Читать книгу The Vertebrate Skeleton - Sidney H. Reynolds - Страница 4

Fig. 1. Diagrammatic sections of various forms of teeth (from Flower).

I. Incisor or tusk of elephant, with pulp-cavity persistently open at base. II. Human incisor during development with root imperfectly formed, and pulp-cavity widely open at base. III. Completely formed human incisor, with pulp-cavity contracted to a small aperture at the end of the root. IV. Human molar with broad crown and two roots. V. Molar of Ox, with the enamel covering the crown, deeply folded and the depressions filled with cement. The surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white, the dentine represented by horizontal lines, and the cement by dots.

The teeth of any animal may be homodont, that is, all having the same general character, or heterodont, that is, having different forms adapted to different functions. The dentition is heterodont in a few reptiles and the majority of mammals.

Succession of teeth. In most fishes, and many amphibians and reptiles the teeth can be renewed indefinitely. In sharks, for example, numerous rows of reserve teeth are to be seen folded back behind those in use (see fig. 15). The majority of mammals have only two sets of teeth, and are said to be diphyodont; some have only a single series (monophyodont).

Development of teeth. A brief sketch of the method in which development of teeth takes place in the higher vertebrates may here be given. Along the surface of the jaws a thickening of the epiblastic epithelium takes place, giving rise to a ridge, which sinks inwards into the tissue of the jaw, and it is known as the primary enamel organ. At the points where teeth are to be developed special ingrowths of this primary enamel organ take place, and into each there projects a vascular dental papilla from the surrounding mesoblast of the jaw. Each ingrowth of the enamel organ forms an enamel cap, which gradually embraces the dental papilla, and at the same time appears to be pushed on one side, owing to the growth not being uniform. The external layer of the dental papilla is composed of long nucleated cells, the odontoblasts, and it is by these that the dentine is formed. Similarly the internal layer of the enamel organ is formed of columnar enamel cells, which give rise to the enamel. The mesoblastic cells surrounding the base of the tooth give rise to the cement.

Bone is in many cases exoskeletal, but it will be most conveniently described with the endoskeleton.

The scales of fish are wholly or in part mesoblastic in origin, being totally different from those of reptiles. The cycloid and ctenoid scales of Teleosteans (see p. 105) are thin plates coated with epidermis. They are sometimes bony, but as a rule are simply calcified. Ganoid scales are flat plates of bone coated with an enamel-like substance, and articulating together with a peg and socket arrangement; they are probably identical with enlarged and flattened placoid scales.

The armour plates of fossil Ganoids, Labyrinthodonts, and Dinosaurs, and of living Crocodiles, some Lizards and Armadillos, are composed of bone. They are always covered by a layer of epidermis.

The antlers of deer are also composed of bone; they will be more fully described in the chapter on mammals. It may perhaps be well to mention them here, though they really belong to the endoskeleton, being outgrowths from the frontal bones.

B. Endoskeletal structures.

I. Hypoblastic.

(a) The notochord is an elastic rod formed of large vacuolated cells, and is surrounded by a membranous sheath of mesoblastic origin. It is the primitive endoskeleton in the Chordata, all of which possess it at some period of their existence; while in many of the lower forms it persists throughout life. Even in the highest Chordata it is the sole representative of the axial skeleton for a considerable part of the early embryonic life. A simple unsegmented notochord persists throughout life in the Cephalochordata, Cyclostomata, and some Pisces, such as Sturgeons and Chimaeroids.

(b) The enamel of the pharyngeal teeth of the Salmon and many other Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum, in which the other teeth are developed, passes into the hypoblast of the mesenteron in which these pharyngeal teeth are formed.

II. Mesoblastic.

The most primitive type of a mesoblastic endoskeleton consists of a membranous sheath surrounding the notochord, as in Myxine and its allies. The first stage of complication is by the development of cartilage in the notochordal sheath, as in Petromyzon. Often the cartilage becomes calcified in places, as in the vertebral centra of Scyllium and other Elasmobranchs. Lastly, the formation of bone takes place; it generally constitutes the most important of the endoskeletal structures.

Bone may be formed in two ways:—

(1) by the direct ossification of pre-existing cartilage, when it is known as cartilage bone or endochondral bone;

(2) by independent ossification in connective tissue; it is then known as membrane or dermal or periosteal bone.

