the time of birth.

The Ischium (Fig. 11), like the ilium and pubis, has the fundamental form of a three-sided prismatic rod. One extremity (the upper) completes the acetabulum, whilst the lower forms the large prominence, or tuber ischii. The surfaces of the bone are internal or pelvic, external, and anterior. The pelvic and external surfaces are separated from each other by a sharp border, on which is seen the ischial spine. The pelvic and anterior surfaces are separated by a border, which forms a part of the boundary of the obturator foramen; but the margin between the external and anterior surfaces is feebly marked. The tuberosity, a thick, rough, and strong process, gives origin to several powerful muscles: on it the body rests in the sitting posture; an offshoot, or ramus, ascends from it to join the descending ramus of the pubis, and completes both the pubic arch and the margin of the obturator foramen.

By the articulation of the two innominate bones with each other in front at the pubic symphysis, and with the sides of the sacrum behind, the osseous walls of the cavity of the PELVIS are formed. This cavity is subdivided into a false and a true pelvis. The false pelvis lies between the expanded wing-like portions of the two ilia. The true pelvis lies below the two pectineal lines and the base of the sacrum, which surround the upper orifice or brim of the true pelvis, or pelvic inlet; whilst its lower orifice or outlet is bounded behind by the coccyx, laterally by the ischial tuberosities, and in front by the pubic arch. In the erect attitude the pelvis is so inclined that the plane of the brim forms with the horizontal plane an angle of from 60° to 65°. The axis of the cavity is curved, and is represented by a line drawn perpendicularly to the planes of the brim, the cavity, and the outlet; at the brim it is directed upwards and forwards, at the outlet downwards and a little forwards. Owing to the inclination of the pelvis, the base of the sacrum is nearly 4 inches higher than the upper border of the pubic symphysis. The female pelvis is distinguished from the male by certain sexual characters. The bones are more slender, the ridges and processes for muscular attachment more feeble, the breadth and capacity greater, the depth less, the ilia more expanded, giving the greater breadth to the hips of a woman than a man; the inlet more nearly circular, the pubic arch wider, the distance between the tuberosities greater, and the obturator foramen more triangular in the female than in the male. The greater capacity of the woman's than the man's pelvis is to afford greater room for the expansion of the uterus

The Femur or Thigh-bone (Fig. 11) is the longest bone Femur. in the body, and consists of a shaft and two extremities. The upper extremity or head possesses a smooth convex surface, in which an oval roughened fossa, for the attachment of the inter-articular ligament of the hip, is found; from the head a strong elongated neck passes downwards and outwards to join the upper end of the shaft; the place of junction is marked by two processes or trochanters: the external is of large size, and to it are attached many muscles; the internal is much smaller, and gives attachment to the psoas and iliacus. A line drawn through the axis of the head and neck forms with a vertical line drawn through the shaft an angle of 30°; in a woman this angle is less obtuse than in a man, and the obliquity of the shaft of the femur is greater in the former than in the latter. The shaft is almost cylindrical about its centre, but expanded above and below; its front and sides give origin to the extensor muscles of the leg; behind there is a rough ridge, which, though called linea aspera, is really a narrow surface and not a line; it gives attachment to several muscles. The lower end of the bone presents a large smooth articular surface for the knee-joint, the anterior portion of which forms a trochlea or pulley for the movements of the patella, whilst the lower and posterior part is subdivided into two convex condyles by a deep fossa which gives attachment to the crucial ligaments of the knee. The inner and outer surfaces of this end of the bone are rough, for the attachment of muscles and the lateral ligaments of the knee.

The Patella or Knee-pan (Fig. 11) is a small triangular Patella. flattened bone developed in the tendon of the great extensor muscles of the leg. Its anterior surface and sides are rough, for the attachment of the fibres of that tendon; its posterior surface is smooth, and enters into the formation of the knee-joint.

