Book - Buchanan's Manual of Anatomy including Embryology 3

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary
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Chapter III Osteology

The skeleton is named from a Greek word which means dry, and is that part of the body which, being richly impregnated with lime salts, persists in a dry condition long after the soft parts have disappeared in the process of decomposition. In the invertebrate animals the skeleton is, for the most part, on the outside, and acts as a protective covering as well as maintaining the shape of the body and serving as a fixed surface from which the muscles may act. In this position it is known as an exoskeleton, and may be calcified, as in the lobster, or composed of chitin, as in the beetle; but in any case it is derived from the outer layer or ectoderm of the embryo.

In the vertebrate animals, of which man is an example, the exoskeleton persists as the horny layer of the epidermis with the so-called epidermal appendages, such as the hair, nails, and teeth; but the shape of the body is maintained, the muscular attachments provided, and protection to delicate internal organs, like the brain and heart, afforded by an endoskeleton derived from the middle layer or mesoderm of the embryo.

To the medical man, a knowledge of the exact position of the bones is of the first importance, since many parts of them can be felt through the skin, and provide landmarks by means of which the soft structures can be found. It is for this reason that the student begins his anatomical course with a study of the bones, and the writer feels that he cannot too early or too often impress upon his reader that it is only from Nature that the skeleton can be learnt. The book can act as a friendly guide, calling attention to the points most worthy of notice, and supplying the conventional names of the parts; but the student must gain for himself the mental picture of the bones as they are, and must find out on his own body what parts can be felt through the skin and how they change their position in different movements. He must learn, too, to visualize those parts of the bones which cannot be felt, and to acquire the anatomist’s sense of how far from the surface they are likely to be.

Remember that the anatomist’s sense is a study of three dimensions, and will never be gained from looking at pictures, however beautifully drawn; indeed, the paradox is not without a good deal of truth that the better illustrated a textbook of anatomy is, the less likely is it to produce a good anatomist, since the temptation to use the clean picture instead of the possibly unattractive ‘ part ’ is so great.

If the student will take as an ideal the power to recognize every bone in the body with his eyes shut, by the feeling alone, and to tell the side to which it belongs, he will find that it will not be so very long before that ideal is attained, that the work gets pleasanter with each attempt, and, when he has reached his goal, he will be surprised to find that he has educated his power of observation and acquired a sense of touch which will help him far in his profession.

Recent development of X rays has made a knowledge of the skeleton more essential than ever to the surgeon and physician, and normal radiograms should be studied whenever possible, otherwise very laughable mistakes in diagnosis may be made later on.

There is another point of view which the student should keep before him—that of forensic medicine; at any time a medical man may be asked whether a particular bone is human, whether it belonged to a man or a woman, and what can be said of the age, the stature, and physique of the individual to whom it belonged. The answer to many of these questions is often ridiculously easy if an interest has been taken in them, and may raise the answerer’s reputation materially.

But apart from the mere technical advantage of a sound knowledge of the skeleton as the foundation of topographical anatomy and surgery, osteology has a wide range of philosophical interest, and brings its followers into touch with many sides of science and art. The study of the evolution and comparative anatomy of the skull and of the limb girdles opens the way to endless discussion on the later phases of life on this planet, while the different proportions of various parts of the skeleton form the basis of physical anthropology, and enable us to understand, year by year, a little more of the interminable wanderings and intermixtures of the different races of mankind before written history began, and to check and criticize the assertions of historians. To take only one small though homely example: the early history of Britain has quite changed its outlook since we have been able to realize the part which the people of the Stone, Bronze, and Iron Ages have played in building up the modern Englishman.

These branches of the subject have to be omitted or merely touched nghtly in a treatise on medical osteology, but they are there in the background, and many a zoologist, anthropologist, and artist envies the expert knowledge and training which most medical men have received in the laborious task of ‘ learning their bones.’


Descriptive Terms.


Apophysis : any projection on a bone centre.

Capitellum : a small head.


not ossified from a separate


lage.


Condyle ( knuckle ’): a round eminence covered by articular carti

hpicondyle : a prominent point, non-articular, on a condyle.

y Sl - S ’ s ^ a ^ a bone, or the part other than the epiphysis. P l p lysis . a process of bone ossifying independently, and attached for a time to the principal part of the bone by cartilage, but subsequently becomes united by bone.

Facet: a small smooth surface.

