reactions (see Appendix). Carbohydrates like glycogen, as well as minute quantities of inorganic substances like calcium, are also present. The vital properties of protoplasm may be studied either in the amoeba, a microscopic unicellular animal found in stagnant water and damp earth, or in the white blood-corpuscle, which is an example of a cell retaining its simple primitive form in an adult higher animal. The chief of these physiological characters are : —

(i.) Power of Movement. — On examining an amoeba with a high power of the microscope, it is seen to consist of an irregular mass of protoplasm with a nucleus (or nuclei), the protoplasm usually containing granular matter and vacuoles. Its shape soon changes ; processes called pseudopodia are extended and retracted ; at times the whole mass seems to follow a process and the animal changes its position. Colourless blood-corpuscles may be noticed to exhibit similar movements, which are hence termed amoeboid.

(2.) Power of Kesponse to Excitation, or Irritability. — Besides the apparent spontaneous movements just described, protoplasm can be excited by external agencies called stimuli. Such stimuli are heat,

Fig. 4. — Human Colourless Blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.)

electricity, foreign particles, &c. , and the protoplasm exhibits its excitability by movement or contraction of its mass when a suitable stimulus is applied.

(3.) Power of Assimilation and Growth. — The amoeba and other similar protoplasmic cells possess the power of taking in material and using it as food. Such matter becomes modified by chemical changes in the protoplasm of the cell, so that part of it becomes converted into protoplasm itself, while the part that is not assimilated is rejected. The power of digestion and nutrition possessed by protoplasm is accompanied with a power of respiration. Oxygen is absorbed and drawn into chemical combination, but it is afterwards exhaled, for the most part united with carbon as carbon dioxide.

(4.) Power of Secretion. — Protoplasmic cells can elaborate new substances from the food supplied, some of which are stored for future use and some of which are excreted. In higher animals special cells in organs called secreting glands are set apart to prepare material known as enzymes or ferments. Living protoplasmic cells are thus the seat of continuous chemical changes, the cell either building up its own substance from the food supplied, or forming materials like giycogen, fat, ferments, &c., within its substance ; while, on the other hand, these constructive processes are always accompanied by destructive changes, of which oxidation forms the chief. The whole series of chemical changes within a cell, or within a body composed of cells, beginning with assimilation and ending with excretion, is termed metabolism. The process of building up living material, or constructive metabolism, is called anabolism ; the breaking down of material into simple products in the protoplasm, or destructive metabolism, is called katabolism.

(5.) Power of Reproduction. — When an amoeba, by the growth of its protoplasm, has reached a certain size, its power of reproduction is manifested by fission, a simple division of the cells preceded by a division of the nucleus taking place. Two independent amoebae are thus produced, each of which may afterwards undergo similar direct division. A more complex process, called karyokinesis or indirect division, is more common both in vegetable and animal cells. This has already been referred to.

The body of man, as well as that of every higher animal, is developed from a single cell, termed the ovum , which after fertilisation first divides into two, each of which again divides. By further division, eight, sixteen, thirty-two, and so on, cells are formed.

After cell-multiplication has proceeded for some time, the cells become arranged in three layers ; the outermost layer is termed the epiblast , the innermost the hypoblast, and the middle layer the vie so ■ blast. From these three layers all the tissuesand organs of the adult are formed.

From the epiblast are formed the epidermis and nervous system ; the hypoblast will form the digestive and absorbent membranes of the digestive and respira- « tory systems ; the mesoblast will give origin to the blood-corpuscles, the connective tissues, the bones and muscles of the body. In this process of development from the three primary embryonic layers there is not merely cell -multiplication, but the cells become modified and metamorphosed from their primitive condition. Some become thin and flat, and cohering at their edges form the blood-vessels ; others become elongated and threadlike to form the fibres of muscular tissue ; others become separated by intercellular substance derived from the cells themselves, and in this intercellular substance fibres such as are found in connective tissue may arise, or calcareous matter be deposited, as in bone. To the study of the varieties of tissue thus produced in the adult we now proceed.