With the exception of the clavicle[4] all the bones of the trunk and limbs, together with a large proportion of those of the skull, are preformed in the embryo in cartilage, and are grouped as cartilage bones; while the clavicle and most of the roofing and jaw-bones of the skull are not preformed in cartilage, being developed simply in connection with a membrane. Hence it is customary to draw a very strong line of distinction between these two kinds of bone; in reality however this distinction is often exaggerated, and the two kinds pass into one another, and as will be shown immediately, the permanent osseous tissue of many of those which are generally regarded as typical cartilage bones, is really to a great extent of periosteal origin. The palatine bone, for instance, of the higher vertebrates in general is preceded by a cartilaginous bar, but is itself almost entirely a membrane bone.

Before describing the development of bone it will be well to briefly describe the structure of adult bone and cartilage.

The commonest kind of cartilage, and that which preforms so many of the bones of the embryo, is hyaline cartilage. It consists of oval nucleated cells occupying cavities (lacunae) in a clear intercellular semitransparent matrix, which is probably secreted by the cells. Sometimes one cell is seen in each lacuna, sometimes shortly after cell-division a lacuna may contain two or more cells. The free surface of the cartilage is invested by a fibrous membrane, the perichondrium.

Bone consists of a series of lamellae of ossified substance between which are oval spaces, the lacunae, giving rise to numerous fine channels, the canaliculi, which radiate off in all directions. The lacunae are occupied by the bone cells which correspond to cartilage cells, from which if the bone is young, processes pass off into the canaliculi. It is obvious that the ossified substance of bone is intercellular in character, and corresponds to the matrix of cartilage.

Bone may be compact, or loose and spongy in character, when it is known as cancellous bone. In compact bone many of the lamellae are arranged concentrically round cavities, the Haversian canals, which in life are occupied by blood-vessels. Each Haversian canal with its lamellae forms a Haversian system. In spongy bone instead of Haversian canals there occur large irregular spaces filled with marrow, which consists chiefly of blood-vessels and fatty tissue. The centre of a long bone is generally occupied by one large continuous marrow cavity. The whole bone is surrounded by a fibrous connective tissue membrane, the periosteum.

The development of bone.

Periosteal ossification. An example of a bone entirely formed in this way is afforded by the parietal. The first trace of ossification is shown by the appearance, below the membrane which occupies the place of the bone in the early embryo, of calcareous spicules of bony matter, which are laid down round themselves by certain large cells, the osteoblasts. These osteoblasts gradually get surrounded by the matter which they secrete and become converted into bone cells, and in this way a mass of spongy bone is gradually produced. Meanwhile a definite periosteum has been formed round the developing bone, and on its inner side fresh osteoblasts are produced, and these with the others gradually render the bone larger and more and more compact. Finally, the middle layer of the bone becomes again hollowed out and rendered spongy by the absorption of part of the bony matter.

Endochondral ossification[5]. This is best studied in the case of a long bone like the femur or humerus. Such a long bone consists of a shaft, which forms the main part, and two terminal portions, which form the epiphyses, or portions ossifying from centres distinct from that forming the shaft or main part of the bone.

In the earliest stage the future bone consists of hyaline cartilage surrounded by a vascular sheath, the perichondrium.

Then, starting from the centre, the cartilage becomes permeated by a number of channels into which pass vessels from the perichondrium and osteoblasts. In this way the centre of the developing shaft becomes converted into a mass of cavities separated by bands or trabeculae of cartilage. This cartilage next becomes calcified, but as yet is not converted into true bone. The osteoblasts in connection with the cavities now begin to deposit true endochondral spongy bone, and then after a time this becomes absorbed by certain large cells, the osteoclasts, and resolved into marrow or vascular tissue loaded with fat. So that the centre of the shaft passes from the condition of hyaline cartilage to that of calcified cartilage, thence to the condition of spongy bone, and finally to that of marrow. At the same time beneath the perichondrium osteoblasts are developed which also begin to give rise to spongy bone. The perichondrium thus becomes the periosteum, and the bone produced by it, is periosteal or membrane bone. So that while a continuous marrow cavity is gradually being formed in the centre of the shaft, the layer of periosteal bone round the margin is gradually thickening, and becoming more and more compact by the narrowing down of its cavities to the size of Haversian canals. The absorption of endochondral and formation of periosteal bone goes on, till in time it comes about that the whole of the shaft, except its terminations, is of periosteal origin. At the extremities of the shaft, however, and at the epiphyses, each of which is for a long time separated from the shaft by a pad of cartilage, the ossification is mainly endochondral, the periosteal bone being represented only by a thin layer.