Foramen : a hole.

Fossa : a depression.

Glenoid : like a shallow socket.

Hiatus : a gap.

Meatus (pi. meatus) : a passage or canal.

Neural: pertaining to the nervous system.

Sinus or antrum : a cavity, usually in the interior of a bone.

Spine : a pointed process.

Styloid : pencil-like; applied to an elongated pointed process.

Trochanter: a term applied to certain muscular levers on the thigh-bone.

Trochlea : a pulley.

Tubercle : a small rounded prominence.

Tuberosity : a larger prominence.

Chemical Composition of Bone.—Osseous tissue belongs to the connective tissues, and it consists of an organic matrix or ground substance, impregnated with mineral matter. The mineral matter is composed chiefly of calcium salts, and so it petrifies the ground substance. The organic matrix is usually spoken of as the animal matter, and it forms about 33 per cent, (or more) of the entire bone. The mineral matter is spoken of as the earthy matter, and it forms about 67 per cent, (or less) of the bone.

The animal matter imparts flexibility and elasticity to the bone, and is composed of very delicate fibres, which are collected into bundles, held together by cement substance. The fibres consist of collagen, which is converted into gelatin by boiling. I he animal matter can be separated from the earthy matter by steeping a bone for some time in dilute hydrochloric acid, the effect of the acid being to dissolve out the earthy matter. When bone is so treated it is said to be decalcified. There is left a tough, flexible, elastic substance, which can be bent and twisted in various directions, and even tied into a knot, but no amount of force applied to it, or pressure laid upon it, would cause it to break. A bone when so treated retains its original shape, but it loses weight to the extent of about two-thirds, and it also loses its property of hardness, so that it is incapable of bearing weight.

The earthy matter imparts hardness and rigidity to the bone, and is composed principally of calcium salts, of which the most abundant is calcium phosphate, there being about 57 P er 9 en ^‘ this salt present. Besides this there are calcium carbonate in the proportion of about 7 per cent., and calcium fluoride in the proportion of about 1 per cent. In addition to the calcium salts there are about 1 per cent, of magnesium phosphate, and about ^ per cent, of sodium chloride. The earthy matter may be separated from the animal matter by burning a bone. The first effect of the heat is to char the animal matter, which is subsequently consumed. When so treated a bone is said to be calcined, and the process is spoken of as combustion or calcination. There is left a white, chalk-like, very brittle substance, which, if of small size and carefully handled, retains its original shape. The slightest rough handling, however, will cause it to break, or crumble into a coarse powder. Calcined bone undergoes no change in shape, but it loses weight to the extent of about one-third. It also loses its flexibility and elasticity. The only property it now possesses is hardness. As stated, it is also very brittle, and, by reason of this, it cannot be bent nor twisted in the slightest degree. The animal and earthy matter, as they form bone, are intimately combined.


The proportion of about one-third of animal matter to about two-thirds of earthy matter applies to the healthy adult. In young children the relative amount of animal matter is much greater, so that in them the bones are very tough and elastic, but not very hard, and therefore not capable of bearing much weight. In old age there is a relatively large amount of earthy matter present. The bones of old persons, therefore, are very hard and brittle, but not very

Fig 75. — Longitudinal Section through the elaS J^ ; hence

Upper Knd of the Femur, showing Compact . e frequency of fractures and Cancellated Osseous Tissues. in old persons from slight causes.


Structure of Bone. There are two varieties of osseous tissue—namely, compact, and spongy or cancellated. Compact osseous tissue is so named TfT 180 \ s constituents are so closely packed together that the bone appears ^11 + 2 akGd e y * (macroscopic) to be dense and close like ivory. Spongy or can llLp a ^ e GT^^ SSeOUS T+ 1 S - SUe, i? n J tke ot ^ er hand, presents an open porous appearance f , £ e ‘ ca ll e d cancellated because it resembles lattice-work. These

Whtn tl poT^r k w Se ^ US tx ® sue .merge very gradually into one another, secti op it r.rpsi 1 ^ * > ° ne p S V1 ? we d under the microscope in thin transverse in thin ^Jrtip f S a d lum .h er °f small round or oval openings, and when viewed eanals tw I sectl ? n 11 presents short longitudinal tubes, called Haversian into the ma™™? TT y ^ ° f com P act bone, and the innermost open orifices on the p-t ^ of l° n g bones, whilst the outermost open by minute theav °l e bpip. f al - SU ? aCe ^u The ^ ran ^ e in diameter from to^fo inch,