Between the two bones of the leg there are no movements of pronation and supination as between the two bones of the fore-arm. The tibia and fibula are fixed in position; the fibula is always external, the tibia internal.

The Tibia or Shin-bone (Fig. 11) is the larger and Tibia. more important of the two bones of the leg; the femur moves and rests upon its upper end, and down it the weight of the body in the erect position is transmitted to the foot. Except the femur, it is the longest bone of the skeleton, and consists of a shaft and two extremities. The upper extremity is broad, and is expanded into two tuberosities, the external of which has a small articular facet inferiorly, for the head of the fibula; superiorly, the tuberosities have two smooth surfaces, for articulation with the condyles of the femur; they are separated by an intermediate rough surface, from which a short spine projects, which gives attachments to the inter-articular crucial ligaments and semilunar cartilages of the knee, and lies opposite the intercondyloid fossa of the femur. The shaft of the bone is three-sided; its inner surface is subcutaneous, and forms the shin; its outer and posterior surfaces are for the origin of muscles; the anterior border forms the sharp ridge of the shin, and terminates superiorly in a tubercle for the insertion of the extensor tendon of the leg; the outer border of the bone gives attachment to the inter-osseous membrane of the leg. The lower end of the bone, smaller than the upper, is prolonged into a broad process, internal malleolus, which forms the inner prominence of the ankle: its under surface is smooth for articulation with the astragalus; externally it articulates with the lower end of the fibula.

The Fibula, or Splint-bone of the leg (Fig. 11), is a Fibula. slender long bone with a shaft and two extremities. The upper end or head articulates with the outer tuberosity of

vertebræ, for they lie in front of, and not behind, the vertebral notches through which the spinal nerves are transmitted. The development of the odontoid process of the axis shows it to be the body of the atlas displaced from its proper bone and fused with the body of the axis.

The development and homology of the skull is a much more difficult problem to solve than that of the spine. The chorda dorsalis extends along the floor of the skull as far forward as the posterior wall of the pituitary fossa. Cartilage is formed around it, without, however, the previous production of proto-vertebræ, and this cartilage is prolonged forward on each side of the fossa, forming two bars, the trabeculæ cranii; these bars then unite, and form the mes-ethmoid cartilage; at the same time the cartilage grows outwards for some distance in the membranous wall of the skull, but it does not mount upwards so as to close it in superiorly, so that the cartilage is limited to the floor of the skull; moreover, the cartilage is not segmented. The roof, side walls, and anterior wall of the cranium retain for a time their primordial membranous structure. This membrane is prolonged downwards into the face proper, where it forms a pair of maxillary lobes or processes, which pass forwards beneath the eyes to form the side parts of the face, and a mid- or frontal-nasal process, into which the cartilaginous mesethmoid extends. Immediately below each maxillary lobe four arches, called branchial or visceral, arise in the ventral aspect of the head, and in each of the three first of these arches a rod of cartilage is formed. The arches on opposite sides unite with each other in the ventral mesial line, but those on the same side are separated from each other by intermediate branchial clefts; these clefts all close up in course of time, except the upper part of the first, which remains as the external meatus of the ear, the tympanum, and the Eustachian tube; whilst the interval between the first visceral arch and the maxillary lobes forms the cavity of the mouth. The conversion of the primordial cartilaginous and membranous cranium into the bones of the head takes place by the formation in it of numerous centres of ossification. The basi-, ex-, and so much of the supra-occipital as lies below the superior curved line, are formed from distinct centres in the cartilaginous floor of the skull; whilst the part of the supra-occipital above the curved line arises from independent centres in the membranous cranium, the whole ultimately fusing together to form the occipital bone. The basi- or post-sphenoid, the pre- with the orbito-sphenoids, the ali-sphenoid with the external pterygoid and the internal pterygoid, also arise in the cartilaginous floor, and they, together with the sphenoidal spongy bones which are formed in the membranous cranium, fuse into the sphenoid bone. The palate is apparently formed by ossification of cartilage continuous with the bar in which the internal pterygoid arises. The central plate and each lateral mass of the ethmoid also arise in the cartilage by distinct centres. The inferior turbinal has also a distinct origin in cartilage. The petro-mastoid part of the temporal arises in cartilage from at least three centres, peri-, pro-, and opisth-otic, and soon blends with the squamous and tympanic elements which arise in the membranous cranium; subsequently the styloid process, which is ossified in the rod of cartilage in the second visceral arch, joins the temporal. The lower end of this same rod forms the lesser cornu of the hyoid; the upper end forms two small bones, the stapes and incus, situated within the cavity of the tympanum. The cartilage of the third visceral arch forms the great cornu and body of the hyoid bone. The name of Meckel's cartilage is applied to the rod found in the first visceral arch; its upper end is ossified into the malleus, a small bone situated in the tympanic cavity; whilst in the membrane surrounding the rest of the cartilage the lower jaw-bone is formed. The parietal and frontal bones arise altogether in the membranous vault; and the nasal, lachrymal, malar, and superior maxillæ arise in connection with the bones which form the face; the vomer is developed in the membrane investing the mes-ethmoid cartilage. The human superior maxilla represents not only the superior maxilla of other vertebrates, but the pre-maxillary bone also; but the two bones become fused together at so very early a period that it is difficult to recognise their original independence. In the deformity of hare-lip and cleft palate, they are not unfrequently separated by a distinct fissure.