Until the adult condition is reached and growth ceases, the pad of cartilage between the epiphysis and the shaft continues to grow, its outer (epiphysial) half growing by the formation of fresh cartilage as fast as its inner half is encroached on by the growth of bone from the shaft. The terminal or articular surfaces of the bone remain throughout life covered by layers of articular cartilage.

Even after the adult condition is reached the bone is subject to continual change, processes of absorption and fresh formation going on for a time and tending to render the bone more compact.

Methods in which bones are united to one another.

The various bones composing the endoskeleton are united to one another either by sutures or by movable joints.

When two bones are suturally united, their edges fit closely together and often interlock, being also bound together by the periosteum.

In many cases this sutural union passes into fusion or ankylosis, ossification extending completely from one bone to the other with the obliteration of the intervening suture. This feature is especially well marked in the cranium of most birds.

The various kinds of joints or articulations[6] may be subdivided into imperfect joints and perfect joints.

In imperfect joints, such as the intervertebral joints of mammals, the two contiguous surfaces are united by a mass of fibrous tissue which allows only a limited amount of motion.

In perfect joints the contiguous articular surfaces are covered with cartilage, and between them lies a synovial membrane which secretes a viscid lubricating fluid.

The amount of motion possible varies according to the nature of the articular surfaces; these include—

a. ball and socket joints, like the hip and shoulder, in which the end of one bone works in a cup provided by another, and movements can take place in a variety of planes.

b. hinge joints, like the elbow and knee, in which as in ball-and-socket joints one bone works in a cup provided by another, but movements can take place in one plane only.

THE ENDOSKELETON.

The endoskeleton is divisible into axial and appendicular parts; and the axial skeleton into—

1. the spinal column,

2. the skull {a. the cranium, {b. the jaws and visceral skeleton, 3. the ribs and sternum[7].

I. The Axial Skeleton.

1. The Spinal column.

The spinal column in the simplest cases consists of an unsegmented rod, the notochord, surrounded by the skeletogenous layer, a sheath of mesoblastic origin, which also envelops the nerve cord. Several intermediate stages connect this simple spinal column with the vertebral column characteristic of higher vertebrates. A typical vertebral column may be said to consist of (1) a series of cartilaginous or bony blocks, the vertebral centra, which arise in the sheath surrounding the notochord. They cause the notochord to become constricted and to atrophy to a varying extent, though a remnant of it persists, either permanently or for a long period, within each centrum or between successive centra. (2) From the dorsal surface of each centrum arise a pair of processes which grow round the spinal cord and unite above it, forming a dorsal or neural arch. (3) A similar pair of processes arising from the ventral surface of the centrum form the ventral or haemal arch. To the ventral arch the ribs strictly belong, and it tends to surround the ventral blood-vessels and the body cavity with the alimentary canal and other viscera.

A neural spine or spinous process commonly projects upwards from the dorsal surface of the neural arch, and a pair of transverse processes project outwards from its sides. When, as is commonly the case, the two halves of the haemal arch do not meet, the ventral surface of the centrum often bears a downwardly-projecting hypapophysis.

The character of the surfaces by which vertebral centra articulate with one another varies much. Sometimes both surfaces are concave, and the vertebra is then said to be amphicoelous; sometimes a centrum is convex in front and concave behind, the vertebra is then opisthocoelous, sometimes concave in front and convex behind, when the vertebra is procoelous. Again, in many vertebrae both faces of the centra are flat, while in others they are saddle-shaped, as in the neck vertebrae of living birds, or biconvex, as in the case of the first caudal vertebra of crocodiles.

In the higher vertebrates pads of fibrocartilage—the intervertebral discs—are commonly interposed between successive centra, these or parts of them often ossify, especially in the trunk and tail, and are then known as inter centra.

Fig. 2. Cervical vertebrae of an Ox (Bos taurus). A, is the fifth; B, the fourth; C, the third. X ¼ (Camb. Mus.)