§ S 500 mch. The smallest lie nearest the external surface, and the largest are nearest the marrow canal. They are very short and longitudinal in direction, and they communicate freely with one another by connecting canals, some of which are oblique and others transverse. These connecting canals are very small, having a diameter of only gnoo inch- The Haversian canals thus form a freely intercommunicating system of tubes throughout compact bone. The largest canals contain each an arteriole, a minute vein, one or two lymphatics, and a small amount of marrow tissue. The smallest canals contain only one bloodvessel, which is of the nature of a capillary. Those nearest the external surface also contain very delicate thread-like processes of the periosteum. Nerve fibrils have been demonstrated in bone, which is extremely sensitive.


The bone around the Haversian canals is arranged in the form of concentric plates, called Haversian or concentric bone lamella. In transverse section these appear as concentric rings, and in longitudinal section as parallel lines. In the interspaces between the systems of Haversian or concentric lamellae there are interstitial, and, near the external surface, circumferential or peripheral lamella, which are parallel with the surface. Some of the canals which pierce the circumferential lamellae are devoid of concentric lamellae, and are known as Volkmann’s canals. In the interspaces between the Haversian or concentric lamellae there are small fusiform cavities, known as the bone lacuna. These are about 2^00 inch in length, and, like the lamellae, they are arranged concentrically round the Haversian canals. Radiating from these lacunae there are minute channels, called bone canaliculi, which pass through perforations in the lamellae, and so serve to connect the various lacunae with one another. Some of those radiating from the innermost ring of lacunae communicate directly with the Haversian canal. The canaliculi thus constitute a system of intercommunicating channels which maintain a connection directly and indirectly between a given Haversian canal and the lacunae arranged concentrically around it, and traverse the lamellae in their course. Each lacuna contains a protoplasmic nucleated cell, called the bone cell, which almost completely fills it, and sends off processes into the canaliculi communicating with it. The canaliculi contain nutritive fluid derived from the arteriole. A given Haversian canal, with its concentric bone lamellae, concentric bone lacunae, and canaliculi, constitutes a Haversian system, and compact bone is simply an aggregation of such systems, with, in addition, the intermediate and circumferential lamellae. Certain lamellae are perforated perpendicularly by fibres, which thus bind them together. These fibres are known as the perforating fibres of Sharpey. Some of them are composed of white fibrous tissue, and others of elastic tissue, and those nearest the periphery are connected with the periosteum from which they are derived. Others, however, more deeply placed, have no apparent direct connection with the periosteum. The perforating fibres are absent from the lamellae of the Haversian systems.


Fig. 76. — Structure of Compact Bone. A, longitudinal section, showing Haversian canals; B, transverse section, showing Haversian systems.



Cancellated bone is composed of very slender trabeculae, which are arranged in a reticular manner so as to enclose spaces, known as the medullary spaces. These spaces, in the recent state, are filled with marrow. The trabeculae consist of superimposed lamellae of compact bone. The strongest lamellae are disposed in the direction in which the greatest pressure has to be borne, and serve to transmit the stress from the cartilage-covered bone end to the thick compact tissue of the shaft. These are known as pressure lamellae. Other lamellae, known as traction lamellae, transmit the strain of the chief muscles from their insertions to the compact bone; while a third kind, known as bolting lamellae, join the others more or less at right angles, keep them from buckling.

Lacuna, with Bone Cell and Marrow Tissue


Fig. 77 -— Diagram of a Haversian System, minus the Concentric Bone Lamellae.


and distribute traction or pressure from one lamella to the next. The use of wdght ted b ° ne 1S t0 impart sufficient strength without adding unduly to

Classification of Bones.—Bones are arranged in four classes, as follows: Long; short; tabular; and irregular.

A long bone consists of a shaft and two articular extremities. The snatt is either cylindrical or prismatic, and contains a medullary cavity, which is surrounded principally by compact bone. The articular

are C T- P ° S ? d 1 ? f cancellated tissue, except at the surface, i ere * s . a thin shell of compact bone. Long bones are found

m the appendicular skeleton.

b ° n I C ° nsistS u mainly of cancellated tissue, except at the are* more of? ther ?, ls a thln covering of compact bone. Short bones tarsus' CSS ° ° n ^ ln s ia P e * and are found in the carpus and

tabular bone is composed of two plates or tables of compact bone, which enclose between them cancellated tissue. The scapula, ilium, and bones of the skull vault belong to this class. In the case of the latter the cancellated tissue is called diploe.