Since the time when Oken and Goethe propounded the theory that the skull was built up of several vertebræ, the vertebral structure of the skull has led to much discussion amongst anatomists. Every one admits that the skull is in series with the spine, that the cranial cavity is continuous with the spinal canal, and that the cranial vault is formed in the wall of the embryonic cerebro-spinal canal. The skull also, like the spine, is transversely segmented, but whether we regard these segments as vertebræ or not will depend upon the conception we entertain of the meaning of the term vertebra. If with Owen we define a vertebra to be "" one of those segments of the endo-skeleton which constitute the axis of the body and the protective canals of the nervous and vascular trunks," then we may support the vertebral nature of the cranial segments on the following grounds:—1st, The presence of a series of bones extending forwards from the foramen magnum along the

basis cranii, in series with the bodies of the spinal vertebræ,-e.g., the basi-occipital, basi-sphenoid, pre-sphenoid, mes-ethmoid (Fig. 7); 2d, The presence of a series of neural arches which enclose and complete the wall of the cranial cavity, and lie in series with the neural arches of the spinal vertebræ,-e.g., the ex- and supra-occipitals, which form the neural arches of the basi-occipital segment; the ali-sphenoids and parietals, which form the neural arches of the basi-sphenoid segment; the orbito-sphenoids and frontal, which form the neural arches of the pre-sphenoid segment; 3d, The presence of a series of visceral arches of which the mandibular and hyoidean enclose the alimentary and vascular canals, just as the ribs enclose them in the thorax; and 4th, The presence of foramina between the cranial segments like the inter-vertebral foramina between the spinal vertebræ for the transmission of nerves,-c.g., the sphenoidal fissure and the jugular foramen.

But if we are to regard a vertebra as a segment of the axial skeleton, which in course of its formation passes through a definite series of developmental changes, then the cranial segments cannot be regarded as vertebræ in the same sense as the spinal segments; for, 1st, The chorda dorsalis is not co-equal in length with the basis cranii, as with the bodies of the spinal vertebræ, so that if the basi-occipital and basi-sphenoid segments, the bodies of which are developed around it, were to be regarded as cranial vertebræ, the pre-sphenoidal and ethmoido-nasal would not be morphologically the same, as they are formed in front of the anterior end of the chorda. 2d, Proto-vertebræ are formed in the spine, but not in the basis cranii. 3d, The spine is transversely segmented in its cartilaginous stage of development, but the skull is not. 4th, The transverse segmentation of the skull only appears when the bones are formed, but the individuality of the segments becomes again concealed by the fusion of the pre- and basi-sphenoids and the basi-occipital into a continuous bar of bone, a condition which is not found in the spine except in the sacro-coccygeal region. 5th, The neural arches in the spine are, like the bodies, ossified in cartilage, but in the cranium they are for the most part ossified in membrane. These differences in the mode of development of the spine and basis cranii may be summarised as below:

1st Stage, Unseginented chorda.