Fig. 5. - Ovum. Magnified 350 limes.

vitelline membrane, zona fiellllcida or zona radiuta. b, external border of the yolk and internal border of the vitelline membrane, c, germinal vesicle and germinal spot.

4. Classification of Tissues. — Having described the structure and properties of the protoplasmic units called animal cells, we now proceed to study the different kinds of material, or tissues, which are formed of or arise from these cells. The number of distinct tissues or textures forming the various organs of the human body has been reduced to five : (x) epithelial or surface-limiting tissues ; (2) connective or supporting tissues ; (3) muscular or contractile tissues ; (4) nervous or sensory tissues; (5) blood and lymph particles, or nutritive tissues. Each of these classes, as will be seen, may be subdivided ; but the various kinds are not always sharply distinguished, forms of transition between them being often recognisable.

5. Epithelial Tissues. — Epithelium is a tissue covering the external and internal free surfaces of the body, and composed of cells placed side by side, with a small amount of cementing intercellular substance. It thus forms — (1) the outer surface of the skin, where it is known as epidermis ; (2) the covering of mucous membranes, i.e. those membranes that line the passages and cavities of the body that communicate with the exterior, including the ducts and tubes of glands opening into these cavities ; (3) the terminal parts of the organs of special sense ;

(4) the inner surface of serous membranes, i.e. the membranes forming what appear closed sacs in the thorax and abdomen ;

(5) the inner surface of the heart, blood-vessels, and lymphatics ; 1 (6) the inner lining of the ventricles of the brain and the central canal of the spinal cord.

Epithelial cells consist of protoplasm and a nucleus, and when multiplying undergo division by karyokinesis in the way already described. Having different functions, as protective, secreting, & c., and being exposed to diverse conditions, they present varieties of structure and form. As regards arrangement, epithelial cells may be stratified , forming several superposed layers ; or simple, forming a tissue of one layer. Where there are but two or three layers fitting into one another the term transitional may be used. As regards shape, epithelial cells are classified as squamous or flat, colutnnar or cylindrical, and cubical. No blood-vessels pass into epithelial tissue, the cells deriving their nourishment by imbibition of the plasma exuded into the subjacent tissue, but in many parts nerve fibrils, as fine filaments, exist among the epithelial cells.

6. Stratified Epithelium. — This epithelium forms the external surface of the body, or epidermis, and is also found lining the cavity of the mouth, pharynx, oesophagus, and the anterior surface of the cornea.

The epidermis consists of (a) the superficial horny layer of flattened cells ; {/>) the stratum lucidum, formed of dense horny scales, showing traces

1 The term endothelium is applied by some authors to the single layer of flattened cells which line the internal free surface of the serous sac?, blood-vessels, and lymphatics (4 and 5 above).

of a nucleus ; (r) the re/e mucosum or Malpighian layer, consisting of several layers of nucleated cells, the deeper ones of which become columnar as they rest on the underlying corium or dermis. It is in the deepest part of a stratified epithelium that new cells are formed by cell-division. Cells are pushed outwards from below, and as they near the surface they not only become compressed and changed in shape, but they undergo a change in chemical composition, their protoplasm becoming converted into a horny substance, keratin , the nucleus also being often involved. Before this conversion occurs there is usually a deposit of granular material within

M

Fig. 6.— Section of Epidermis from the Human Hand. Highly magnified.

//, horny layer, consisting of s, superficial horny scales ; sw, swollen-out horny cells ; s.l, stratum lucidum ; M , rete mucosum or Malpighian layer, consistmg ot, p, prickle-cells, several rows deep, and c, elongated cells forming a single stratum near the corium; «, part of a plexus of nerve-fibres in the superficial ayer of the cutis vera. From this plexus fine varicose nervehbrils may be traced passing up between the cells of the Malpighian layer.

the cells, and the upper layer of cells of the rete mucosum has been termed the stratum granulosum. In the deeper layers of a stratified epithelium

CU aces of , cells are not close together, but a radiating system of fibrils connects the protoplasm of adjacent cells, and between these lie very fine channels. The exact purpose of these intercellular bridges and canals is not fully understood. &

When such cells, known as prickle-cells , are isolated, the broken fibrils give them a spiked or dentated appearance. In coloured races the colour is due to pigment-granules, contained chiefly in the cells of the deeper

layer of the rete mucosum. Pigment epithelial cells also occur on the internal surface of the choroid coat and iris of the eyeball. The older superficial cells of the epidermis are continually being removed by wasting and friction, a constant renewal taking place from below.