An irregular bone is one which is so irregular in form and in the relative distribution of the compact and cancellated tissues as to be excluded from any of the preceding classes. The vertebrae belong to this class.

Ossification.—All bones are originally membranous. Some of them for example, the tegmental bones of the cranium and most of the bones of the face—ossify in membrane, but the majority pass through a cartilaginous stage before becoming ossified. There are, therefore, two modes of ossification—namely, intramembranous and intracartilaginous—and bones are consequently spoken of as membrane and cartilage-bones.



Fig. 78. — Ossification in Membrane.


Centres of Ossification. — These are primary and secondary. The primary centre, which as a rule appears about the end of the second month of intra-uterine life, is that from which the ossification of the principal part of the bone proceeds. This, in the case of a long bone, is the shaft or diaphysis. The secondary centres, which for the most part do not appear until after birth, are those from which the ossification of outgrowths of a bone proceeds, these forming what are known as the epiphyses.


Ossification in Membrane. — In this mode of ossification the bone is preceded by fibrous tissue. The fibres of this tissue are known as osteogenetic fibres, and they are arranged in small bundles. The tissue is very vascular, and contains many nucleated cells, called osteoblasts. At the centre of ossification the osteogenetic fibres, which have a covering of osteoblasts, become calcified, and bony spicula are thus formed, which radiate towards the circumference of the bone. These radiating spicula are connected at frequent intervals, and so build up a bony reticulum. As the osteogenetic fibres grow and shoot out they carry with them coatings of osteoblasts, and the process of calcareous incrustation goes on, so that the bony spicula increase in length, and gradually approach the periphery. During this process of spicular bony formation many of the osteoblasts are left behind, and become imprisoned in the lacunar spaces of the forming bone, where they represent the future bone cells. The ossification of a membrane bone is thus effected by means of osteogenetic fibres plus osteoblasts, the fibres acting as outrunners and becoming calcified.


Ossification in Cartilage. — The cartilage is covered by a membrane, called the perichondrium, which corresponds to the periosteum, and the process of ossification takes place in three stages.


First Stage. — In this stage the ossification is partly endochondral, and partly ectochondral, or on the surface beneath the perichondrium. In the endochondral form the cartilage cells at the centre become enlarged, and the intervening matrix becomes calcified. Above and below the centre the cartilage cells are arranged in long columns, directed towards each extremity. The matrix between these columns becomes calcified by an extension of the calcareous matter at the centre, which now surrounds the cell-columns. The spaces in the calcified matrix, which contain these columns, are known as the primary areolce. At the same time, ectochondral or sub* perichondral ossification is proceeding in a manner similar to what takes place in membranous ossification—that is to say, by osteogenetic fibres, osteoblasts, and calcareous impregnation. In this way several layers of bone are laid down at the surface beneath the perichondrium, and these constitute the circumferential lamellae. During this process some of the osteoblasts are detained in lacunar spaces, and form the bone cells.


Second Stage. — This is known as the stage of irruption. The inner or osteogenetic layer of the perichondrium IG - 79 - Ossification bursts in through openings in the circumferential lamellae in Cartilage. in the form of osteogenetic fibres, osteoblasts, and osteo. : . clasts or bone destroyers. These incursions reach the calcified matrix, and the osteoclasts now commence their destructive work. The cartilage cells of the primary areolae, as well as the walls of these areolae, are a sor ed, and larger spaces, called secondary areolcB, or medullary spaces, are or me ^ m the original calcified matrix. These spaces contain osteogenetic fibres &n an< ^ tatter now build up lamellae of bone.


Third stage. — This is a repetition of the preceding two stages. The cartilage - S ^■* 1 ^ nse ^ ves iu rows; the intervening matrix becomes calcified and

j S . j-if 111, the enclosed cartilage cells atrophy and give rise to primary • e osteoclasts produce partial absorption of the calcified cartilage, 1S f ° me( ta^ ar y spaces; and the osteogenetic fibres and osteoblasts ar - ae k° ne - Simultaneously with these processes, subperiosteal

osteorl h rvf + ^° ln ^ on ' . me dullary cavity is due to absorption by the

osteoclasts of the osseous tissue m the centre of the shaft.

