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like bone, the fundamental form of which is obscured by during pregnancy, and for the expulsion of the child at
The body and horizontal ramus form the funda- upper extremity or head possesses a smooth convex surface, mental prismatic rod, and the descending ramus is merely in which an oval roughened fossa, for the attachment of a special offshoot from the inner end of the rod. The the inter-articular ligament of the hip, is found; from the outer end of the rod takes a part in the formation of the head a strong elongated neck passes downwards and outacetabulum; the inner end is expanded into the body of wards to join the upper end of the shaft; the place of the pubis, and has a broad margin, or symphysis, for junction is marked by two processes or trochanters: the articulation with the corresponding bone on the opposite external is of large size, and to it are attached many side of the pelvis. The three surfaces are—a superior, for muscles; the internal is much smaller, and gives attachthe origin of the pectineus muscle; a posterior, which ment to the psoas and iliacus. A line drawn through the enters into the wall of the true pelvis ; and an inferior, axis of the head and neck forms with a vertical line drawn which forms the upper boundary of the obturator foramen. through the shaft an angle of 30°; in a woman this angle The descending ramus is merely a downward prolongation is less obtuse than in a man, and the obliquity of the of the inner end of the bone which joins the ischium, and shaft of the femur is greater in the former than in the aids in forming the side of the pubic arch. The junction latter. The shaft is almost cylindrical about its centre, of the outer end of the pubis with the ilium is marked by but expanded above and below; its front and sides give the pectineal eminence. The superior and posterior sur-origin to the extensor muscles of the leg; behind there is faces are separated by the sharp pectineal line, which, a rough ridge, which, though called linea aspera, is really starting from the spine of the pubis, runs outwards to aid a narrow surface and not a line; it gives attachment to in forming the brim of the true pelvis.
several muscles. The lower end of the bone presents The Ischium (Fig. 11), like the ilium and pubis, has a large smooth articular surface for the knee-joint, the the fundamental form of a three-sided prismatic rod. One anterior portion of which forms a trochlea or pulley for the extremity (the upper) completes the acetabulum, whilst movements of the patella, whilst the lower and posterior the lower forms the large prominence, or tuber ischii. The part is subdivided into two convex condyles by a deep surfaces of the bone are internal or pelvic, external, and fossa which gives attachment to the crucial ligaments of anterior. The pelvic and external surfaces are separated the knee. The inner and outer surfaces of this end of the from each other by a sharp border, on which is seen the bone are rough, for the attachment of muscles and the ischial spine. The pelvic and anterior surfaces are sepa- lateral ligaments of the knee. rated by a border, which forms a part of the boundary of The Patella or Knee-pan (Fig. 11) is a small triangular Patella. the obturator foramen; but the margin between the external flattened bone developed in the tendon of the great extensor and anterior surfaces is feebly marked. The tuberosity, muscles of the leg. Its anterior surface and sides are a thick, rough, and strong process, gives origin to rough, for the attachment of the fibres of that tendon; its several powerful muscles : on it the body rests in the posterior surface is smooth, and enters into the formation sitting posture; an offshoot, or ramus, ascends from it of the knee-joint. to join the descending ramus of the pubis, and com Between the two bones of the leg there are no movepletes both the pubic arch and the margin of the obturator ments of pronation and supination as between the two foramen.