7. Transitional Epithelium —

This variety is found in several places. A squamous transitional bladder and ureters, and consists of three or four

Fig. 7. — Two ‘ Prickle-cells from the deeper part of the Epidermis. (Ranvier.) d, space around the nucleus, probably caused by shrinking of the latter.

epithelium lines th layers of cells with distinct nuclei

Fig. 8.— Section of the Transitional Epithelium lining the Bladder. (E. A. S.) a, superficial, b, intermediate, and c, deep layer of cells.

Fig. 9.— Pavement Epithelium, scraped from a Serous Membrane.

«, cell-body ; b, nucleus ; c, nucleoli. (Henle.

8, Simple Squamous or Pavement

The inner or most superficial layer consists of somewhat flattened cells, each of which overlies two or three pear shaped cells, which form the second layer. A third layer fills up the intervals between the cells of the second layer. A columnar transitional epithelium of three or four layers of cells is found on the lining membrane of the larynx, trachea, and large bronchi, but 1 these will be described with the corresponding simple epi- ' thelium below.

Epithelium.— A single layer of simple scaly epithelium, consisting of flattened polygonal cells fitting together at their edges, lines [a) the air-cells of the lungs, certain looped tubules of the kidney, and the inner surface of the iris and choroid ; (/>) the free surfaces of serous membranes — as the pleura, pericardium, peritoneum, arachnoid— and the interior of the heart, blood-vessels, and lymphatics. It is to the epithelial tissue of the latter division (/>) that the term endothelium is sometimes applied. The outline of these simple squamous epithelial cells is readily brought out by treatment with silver nitrate, as this salt darkens the small amount of intercellular substance.

9. Simple Columnar Epithelium. — This kind of epithelium consists of cells of cylindrical or prismatic form set upright on a surface, and, except in the cases mentioned above, the cells form but one layer side by side. Owing to mutual compression

Fig. 10. — Columnar Epithelium Cells of the Rabbit's intestine. (E. A. S.)

The cells have been isolated after maceration in very weak chromic acid. The protoplasm is reticular and vacuolated ; the striated border (str.) is well seen, and the bright disc separating this from the cell-protoplasm ; «, nucleus with intranuclear network ; a, a thinned out wing-like projection of the cell, which probably fitted between two adjacent cells.

Fig. ii.— A row of Columnar Cells from an Intestinal Villus of the Rabbit. (E. A. S.) sir, striated border : m, wander-cells between the epithelium cells.

and the frequent presence of lymphoid or wander-cells, their shape is often very irregular. Columnar epithelium is found lining the alimentary canal from the lower end of the oesophagus to the anus, as well as on the free surface of the trachea and largest air-tubes. In some cells of columnar epithelium, as those of the small intestine, the free edge is finely striated. Columnar cells often undergo a modification of shape owing to the secretion in their interior of mucin , the chief organic constituent of mucus. The mucus distends the upper part of the cell until a rupture occurs and the mucus is discharged on the surface. Such mucus-secreting cells are called goblet or chalice cells, and the columnar epithelial cells of the intestinal and respiratory tracts may at any time undergo this transfermation into goblet-cells temporarily, while the epithelial cells of mucus-secreting glands have this function permanently.

io. Ciliated Epithelium.— This variety of epithelium consists of cells, usually columnar, having at their free ends

Fig. 12.— Simple Columnar Epithelium from the Mucous Membrane oi the Intestine with goblet-cells pouring out their contents. (Klein and Noble Smith.)