1. neural spine.	6. prezygapophysis.
2. transverse process.	7. postzygapophysis.
3. hypapophysis.	8. vertebrarterial canal.
4. convex anterior face of the centrum.	9. neural canal.
5. concave posterior face of the centrum.	10. inferior lamella of transverse process.

The vertebrae of the higher forms can generally be arranged in the following five groups, each marked by certain special characteristics:

1. The cervical or neck vertebrae. These connect the skull with the thorax, and are characterised by relatively great freedom of movement. They often bear small ribs, but are distinguished from the succeeding thoracic vertebrae by the fact that their ribs do not reach the sternum. The first cervical vertebra which articulates with the skull is called the atlas, but a study of the nerve exits shows that the first vertebra is not serially homologous throughout the Ichthyopsida, so that it is best to reserve the term atlas for the first vertebra in Sauropsida and Mammalia.

2. The thoracic vertebrae (often called dorsal) bear movably articulated ribs which unite ventrally with the sternum.

3. The lumbar vertebrae are generally large, and are often more movable on one another than are the thoracic vertebrae. They bear no ribs.

4. The sacral vertebrae are characterised by the fact that they are firmly fused together, and are united with the pelvic girdle by means of their transverse processes and rudimentary ribs.

5. The caudal or tail vertebrae succeed the sacral. The anterior ones are often fused with one another and with the sacrals, but they differ from true sacral vertebrae in that there are no rudimentary ribs between their transverse processes and the pelvic girdle. They often bear V-shaped chevron bones.

In fish and snakes the vertebral column is divisible into only two regions, an anterior trunk region, whose vertebrae bear ribs, and a posterior tail region, whose vertebrae are ribless.

2. The Skull.

Before giving a general account of the adult skull it will be well to briefly describe its development.

General development of the Cranium[8].

Shortly after its appearance, the central nervous system becomes surrounded by a membranous mesodermal investment which in the region of the spinal cord is called the skeletogenous layer or perichordal sheath, while in the region of the brain it is called the membranous cranium. Ventral to the central nervous system is the notochord, which extends far into the region of the future cranium, and like the nervous system, is enclosed by the skeletogenous layer. The primitive cartilaginous cranium is formed by histological differentiation within the substance of the membranous cranium and always consists of the following parts:

(a) the parachordals. These are a pair of flat curved plates of cartilage, each of which has its inner edge grooved where it comes in contact with the notochord. The parachordals, together with the notochord, form a continuous plate, which is known as the basilar plate. The basilar plate is the primitive floor below the hind- and mid-brain. In front the parachordals abut upon another pair of cartilaginous bars, the trabeculae, the two pairs of structures being sometimes continuous with one another from the first;

(b) the trabeculae which meet behind and embrace the front end of the notochord. Further forwards they at first diverge from one another, and then converge again, enclosing a space, the pituitary space. After a time they generally fuse with one another in the middle line, and, with the parachordals behind, form an almost continuous basal plate. The trabeculae generally appear before the parachordals. They form the primitive floor below the fore-brain;

(c) the cartilaginous capsules of the three pairs of sense organs. At a very early stage of development involutions of the surface epiblast give rise to the three pairs of special sense organs—the olfactory or nasal organs in front, the optic in the middle, and the auditory behind. The olfactory and auditory organs always become enclosed in definite cartilaginous capsules, the eyes often as in the Salmon, become enclosed in cartilaginous sclerotic capsules, while sometimes, as in mammals, their protecting capsules are fibrous.

Each pair of sense capsules comes into relation with part of the primitive cranium, and greatly modifies it. Thus the auditory or periotic capsules press on the parachordals till they come to be more or less imbedded in them. Perhaps owing to the pressure of the nasal capsules the trabeculae fuse in front, and then grow out into an anterior pair of processes, the cornua trabeculae, and a posterior pair, the antorbital processes, which together almost completely surround the nasal capsules. The sclerotic capsules of the eyes greatly modify the cranium, although they never become completely united with it.