I he ossification of the epiphyses of a bone is endochondral.


Periosteum. — The periosteum is a fibrous, vascular membrane, which closely invests bones, except where there is articular cartilage, at the margin of which it ceases. It consists of two layers—outer and inner. The fibres of the outer layer are arranged closely, and it is therefore dense. The inner layer consists chiefly of ramifying elastic fibres. Between the inner layer and the surface of the shaft of a long bone there is, especially during the development and growth of the bone, a layer of subperiosteal areolar tissue. Within its meshes cells, called osteoblasts, accumulate during the period of growth. These cells emerge from the outermost Haversian canals along the course of the entering arteries, and they represent the bone cells of the bone lacunae of compact bone which have migrated outwards. The osteoblasts take an important part in the formation of bone during its growth.

The periosteum is richly supplied with arteries which, after ramifying, enter the outermost Haversian canals, along with fine processes of the inner periosteal layer. The periosteum serves (i) as a bed in which the arteries subdivide before entering the bone, and (2) to give a firm hold to tendons and ligaments. It also takes part in ossification and regeneration of bone. When the periosteum is stripped from a bone, the uncovered portion is liable to necrosis and exfoliation.

Marrow or Medulla Ossium.—The marrow fills the marrow canals of long bones and the medullary spaces of cancellated bone, and it also sends processes into the innermost Haversian canals. It is composed of a reticular fibrous matrix, which is pervaded by many bloodvessels and cells, the latter being called myelocytes (marrow cells). There are two kinds of marrow, yellow and red, which differ as to the character of the cells. In yellow marrow most of the cells have become transformed into fat cells, so that the marrow resembles adipose tissue. Red marrow contains very few fat cells. Many of its cells are colourless, protoplasmic, nucleated cells, which resemble the leucocytes of the blood, though of larger size, and like them are capable of amoeboid movement. Other reddish cells, called erythroblasts, are present, which are the sources from which large numbers of red blood-corpuscles are formed. In addition to these two sets of cells, there are large multinucleated, protoplasmic cells, called the megakaryocytes (myeloplaxes of Robin), which play an important part in the absorption of bone.

Yellow marrow is found in the marrow canals of long bones, whilst red marrow occurs in (1) articular ends of long bones; (2) medullary spaces of cancellated bone; (3) bodies of the vertebrae; (4) sternum; (5) ribs; and (6) the diploe of the cranial bones. Marrow serves the following uses: (1) it (red marrow) is an important blood-forming organ (red corpuscles); (2) it contributes to the nourishment of bone; and (3) it serves as a light packing material for all hollow spaces within bones, with the exception of the air-sinuses in the bones of the head.

The wall of the marrow canal of all long bones and that of the medullary spaces of all cancellated bone are lined with a very delicate layer of areolar tissue, which is richly provided with bloodvessels. This is known as the endosteum or medullary membrane.

Osseous tissue is richly supplied with bloodvessels, which are derived from the periosteum and marrow.


General Principles of the Skeleton

Before beginning a detailed description of the bones, it will be well to impress upon the reader a few general principles in order to avoid misunderstandings later on.

In the first place, it must be very clearly understood that all parts of the human body are described by the anatomist as if the body were in the upright position, with the arms by the sides, and the palms of the hands turned forwards. The commonest cause of misunderstanding is in careless use of the terms ‘ over ’ and ‘ under.’ There are certain terms, however, which free the describer from any thought of the position of the body. Among these are dorsal, towards the back; ventral, towards the front; cephalic or cephalad, towards the head; and caudal or caudad, towards the tail or its site. A structure nearer the middle line is internal or medial, but if it is actually in the middle line it is median, while one farther from the middle line is external or lateral.


Superficial and deep are used in relation to the nearest skin, but they are only useful when there is no doubt as to where the nearest skin really is. Proximal and distal, too, are very useful terms, and are used relatively to the centre of the body, which, for practical purposes, may be taken as the heart.


Homologous and analogous are terms which are liable to be confounded one with the other. Homology suggests structure, and means that the parts compared are derived from the same embryological structures, though they may be used for very different purposes. Analogy, on the other hand, suggests function, and implies that the parts are used loi the same purpose, though they may be derived from totally different parts of the embryo. The wing of a bird, for example, is homologous with the fore-limb of a horse and the upper extremity of a man, though t ley are not analogous; while the wing of a beetle is analogous, though not homologous, with the wing of a bird. When similar structures are repeated m the body, they are said to be serially homologous or homoynamous, as m the successive legs of a centipede. The human arm is, therelore serially homologous with the thigh, the forearm with the leg, and the hand with the foot.