fine hair-like processes called cilia. Each cell bears a brush of cilia, about ^Vo °f an inch long in the windpipe, and during life or for a short time after removal from the body the cilia exhibit a rapid whip-like movement, the cells of the surface of a membrane moving simultaneously or in quick succession in one direction. Bending swiftly in one direction and returning to the upright position more slowly, these cilia set in motion in a definite direction the fluid which bathes the ciliated surface. It is thus that mucus is moved along the bronchial tubes and trachea to the pharynx. Ciliated epithelium is found in the respiratory region of the nose, on the upper

F,c. ,3-Cili.ted Epithelium Cells from half °f the larynx, along the the Trachea of the Rabbit. Highly Eustachian tubes, ill tile lower

,n\ m', m\ mucus-secreting cells, lying Part °f the larynx, except Over

between the ciliated cells, and seen in VOCal COrds, ill the trachea

various stages of mucin-formation. ’

and bronchial tubes, lining the ventricles of the brain and the central canal of the spinal cord, and on the mucous lining of the uterus and Fallopian tubes. As to the nature and cause of ciliary movement little is definitely known, except that the source of motion is contained in the ciliated cell itself and is independent of any connection with nerve-fibres. Ciliary movement may be readily studied by taking a piece of the gill of a salt-water mussel, mounting it in sea water, and examining it with a power of about 200 diameters.1

11. Sensory Epithelium.-- Various forms of modified epithelial cells are connected with the terminations of certain sensory nerves to form the receptive end-organ for different kinds of vibrations. Examples of such sensory epithelium are the rods and cones of the retina, the auditory hair-cells, &c. These will be best studied in connection with the different senses.

12. Functions of Epithelium.— Putting aside the varieties of sensory epithelium, it will be seen that the functions of epithelium are either protective or secretive. Thus the layers of epithelium forming the epidermis, the epithelium lining the air-passages and that lining the eyelids, serve mainly as a protective covering. But the epithelial cells of the salivary glands, those of tue gastric glands which secrete gastric juice, those of the liver, of the sweat-glands, &c., are composed of protoplasm which is the seat of active chemical operations. New substances are formed from the blood and poured out as secretions to fulfil important functions, or discharged from the body as excretions. ‘A secreting epithelium may be regarded as a partition between the blood, or, more properly speaking, between the lymph on the one side and the lumen of the secreting gland on the other. From the lymph the materials are taken by the secreting cells and then worked up into the components of the secretion, and finally discharged on the other side into the lumen, and thence by the ducts of the secreting gland to their destination. 1 he amount of secretion is in some cases, as in that of the kidney, very largely influenced by the amount of blood reaching the organ, and by the blood pressure ; this again is dependent on the size of the blood-vessels, which is regulated by the vaso-motor nerves that supply their muscular tissue.’— Halliburton. Ciliated epithelium, besides being protective, also aids by its movements in propelling fluids and minute particles from the body.

1 Sec note on Microscope in Appendix.

13. Basement Membrane. — Beneath the epithelium of certain parts appears a fine homogeneous membrane spoken of as basement membrane or membrana propria. That at the base of most mucous membranes and beneath the epithelium of secreting glands, however, is found to consist of a very thin layer of flattened cells belonging to the variety of tissue called connective tissue, now to be treated of.

14. Connective Tissues. — The term connective tissues includes a number of tissues which at first sight appear to differ greatly, but which are, nevertheless, properly grouped together owing to their common function, origin, histological and chemical similarities. Their common function is that of connecting and supporting the other tissues, and they have a common origin, as they are all derived from the mesoblast in the developing body. The histological similarities or points of structure in common are three microscopic elements which exist in varying proportion in each, viz. a ground-substance or matrix, cells, and fibres. All varieties of connective tissue that contain white fibres yield gelatine on boiling. Moreover, these tissues may under certain circumstances replace each other or merge into one another. There are three chief varieties of connective tissue, the first variety being separated into six kinds. The following table gives a list of these tissues,