The cartilaginous cranium formed of the basal plate, together with the sense capsules, does not long remain merely as a floor. Its sides grow vertically upwards, forming the exoccipital region of the cranium behind, and the alisphenoidal and orbitosphenoidal regions further forwards. In many forms, such as Elasmobranchs, all these upgrowths meet round the brain, roofing it in and forming an almost complete cartilaginous cranium. But in most vertebrata, while in the occipital region, the cartilaginous cranium is completed dorsally, in the alisphenoidal and orbitosphenoidal regions the cartilage merely forms the lateral walls of the cranium, the greater part of the brain having dorsal to it a wide space, closed by merely membranous tissue in connection with which the large frontal and parietal bones are subsequently formed.

The Skull includes

a. the cranium,

b. the jaws and visceral skeleton.

The cranium can be further subdivided into

(1) an axial portion, the cranium proper or brain case;

(2) the sense capsules. The capsules of the auditory and olfactory sense organs are always present, and as has been already mentioned, in many animals the eye likewise is included in a cartilaginous capsule.

(1) The cranium proper or brain case.

The cranium varies much in form and structure. In lower vertebrates, such as Sharks and Lampreys, it remains entirely cartilaginous and membranous, retaining throughout life much of the character of the embryonic rudiment of the cranium of higher forms. The dogfish's cranium, described on pp. 73 to 76, is a good instance of a cranium of this type. But in the majority of vertebrates the cartilage becomes more or less replaced by cartilage bone, while membrane bones are also largely developed and supplant the cartilage.

The cranium of most vertebrates includes a very large number of bones whose arrangement varies much, but one can distinguish a definite basicranial axis formed of the basi-occipital, basisphenoid, and presphenoid bones, which is a continuation forwards of the axis of the vertebral column. From the basicranial axis a wide arch arises, composed of a number of bones, which form the sides and roof of the brain-case These bones are arranged in such a manner that if both cartilage and membrane bones are included they can be divided into three rings or segments. The hinder one of these segments is the occipital, the middle the parietal, and the anterior one the frontal.

The occipital segment is formed of four cartilage bones, the basi-occipital below, two exoccipitals at the sides, and the supra-occipital above. The parietal segment is formed of the basisphenoid below, two alisphenoids at the sides and two membrane bones, the parietals above, and the frontal segment in like manner consists of the presphenoid below, the two orbitosphenoids at the sides, and two membrane bones, the frontals, above. The parietals and frontals, being membrane bones, are not comparable to the supra-occipital, in the way that the presphenoid and basisphenoid are to the basi-occipital.

The cartilage bones of the occipital segments are derived from the parachordals of the embryonic skull, those of the parietal and frontal segments from the trabeculae.

In front of the presphenoid the basicranial axis is continued by the mesethmoid.

(2) The sense capsules.

These enclose and protect the special sense organs.

(a) Auditory capsule.

The basisphenoid is always continuous with the basi-occipital, but the alisphenoid is not continuous with the exoccipital as the periotic or auditory capsule is interposed between them. Each periotic capsule has three principal ossifications; an anterior bone, the pro-otic, a posterior bone, the opisthotic, and a superior bone, the epi-otic.

These bones may severally unite, or instead of uniting with one another they may unite with the neighbouring bones. Thus the epi-otic often unites with the supra-occipital, and the opisthotic with the exoccipital.

Two other bones developed in the walls of the auditory capsule are sometimes added, as in Teleosteans; these are the pterotic and sphenotic.

(b) Optic capsule.

The eye is frequently enclosed in a cartilaginous sclerotic capsule, and in this a number of scale-like bones are often developed.

Several membrane bones are commonly formed around the orbit or cavity for the eye. The most constant of these is the lachrymal which lies in the anterior corner; frequently too, as in Teleosteans, there is a supra-orbital lying in the upper part of the orbit, or as in many Reptiles, a postorbital lying in the posterior part of the orbit.

(c) Nasal capsule.

In relation to the nasal capsules various bones occur.

The basicranial axis in front of the presphenoid is ossified, as the mesethmoid, dorsal to which there sometimes, as in Teleosteans, occur a median ethmoid and a pair of lateral ethmoids[9]. Two pairs of membrane bones very commonly occur in this region, viz. the nasals which lie dorsal to the mesethmoid, and the vomers (sometimes there is only one) which lie ventral to it.

The part of the skull lying immediately in front of the cranial cavity and in relation to the nasal capsules constitutes the ethmoidal region.