Structures which in the early embryonic position, in which the exremities project at right angles to the central axis of the trunk, lie nearer the head are said to be preaxial, whatever position they may

TiT Py n ln the • and those farther away from the head are postaxial.

Ssta^W Tk’ t f heref0re ' are P reaxial - and the ulna and fibula postaxial bones of the forearm and leg respectively.

iuom the foregoing it will have been noticed incidentally that,


to the anatomist, the arm is only that region between the shoulder and elbow, while the leg lies only between the knee and the ankle.

It is well to think, when studying the details of any particular bone, what that bone has to do, and, having settled that, whether it is not wonderfully well adapted for the work it has to perform; whether any part of it seems useless at first sight; and whether, as one’s knowledge increases, a use does not appear for the seemingly useless part.

At first we marvel at the wonderful way in which bones are designed for the work they have to do, but after more patient observation we realize that much that we regarded as design is really adaptation, that bones are moulded by the surrounding muscles to suit the needs of those muscles, and that the muscles again increase or diminish according to the work which they are called on to do in the particular animal or individual.

Sometimes, it is true, no mechanical advantage is to be found for certain marks or projections, and, if these were parts of a machine, they would be scrapped and the machine made lighter and less cumbrous thereby; but it must be realized that Nature’s method of scrapping is very slow, and that structures which have ceased to be useful take ages to disappear, and, even then, are liable to reappear in individual cases as abnormalities without any advantage, it may be with positive disadvantage, to the individual.

In the case of the vertebral column, which is the part of the skeleton to be described immediately, there is a jointed rod which in man’s case supports the weight of the head by means of the bodies of the vertebrae; it is sometimes said that these are wonderfully designed for this purpose, but all the teaching of comparative anatomy shows rather that they are wonderfully adapted, for we are enabled to follow the evolution of an invertebrate, sea-living animal, whose young could only find food enough by dispersal, developing an unjointed elastic rod in its tail, the effect of which is to bring the tail back to the mid-line after each lash produced by the muscles, thus enabling the creature to swim away.

This is the notochord, around which a fibrous sheath is developed, and, as the creature becomes larger and more complex, the sheath requires stiffening, for muscular attachments, by conversion into cartilage and finally bone. The stiffening would mean loss of mobility were the now cartilaginous rod not segmented into the bodies of vertebrae. At the same time the nerve cord, which runs along the dorsal side of the notochord, becomes protected, and made to follow the movements of the backbone, by a series of cartilaginous and, later on, bony rings which join the bodies of the vertebrae.

This is the condition of things in the fish, but, up to now, the head has derived little or no benefit from the backbone, save that it is propelled through the water with the rest of the creature.

The pectoral and pelvic fins, which are the primitive vertebrate limbs, are borne on limb girdles which at this stage are unconnected with the backbone. On leaving the water for a terrestrial existence the hind-limb girdle becomes connected with one or more vertebrae, and so the hind fin is converted into an organ for propulsion of the body on land, through its central axis or backbone. The pectoral limb girdle, on the other hand, does not join the vertebral column, but fastens itself on to the ventral ends of the ribs, which fuse to form a breast-bone, or sternum. In this way the fore-limbs are specialized for the support of the front part of the body and, should the possessor take to the air, become gradually modified to form wings; or, should he take to an arboreal life, are useful organs for climbing.

When the aquatic life is relinquished the tail, which in the fish forms the greater part of the body, is no longer needed as an organ of propulsion, and is modified into a balancing or prehensile organ, or may merely serve as a wisp to keep off flies. Finally, in man and his nearest relations it ceases to project at all, and is only represented by a few rudimentary vertebrae, fused together to form the coccyx.

Finally, with the gradual assumption of the erect position, the union of the pelvis with the vertebral column is most advantageous for transmitting the whole weight of the trunk to the thighs, and only slight adaptation is needed here, though it is quite obvious that the arrangement was not one of design, and now, at last, the bodies of the vertebrae support the weight of the head, a work for which they already happen to be almost completely adapted, though not designed.