bones of the fore-arm. The tibia and fibula are fixed in By the articulation of the two innominate bones with position; the fibula is always external, the tibia internal. each other in front at the pubic symphysis, and with the The Tibia or Shin-bone (Fig. 11) is the larger and Tibia. sides of the sacrum behind, the osseous walls of the cavity more important of the two bones of the leg; the femur of the PELVIS are formed. This cavity is subdivided into moves and rests upon its upper end, and down it the a false and a true pelvis. The false pelvis lies between weight of the body in the erect position is transmitted to the expanded wing-like portions of the two ilia. The true the foot. Except the femur, it is the longest bone of the pelvis lies below the two pectineal lines and the base of skeleton, and consists of a shaft and two extremities. The the sacrum, which surround the upper orifice or brim of upper extremity is broad, and is expanded into two tuberthe true pelvis, or pelvic inlet; whilst its lower orifice or osities, the external of which has a small articular facet outlet is bounded behind by the coccyx, laterally by the inferiorly, for the head of the fibula; superiorly, the tuberischial tuberosities, and in front by the pubic arch. In osities have two smooth surfaces, for articulation with the the erect attitude the pelvis is so inclined that the plane condyles of the femur; they are separated by an intermediate of the brim forms with the horizontal plane an angle of rough surface, from which a short spine projects, which gives from 60° to 65°. The axis of the cavity is curved, and is attachments to the inter-articular crucial ligaments and represented by a line drawn perpendicularly to the planes semilunar cartilages of the knee, and lies opposite the interof the brim, the cavity, and the outlet; at the brim it is condyloid fossa of the femur. The shaft of the bone is directed upwards and forwards, at the outlet downwards three-sided; its inner surface is subcutaneous, and forms and a little forwards. Owing to the inclination of the the shin; its outer and posterior surfaces are for the origin pelvis, the base of the sacrum is nearly 4 inches higher of muscles; the anterior border forms the sharp ridge of the than the upper border of the pubic symphysis. The female shin, and terminates superiorly in a tubercle for the inserpelvis is distinguished from the male by certain sexual tion of the extensor tendon of the leg; the outer border characters. The bones are more slender, the ridges and of the bone gives attachment to the inter-osseous membrane processes for muscular attachment more feeble, the breadth of the leg. The lower end of the bone, smaller than the and capacity greater, the depth less, the ilia more expanded, upper, is prolonged into a broad process, internal malleolus, giving the greater breadth to the hips of a woman than a which forms the inner prominence of the ankle: its under man; the inlet more nearly circular, the pubic arch wider, the surface is smooth for articulation with the astragalus; exterdistance between the tuberosities greater, and the obturator nally it articulates with the lower end of the fibula. foramen more triangular in the female than in the male. The Fibula, or Splint-bone of the leg (Fig. 11), is a Fibula. The greater capacity of the woman's than the man's pelvis slender long bone with a shaft and two extremities. The is to afford greater room for the expansion of the uterus | upper end or head articulates with the outer tuberosity of
the tibia. The shaft is three-sided, and roughened for the bird, both of which, with the same fundamental type of structure, origins of muscles ; along the inner surface is a slender
are subservient to flight. In other cases analogous parts are not ridge for the attachment of the interosseous membrane. homologues, as is illustrated by the wing of the insect, which,
though subservient to flight, is fundamentally different in structure The lower end has a strong process (external malleolus) from the wing of the bat or bird. projecting downwards to form the outer prominence of the In the germinal area of the fertilised vertebrate ovum a longitudi. ankle, and possesses a smooth inner surface for articulation nal groove appears which marks the beginning of the cranial cavity with the astragalus, above which is a rough surface for the spinal groove a slender rod is formed, called chorda dorsalis or noio
and spinal canal of the young embryo. At the bottom of this cranioattachment of ligaments which bind together the tibia and chord. Each side of the groove then becomes elevated as a thin memfibula.
brane, to meet behind to enclose a canal in which the brain and spinal The Foot consists of the Tarsus, the Metatarsus, and the
marrow are developed. Small dark masses, the primordial or protofive free Digits or Toes, which is the maximum number vertebrae, next form on each side of the chcrda dorsalis. In these
proto-vertebræ, about the sixth or seventh week of intra-uterine found in mammals. The human foot is placed in the prone life of the human ovum, little masses of cartilage appear, which position, with the sole or plantar surface in relation to the correspond in number and position to the future spinal vertebra. ground;
the dorsum or back of the foot directed upwards ; The part of the cartilage which forms the body of the future vertebra the axis of the foot at about a right angle to the axis of the substance, whilst the cartilaginous neural arch forms in the memleg; and the great toe or hallux, which is the corresponding brane which closes in the spinal canal. The formation of these digit to the thumb, at the inner border of the foot. The cartilaginous vertebræ is completed in the human embryo about the human foot, therefore, is a pentadactylous, plantigrade foot.