There remain certain other membrane bones which are often found connected with the cranium. Of these, one of the largest is the parasphenoid which, in Ichthyopsids, is found underlying the basicranial axis. Prefrontals often, as in most reptiles, occur lying partly at the sides and partly in front of the frontal, and postfrontals similarly occur behind the orbit lying partly behind the frontals and partly at their sides. Lastly a squamosal bone is, as in Mammals, very commonly developed, and lies external and partly dorsal to the auditory capsules.

The Jaws and Visceral Skeleton.

In the most primitive fish these consist of a series of cartilaginous rings or arches placed one behind another and encircling the anterior end of the alimentary canal. Originally they are mainly concerned with branchial respiration.

The first or maxillo-mandibular arch forms the upper jaw and the lower jaw or mandible.

The second or hyoid arch bears gills and often assists in attaching the jaws to the cranium. The remaining arches may bear gills, though the last is commonly without them.

The above condition is only found in fishes, in higher animals the visceral skeleton is greatly reduced and modified.

The first or maxillo-mandibular arch is divisible into a dorsal portion, the palato-pterygo-quadrate bar, which forms the primitive upper jaw and enters into very close relations with the cranium, and a ventral portion, Meckel's cartilage, which forms the primitive lower jaw. The cartilaginous rudiments of both these portions disappear to a greater or less extent and become partly ossified, partly replaced by or enveloped in membrane bone.

The posterior part of the palato-pterygo-quadrate bar becomes ossified to form the quadrate, the anterior part to form the palatine and pterygoid, or the two latter may be formed partially or entirely of periosteal bone, developed round the cartilaginous bar. Two pairs of important membrane bones, the premaxillae and maxillae form the anterior part of the upper jaw, and behind the maxilla lies another membrane bone, the jugal or malar, which is connected with the quadrate by a quadratojugal. The premaxillae have a large share in bounding the external nasal openings or anterior nares.

In lower vertebrates the nasal passage leads directly into the front part of the mouth cavity and opens by the posterior nares. In some higher vertebrates, such as mammals and crocodiles, processes arise from the premaxillae and palatines, and sometimes from the pterygoids, which meet their fellows in the middle line and form the palate, shutting off the nasal passage from the mouth cavity and causing the posterior nares to open far back.

The cartilage of the lower jaw is in all animals with ossified skeletons, except the Mammalia, partly replaced by cartilage bone forming the articular, partly overlain by a series of membrane bones the dentary, splenial, angular, supra-angular and coronoid. In many sharks large paired accessory cartilages occur at the sides of the jaws; and in a few reptiles and some Amphibia, such as the Frog, the ossified representative of the anterior of these structures occurs forming the mento-meckelian bone. In mammals the lower jaw includes but a single bone.

The quadrate in all animals with ossified skeletons, except the Mammalia, forms the suspensorium of the mandible or the skeletal link between the jaw and the cranium; in the Mammalia, however, the mandible articulates with the squamosal, while the quadrate is greatly reduced, and is now generally considered to be represented by the tympanic ring of the ear.

The second visceral or hyoid arch in fishes consists of two pieces of cartilage, a proximal piece the hyomandibular, and a distal[10] piece the cerato-hyal. The cerato-hyals of the two sides are commonly united by a median ventral plate, the basi-hyal. The hyoid arch bears gills on its posterior border, but its most important function in most fishes is to act as the suspensorium. In higher vertebrates the representative of the hyomandibular is much reduced in size, and comes into relation with the ear forming the auditory ossicles; the cerato-hyal looses its attachment to the hyomandibular and becomes directly attached to the cranium, forming a large part of the hyoid apparatus of most higher vertebrates.

Behind the hyoid arch come the branchial arches. They are best developed in fishes, in which they are commonly five in number and bear gills. Their ventral ends are united in pairs by median pieces, the copulae.

In higher vertebrates they become greatly reduced, and all except the first and second completely disappear. In the highest vertebrates, the mammals, the second has disappeared, but in birds and many reptiles it is comparatively well developed.