1st Stage, Unsegmented chorda in part.

Spine.

It is evident, therefore, that, although both skull and spine are developed in the walls of the cerebro-spinal groove, yet, to quote the words of Huxley, "though they are identical in general plan of construction, the two begin to diverge as soon as the one puts on the special character of a skull and the other that of a vertebral column; the skull is no more a modified vertebral column than the vertebral column is a modified skull."

The limbs, at their first appearance, sprout like little buds or lappets from the sides of the trunk; cartilage forms within them, which assumes the shape of the future bones, and as the limbs grow outwards, manifestations of joints appear, and the subdivision of each limb into its several segments takes place. The clavicle, which ossifies before any of the other bones, begins to form, however, in fibrous membrane; and at a much later period the ends of the bone, which are formed in cartilage, unite with the intermediate shaft. The scapula ossifies from one centre for its expanded plate and spine, two small centres each for the acromion and vertebral border, and one for the coracoid. In many vertebrates, more especially birds and reptiles, the coracoid is a distinct bone from the scapula, but they articulate with each other to form the glenoid fossa. Each of the three rod-like bones of which the innominate bone is composed, ossifies from one centre for the shaft of the bone, and one for each extremity; in the ilium these terminal centres are situated at the crest and acetabulum; in the ischium, at the tuber and acetabulum; and in the pubis, at the symphysis and acetabulum. Each of the long bones of the shafts of the limbs ossifies from a single centre for the shaft, and one or more centres for each articular extremity. Each carpal and tarsal bone ossifies from a single centre, except the os calcis, which possesses an independent centre for its posterior surface. The metacarpal and metatarsal bones and the phalanges ossify each from two centres, one for the shaft and one for one of the extremities. In the metacarpal bones of the fingers and the four outer metatarsals, the distal end is that which ossifies independently; in the metacarpal of the thumb, in the metatarsal of the great toe, and in all the phalanges, the proximal end is that which ossifies independently. As the method of ossification of the first metacarpal and first metatarsal corresponds with that of the phalanges, some anatomists hold that these bones are really the first phalanges of their respective digits, and that the bone which is absent in these digits, when compared with the other digits, is not s phalanx, but a meta-carpal or tarsal bone. When the extremity