fourth month of intra-uterine life. The bodies of the cartilaginous The bones of the Tarsus, or Ankle (Fig. 11, Tr), are seven
vertebræ are connected together by plates or discs of intervening
fibro-cartilage, which are also developed around the chorda dorsalis. in number, and are arranged in two transverse rows, -a After the enclosure of the rod-like chorda by the cartilaginous proximal, next the bones of the leg, consisting of the vertebræ and the inter-vertebral discs it disappears, no remains astragalus, os calcis, and scaphoid; a distal, next the being found in the adult human body, or in that of the higher metatarsus, consisting of the cuboid, ecto-, meso-, and
vertebrates, except perhaps some slight traces in the soft pulpy ento-cuneiform. If the tarsal bones be looked at along it remains as a more or less complete structure throughout life.
centres of the inter-vertebral discs ; although in the cartilaginous fish with those of the metatarsus and toes, the bones of the In each of the cartilaginous vertebræ bone begins to form and foot may be arranged in two longitudinal columns,-an to spread beyond
to spread beyond its original point of formation, which is called outer, consisting of the os calcis, cuboid, and the metatarsal
a centre or nucleus of ossification; the greater part of the body is
formed from one of these centres, and each half of the neural arch bones and phalanges of the fourth and fifth toes; an inner from another; whilst small ossific centres arise for the tips of the column consisting of the astragalus, scaphoid, three cunei- spinous, transverse, and mammillary processes, and a special plate form, and the metatarsal bones and phalanges of the first, appears for both the upper and lower surfaces of the body; the second, and third toes. The tarsal, like the carpal bones, fusion of the various centres together to form a complete vertebra
The are short and irregularly cuboidal ; the dorsal and plantar atlas has a separate centre for each lateral mass and one for the surfaces are as a rule rough for ligaments, but as the anterior boundary of the ring. The axis, in addition to the ossific astragalus is locked in between the bones of the leg and centres found in the vertebræ generally, has one or two for the the os calcis, its dorsal and plantar surfaces, as well odontoid process. The seventh cervical vertebra has the anterior as the dorsum of the os calcis, are smooth for articu- coccygeal vertebra possesses only a single centre, which represents
bar of its transverse process developed from a separate centre. Each lation; similarly, its lateral surfaces are smooth for the body of the bone. articulation with the two malleoli. The posterior surface At the time when the cranio-spinal canal is being closed in by of the os calcis projects backwards to form the prominence the development of its membranous walls, the germinal layers of
the young embryo grow towards its anterior or ventral surface, and of the heel. With this exception, the bones have their
meet in the ventral mesial line, so as to enclose the cavities in which anterior and posterior surfaces smooth for articulation. the thoracic and abdominal viscera are developed. In the membranous Their lateral surfaces are also articular, except the outer wall on each side of the thoracic cavity twelve cartilaginous rods, surface of the os calcis and cuboid, which form the outer the future ribs, are developed; and, connected with the anterior ends border ; and the inner surface of the os calcis, scaphoid, Each rib ossifies from one centre for its shaft, and one each for the
of the seven pairs of upper ribs, the cartilaginous sternum is formed and ento-cuneiform, which form the inner border of the head and tubercle. The sternum ossifies in transverse segments,tarsus. A supernumerary bone is sometimes found in the one for the præ-sternum, one or sometimes two for each of the four human tarsus, from a subdivision of either the ento-cunei- subdivisions of the meso-sternum, and one for the xiphi-sternum. form, astragalus, os calcis, or cuboid into two parts. In
The complete ossification and fusion of the different parts of the
sternum into a single bone does not take place until an advanced age. some rodents and other mammals eight is the normal The axial part of the skeleton, formed by the vertebræ, ribs, and number of bones in the tarsus.
sternum, is built up of a series of thirty-three transverse segments, The Metatarsal bones and the Phalanges of the toes equal in number, therefore, to the bones of the spine ; so that each agree in number and general form with the metacarpal ribs and a segment of the sternum, constitutes a complete or
vertebra, according as it is, or is not, articulated with a pair of bones and the phalanges in the hand. The bones of the incomplete transverse segment. These several segments are serially great toe or hallux are more massive than those of the homologous with each other, but the homology is not so complete other digits, and this digit, unlike the thumb or pollex, in some of the segments as in the others. In the coccygeal, sacral, does not diverge from the other digits, but lies almost bral portion of each skeletal segment is represented, though in the parallel to them.