3. The Ribs and Sternum.

The ribs are a series of segmentally arranged cartilaginous or bony rods, attached to the vertebrae; they tend to surround the body cavity, and to protect the organs contained within it. Ribs are very frequently found attached to the transverse processes of the vertebrae, but a study of their origin in fish shows that they are really the cut off terminations of the ventral arch, not of the transverse processes which are outgrowths from the dorsal arch. In the tail their function is to surround and protect structures like the ventral blood-vessels which do not vary much in size, consequently they meet one another, and form a series of complete ventral or haemal arches. But the trunk contains organs like the lungs and stomach which are liable to vary much in size at different times, consequently the halves of the haemal arch do not meet ventrally, and then the ribs become detached from the rest of the haemal arch. Having once become detached, they are able to shift about and unite themselves to various points of the vertebra. They frequently, as has been already mentioned, become entirely attached to the transverse process, or they may be attached to the transverse process by a dorsal or tubercular portion and to the centrum or to the ventral arch by a ventral or capitular portion.

In all animals above fishes the distal ends of the thoracic ribs unite with a median breast bone or sternum which generally has the form of a segmented rod. The sternum is really formed by the fusion of the distal ends of a series of ribs. In many animals elements of the shoulder girdle enter into close relation with the rib elements of the sternum.

II. The Appendicular Skeleton.

This consists of the skeleton of the anterior or pectoral, and the posterior or pelvic limbs, and their girdles. In every case (except in Chelonia) the parts of the appendicular skeleton lie external to the ribs.

1. The Limb girdles.

The Pectoral girdle[11]. In the simplest case the pectoral or shoulder girdle consists of a hoop of cartilage incomplete dorsally. It is attached by muscle to the vertebral column, and is divided on either side into dorsal and ventral portions by a cavity, the glenoid cavity, at the point where the anterior limb articulates. In higher fishes this hoop is distinctly divided into right and left halves; it becomes more or less ossified, and a pair of important bones, the clavicles, are developed in connection with its ventral portion.

In higher vertebrates ossification sets up in the cartilage and gives rise on each side to a dorsal bone, the scapula, and frequently to an anterior ventral bone, the precoracoid, and a posterior ventral bone, the coracoid. The precoracoid is often not ossified, and upon it is developed the clavicle which more or less replaces it. In some forms a T shaped interclavicle occurs, in others epicoracoids are found in front of the coracoids. In all vertebrata above fish, except the great majority of mammals, the coracoids are large and articulate with the sternum. But in mammals the coracoids are nearly always quite vestigial, and the pectoral girdle is attached to the axial skeleton by the clavicle or sometimes by muscles and ligaments only.

The Pelvic girdle[12] like the pectoral consists primitively of a simple rod or hoop of cartilage, which in vertebrata above fishes is divided into dorsal and ventral portions, by a cavity, the acetabulum, with which the posterior limb articulates. In the pelvic girdle as in the pectoral one dorsal, and (commonly) two ventral ossifications take place. The dorsal bone is the ilium and corresponds to the scapula. The posterior ventral bone is the ischium corresponding to the coracoid. The anterior ventral bone is the pubis and is generally compared to the precoracoid, but in some cases a fourth pelvic element, the acetabular or cotyloid bone is found, and this may correspond to the precoracoid.

The pelvic girdle differs from the pectoral in the fact that the dorsal bones—the ilia—are nearly always firmly united to transverse processes of the sacral vertebrae, by means of rudimentary ribs. The pubes and ischia generally meet in ventral symphyses.

2. The Limbs.

It will be most convenient to defer a discussion of the limbs of fishes to chap. VIII.

All vertebrates above fishes have the limbs divisible into three main segments:—

	Anterior or Fore limb.	Posterior or Hind limb.
Proximal segment.	upper arm or brachium.	thigh.
Middle segment.	fore-arm or antibrachium.	shin or crus.
Distal segment.	manus.	pes.

The proximal segments each contain one bone, the humerus in the case of the upper arm, and the femur in the case of the thigh. The middle segments each contain two bones, the radius and ulna in the case of the fore-arm, and the tibia and fibula in the case of the shin.

The manus and pes are further subdivided into

(a) two or three proximal rows of bones forming the wrist or carpus in the case of the manus, and the ankle or tarsus in the case of the pes.

(b) a middle row called respectively the metacarpus and metatarsus.

(c) a number of distal bones called the phalanges which form the skeleton of the fingers and toes, or digits.

Typically the manus and pes both have five digits (pentedactylate). The first digit of the manus is commonly called the pollex, and the first digit of the pes the hallux.