of a bone ossifies from a centre distinct from the centre from which the shaft arises, it is called an epiphysis. The epiphysis is united to the shaft of the growing bone by an intermediate plate of cartilage, and so long as any of this cartilage remains unossified the bone can continue to grow in length. The ossification is not completed in the different bones until from the twentieth to the twenty-fifth year. In the case of the long bones, the epiphysis situated at the and of the bone, towards which the canal in the shaft which transmits the nutrient artery is directed, ossifies to the shaft before the epiphysis at the other end. In the humerus, tibia, and fibula, where the canal is directed downwards, the epiphyses at the lower ends of the bones first unite with the shaft; whilst in the femur, radius, and ulna, where the canal is directed upwards, the ossification first takes place between the upper epiphysis and the shaft. All anatomists hold that the bones of the shaft and distal part of a limb belong to the appendicular part of the skeleton, but there is a difference of opinion as to the place in the skeleton to which the bones of the shoulder girdle and haunch are to be referred. Owen considers that the scapular and pelvic arches belong to the axial skeleton, and are homologous with the ribs; the scapula and coracoid as the visceral or rib-arch of the occipital vertebra, the clavicle of the atlas, and the innominate bone of the upper sacral vertebræ. Goodsir objected to this conclusion of Owen's on the ground that the shoulder girdle was not in series with the visceral arches, but was developed outside the visceral wall, at the junction of the cervical and thoracic regions, from which region the upper limb receives its nerves, and not from the occipito-atlantal region, whence they would have proceeded had it been an appendage of the rib-arches of those segments. Owen's chief argument for regarding the scapula and coracoid as the costal arch of the occipital vertebra is because in fish the scapula is attached to the occipital bone by a bone which Cuvier called the supra-scapula, and which he believed to be homologous with the supra-scapular cartilage of many other vertebrates. Parker, however, has recently pointed out that the so-called supra-scapula of a fish is not homologous with the supra-scapula of a reptile or mammal, that it is not a is not a cartilage bone, but is a splint or scale-like bone, developed as a as a part of the dermo-skeleton. Between the scapula and coracoid and the innominate bone, anatomists have long recognised homologies to exist; the scapula is generally admitted to be the homotype of the ilium and the coracoid of the ischium, so that if these elements of the shoulder girdle be not a costal arch, neither can those of the pelvic girdle. The clavicle has by some been regarded as the homotype of the pubis; but in all probability the pubis is homologous with the procoracoid bone which is found in the amphibia and some reptiles, but is absent in crocodiles, birds, and mammals; whilst the clavicle is represented in the pelvic girdle, not by a bone, but by a fibrous band called Poupart's ligament. Between the bones of the shafts of the limbs homologies exist: the humerus is the homotype of the femur, the radius of the tibia, the ulna of the fibula; whilst the patella has no representative in the human upper limb. The scaphoid and semilunar bones in the carpus are homotypes of the astragalus in the tarsus, the cuneiform is the homotype of the os calcis, the cuboid of the unciform; the trapezium of the ento-cuneiform, the trapezoid of the meso-, and the os magnum of the ecto-cuneiform. The tarsal scaphoid is not, as a rule, represented in the human carpus, but its homotype is the os intermedium, found in many mammals. The carpal pisciform is a sesamoid bone developed in the tendon of a muscle. The metacarpal bones and phalanges are homologous with the metatarsal bones and phalanges; the thumb with the great toe, and the fingers with the four outer toes. During the growth of the limbs outward, and their change from the simple lappet-like form to their elongated condition, a rotation of the proximal segment of the shaft takes place that of the upper limb a quarter of a circle backward, that of the lower limb a quarter of a circle forward-to produce in the former case a supine position of the fore-arm and hand, with the thumb as the outermost digit; in the latter case, a prone condition of the leg and foot, with the great toe as the innermost digit. The range of movement at the radio-ulnar joints enables us, however, to pronate the hand and fore-arm by throwing the radius across the ulna, so as to make the thumb the innermost digit. In many quadrupeds the fore-leg is fixed in this position, so that these animals walk on the soles of both the fore and hind feet.

GENERAL OCSERVATIONS ON THE ARTICULATORY AND MUSCULAR SYSTEMS.

A JOINT OF ARTICULATION is the junction or union of any two adjacent parts of the body. Most usually the term is employed to signify the connection established between contiguous bones. It is by the joints that the various bones are knit together to form the skeleton. Joints may be either immovable or movable.

The immovable joints are divided into the synchondroses

and the sutures. A synchondrosis is the junction of two Synchonbones by the interposition of an intermediate plate of droses. cartilage, the fibrous membrane or periosteum which invests the bones being prolonged from one bone to the other over the surface of the cartilage. A suture is the Sutures. interlocking of adjacent toothed connection of two bones by the margins; the periosteal fibrous membrane is prolonged from one

8

a cranial suture. b, b, the two bones; s, opposite the suture; 1, the fibrous membrane, or periosteum, passing between the of a ligament, and which is continuous with the interposed fibrous membrane.