In a very simple carpus such as that of Chelydra, there are nine bones. They are arranged in a proximal row of three, the radiale, intermedium, and ulnare—the first being on the radial side of the limb, and a distal row of five called respectively carpale 1, 2, 3, 4, 5, beginning on the radial side. Between these two rows is a single bone the centrale, or there may be two.

Similarly there are nine bones in a simple tarsus such as that of Salamandra. They form a proximal row of three, the tibiale, intermedium and fibulare, and a distal row of five, called respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side. Between the two rows there is a centrale as in the carpus, or there may be two.

The following names derived from human anatomy are commonly applied to the various carpal and tarsal bones:

Carpus.	Tarsus.
radiale = scaphoid	tibiale}
intermedium = lunar	intermedium } astragalus
ulnare = cuneiform	fibulare = calcaneum
centrale = central	centrale = navicular
carpale 1 = trapezium	tarsale 1 = internal cuneiform
"2 = trapezoid	"2 = middle "
"3 = magnum	"3 = external "
"4 }	"4 }
"5 } = unciform	"5 }= cuboid

Note. The above is the view commonly accepted concerning the homology of the carpal and tarsal bones. But with regard to the proximal row of tarsal bones there is difference of opinion. All anatomists are agreed that the calcaneum is the fibulare and that the intermedium is contained in the astragalus, but while the majority regard the astragalus as the fused tibiale and intermedium, Baur considers that a small bone found on the tibial side of the tarsus in Procavia, many Rodents, Insectivores, and the male Ornithorhynchus, is the vestigial tibiale, and regards the astragalus as the intermedium alone[13]. He also considers that the mammalian scaphoid represents a centrale.

Modifications in the positions of the limbs[14].

In their primitive position the limbs are straight and are extended parallel to one another at right angles to the axis of the trunk. Each limb then has a dorsal surface, a ventral surface, an anterior or pre-axial edge, and a posterior or postaxial edge.

In the anterior limb the radius and the pollex are pre-axial, the ulna and the fifth finger are postaxial. In the posterior limb the tibia and the hallux are pre-axial, the fibula and the fifth toe are postaxial. The Cetacea and various extinct reptiles, such as Ichthyosaurus and Plesiosaurus, have their limbs in practically this primitive position.

The first modification from it is produced by the bending ventrally of the middle segments of both limbs upon the proximal segments, while the distal segment is bent in the opposite direction on the middle segment. Then the ventral surfaces of the antibrachium and crus come to look inwards, and their dorsal surfaces to look outwards. The brachium and manus, thigh and pes still have their dorsal surfaces facing upwards and their ventral surfaces facing downwards as before, and the relations of their pre- and postaxial borders remain as they were. Many Amphibians and Reptiles, such as tortoises, carry their limbs in this position.

In all higher vertebrates, however, a further change takes place, each limb is rotated as a whole from its proximal end, the rotation taking place in opposite directions in the fore and hind limbs respectively. The anterior limb is rotated backwards from the shoulder, so that the brachium lies nearly parallel to the body, and the elbow points backwards, the antibrachium downwards, and the manus backwards; the pre-axial surface of the whole limb with the radius and pollex now faces outwards, and the postaxial surface with the ulna and fifth finger now faces inwards. In the Walrus and, to a certain extent, in the Sea lions the anterior limb remains throughout life in this position. The posterior limb is also rotated, but the rotation in this case takes place forwards, so that the thigh lies nearly parallel to the body, the knee-joint pointing forwards; the crus downwards and the pes forwards. The pre-axial surface of the whole limb with the tibia and hallux looks towards the middle of the body, the postaxial surface with the fibula and fifth toe looks outwards. This is the position in which the hind limb is carried in nearly all mammals.

In nearly all mammals a further change takes place in the position of the anterior limb. The radius and ulna have hitherto been parallel to one another, but now the lower end of the radius, carrying with it the manus, comes to be rotated forwards round the ulna, so that the manus, as well as the pes, comes to be forwardly-directed, and its pre-axial surface faces inwards.

In the majority of mammals the radius and ulna are permanently fixed in this, which is known as the prone position, but in man and some other mammals the manus can be pronated or turned into this position at will. When the radius and ulna are parallel throughout their whole length the manus is said to be in the supine position.

The extensor side of a limb is that to which the muscles which straighten it are attached, the flexor side is that to which the muscles which bend it are attached.