tion through a synchondrosis. b, b, the two bones; Sc, the interposed cartil

age;, the fibrous

membrane which plays the part of a

As

bone to the other, and is also FIG. 12.-Vertical section through interposed between their adjacent margins. In a young skull the basi-occipital and basi-sphenoid two bones, which plays the part are united by synchondrosis, but junction by sutures is the mode of union which prevails in the bones of the head. In old persons the sutures become obliterated by the ossification of the intermediate fibrous membrane, and the bones are permanently fused together. The cranial sutures may conveniently be arranged in three groups: a, Median longitudinal, consisting of FIG. 13.-Vertical secthe frontal suture, which connects the two halves of the frontal bone, and the sagittal suture, between the two parietal bones; b, Lateral longitudinal, consisting on each side of the head of the ligament. fronto-nasal, fronto-maxillary, fronto-lachrymal, frontoethmoidal, fronto-malar, fronto-sphenoidal, parieto-sphenoidal, parieto-squamous, parieto-mastoid sutures; c, Vertithe lambdoidal or parieto-occipital, the sphenoido-malar, cal transverse, consisting of the coronal or fronto-parietal, sphenoido-squamous and occipito-mastoid sutures. the skull grows by ossification of the cartilage of the base and the membranous vault, the direction of growth is perpendicular to the margins of the bones and the sutures and synchondroses which connect them together. The growth of the skull in length is perpendicular, therefore, to the basi-cranial synchondrosis and the vertical transverse group of sutures; its growth, in breadth, to the median longitudinal group, and in height to the lateral longitudinal group. So long as any of the cartilage or membrane between the margins of the bones remains unossified, bone may continue to form, and the skull may increase in size. It sometimes happens that the cartilage or membrane is prematurely ossified in a particular locality, and the further growth of the skull put a stop to in that region; if the brain is still growing, the skull must increase in other directions to permit of the expansion of the cranial cavity, and deformities of the skull are thereby occasioned. One of the most usual of these deformities is due to premature closure of the sagittal suture, causing

ོ་

cephalic skull, showing the complete disappear

FIG. 14.-Vertex view of a boat-shaped or scaphoance of the sagittal suture.

The movable joints are divided into the amphiarthrodial and the diarthrodial joints. An amphiarthrosis or halfjoint has only a feeble range of movement. It consists of two bones, each of which has its articular surface covered by a plate of cartilage, and which plates are firmly connected together by an intermediate disc of fibro-cartilage. The centre of this FIG. 15.-Vertical section through disc is soft, or may even be an amphiarthrodial joint. b, b. hollowed out into a cavity, lined cartilage on the articular surthe two bones; c, c, the plate of by a smooth synovial membrane, face of each bone; Fe, the intermediate fibro-cartilage; 4, 4, and containing a little fluid. the external ligaments. Ligamentous bands, continuous with the periosteum investing the bones, invest the fibro-cartilage, and assist in binding the bones together. The best examples of amphiarthrodial joints are furnished by the articulations between the bodies of the true vertebræ.

FIG. 16.-Vertical section through

a diarthrodial joint. b, b, the two bones; c, c, the plate of

vesting ligament, the dotted

the synovial membrane. The

A diarthrosis admits of more or less perfect movement. In it the two articular surfaces are each covered by a plate of encrusting cartilage, the free surface of which is smooth and polished; between these surfaces is a cavity containing a glairy fluid, the synovia, for lubricating the smooth surfaces of the cartilage and facilitating the movements of the joint. This cavity is enclosed by ligaments, which are attached to the bones, and the inner surface of these ligaments is lined by a synovial membrane which secretes the synovia. Sometimes a plate or meniscus of fibrocartilage is interposed between, cartilage on the articular surwithout, however, being attached face of each bone; 1, 1, the into the encrusting cartilages of a line within which represents diarthrodial joint, so as more or letters is placed in the cavity less perfectly to subdivide the of the joint. cavity enclosed by the ligaments into two spaces. The articular surfaces of diarthrodial joints are retained in apposition with each other, sometimes by investing ligaments, at others by surrounding muscles and tendons; at others by atmospheric pressure, aided by the adhesive character of the interposed synovia. The form of the articular or movable surfaces varies very materially in different examples of these joints, and the modifications in form determine the direction of the movements of the joints. In some, as the carpal and tarsal joints, the surfaces are almost flat, so that they glide on each other; the movement is comparatively slight, and about an axis perpendicular to the moving surfaces: these are called planesurfaced joints. In other joints the articular surfaces may be regarded as produced by the rotation of a straight or curved line about an axis lying in the same plane; these are called rotation joints, and they present various modifications according to the direction and relation of the rotating line to the axis. One form of a rotation joint is the

FIG. 17.-Vertical section through

a diarthrodial joint, in which the cavity is subdivided into tilage or meniscus, Fc. The

two by an interposed fibro-car

other letters as in Fig. 16.

axis of one of the bones, which is the axis of rotation of the joint; examples of this joint are found in the joint between the atlas and odontoid process of the axis and the radio-ulnar joint. Another form is the ginglymus or hinge joint, in which the axis of rotation of the joint is perpendicular to the axis of the two bones; the movements of the hinge are called flexion when the angle between the two bones is diminished, and extension when the angle is increased. An important modification of the ginglymus is the screwed-surfaced joint, examples of which are found in the elbow and ankle; here the plane of flexion is not perpendicular, but oblique to the axis of the joint. The saddle-shaped and oblong joints are also modified hinges, but allow motion about two axes; in the oblong both axes are on the same side of the joint; but in the saddle-shaped there is an axis of rotation on each side of the joint. The best example of the saddle-shaped is found between the metacarpal bone of the thumb and the trapeum; of the oblong between the fore-arm and the carpus. In the ball-and-socket joint a spheroidal head fits into a cup, and rotation takes place about any diameter of the sphere; the joint therefore is multi-axial; the hip and shoulder joints are the best examples. Some joints, in which the forms of the articular surfaces are more complex, are called composite; in them the movements of a hinge and of a ball-and-socket joint may be combined; the knee may be cited as an example of this form of articulation. In a large number of movable joints only portions of the opposite articular surfaces are in contact with each other at a given time; but, as the joint describes its path of movement, different parts of the surfaces come into contact with each other successively, and it is not unusual to find the articular surface both of the cartilage and the subjacent bone mapped out into distinct areas or facets, which are adapted to corresponding facets on the opposite articular surface in particular positions of the joint. When the corresponding facets on opposite articular surfaces break contact with each other, the space between becomes occupied by synovia, or in some joints, more especially the knee, by folds of synovial membrane enclosing clumps of fat, which have been called synovial pads. In the simple hinge, in that with screwed surfaces, in the oblong and composite joints, the principal ligaments are situated at the sides of the joint, and are called lateral; they not only prevent lateral displacement of the bones, but, by a tightening of their fibres, check excessive movement forwards or backwards during flexion and extension. In the saddle-shaped and ball-and-socket joints, the joint is included within a bag-like ligament called capsular. In the pivot joints the cavity in which the pivot fits is completed by a transverse or a ring-shaped ligament.

The MUSCLES are the organs which, by their contraction Muscles. or shortening, move the bones on each other at the joints. The muscles constitute the flesh of the body. They are so arranged as to be capable not only of moving the various bones on each other, but the entire body from place to place. Hence the muscles are organs both of motion and locomotion. As they can be brought into action at the will of the individual, they are called voluntary muscles. Some of the muscles are engaged in the movement of other structures than the bones, such as the eye-ball, tongue, cartilages of the larynx, &c. About 400 muscles are usually enumerated, and the names applied to them express either their position, or relative size, or shape, or direction, or attachments, or mode of action. The word muscle is itself derived from the Latin musculus, a little mouse, from a fancied resemblance between that animal and some of the most simply formed muscles. It is customary to distinguish in a muscle a central part, or belly, and two ex