abdominal wall of the crocodiles abdominal ribs and a sternum are
developed. In the thoracic region the five lowest dorsal vertebræ Development and Ilomologres of the Skeleton.
have five pairs of ribs developed in connection with them; whilst
the seven highest vertebræ have not only their corresponding pairs It will now be advisable to consider briefly the mode of develop- of ribs, but also a sternum, which bone, however, has only six transment of the skeleton, and along with the study of its genesis to verse segments. In the cervical region seven vertebræ are found, compare its several parts with each other, in order to ascertain if but the anterior bar of the transverse process, although fused with correspondences in their arrangement and mode of origin exist, even the vertebral body, is homologous with a rib, for in man it someif they differ in the function or office which they perform. When times develops as a distinct movable rib in connection with the two or more parts or organs correspond with each other in structure, seventh cervical ; and in the crocodiles small movable ribs are relative position, and mode of origin, we say they are homologous regularly developed in connection with the different cervical verteparts, or homologues; whilst parts which have the same function, bre. The bodies and neural arches of the vertebre are serially but do not correspond in structure, relative position, and mode of homologous with each other; as a rule this is also the case with origin, are analogous parts, or analognies. Homologous parts have their processes, but the articular processes of the atlas and the therefore a morphological identity with each other, whilst analogous superior pair of the axis, although functionally analogous, are not parts have a physiological agreement. The same parts may be homologous with the articular processes of the other vertebræ, but both homologous and analogous, as the fore-limbs of a bat and a with the articular surfaces for the ribs on the bodies of the dorsal
vertebre, for they lie in front of, and not behind, the vertebral basis cranii, in series with the bodies of the spinal vertebræ,-.g., notches through which the spinal nerves are transmitted. The the basi-occipital, basi-sphenoid, pre-sphenoid, mes-ethmoid (Fig. 7); development of the odontoid process of the axis shows it to be the 2d, The presence of a series of neural arches which enclose and body of the atlas displaced from its proper bone and fused with the complete the wall of the cranial cavity, and lie in series with the body of the axis.
neural arches of the spinal vertebræ,-e.g., the ex- and supra-occi. The development and homology of the skull is a much more pitals, which form the neural arches of the basi-occipital segment; difficult problem to solve than that of the spine. The chorda the ali-sphenoids and parietals, which form the neural arches of dorsalis extends along the floor of the skull as far forward as the the basi-sphenoid segment; the orbito-sphenoids and frontal, which posterior wall of the pituitary fossa. Cartilage is formed around it, form the neural arches of the pre-sphenoid segment; 3d, The without, however, the previous production of proto-vertebræ, and presence of a series of visceral arches of which the mandíbular and this cartilage is prolonged forward on each side of the fossa, forming hyoidean enclose the alimentary and vascular canals, just as the two bars, the trabeculæ cranii; these bars then unite, and form the ribs enclose them in the thorax; and 4th, The presence of foramina mes-ethmoid cartilage; at the same time the cartilage grows out between the cranial segments like the inter-vertebral foramina wards for some distance in the membranous wall of the skull, but between the spinal vertebræ for the transmission of nerves, -c.g., it does not mount upwards so as to close it in superiorly, so that the sphenoidal fissure and the jugular foramen. the cartilage is limited to the floor of the skull; moreover, the But if we are to regard a vertebra as a segment of the axial cartilage is not segmented. The roof, side walls, and anterior wall skeleton, which in course of its formation passes through a definite of the cranium retain for a time their primordial membranous struc series of developmental changes, then the cranial segments cannot ture. This membrane is prolonged downwards into the face proper, be regarded as vertebræ in the same sense as the spinal segments; where it forms a pair of maxillary lobes processes, which pass for, 1st, The chorda dorsalis is not co-equal in length with the basis forwards beneath the eyes to form the side parts of the face, and cranii, as with the bodies of the spinal vertebræ, so that if the a mid. or frontal-pasal process, into which the cartilaginous mes basi-occipital and basi-sphenoid segments, the bodies of which are ethmoid extends. Immediately below each maxillary lobe four developed around it, were to be regarded as cranial vertebra, the arches, called branchial or visceral, arise in the ventral aspect of the pre-sphenoidal and ethmoido-nasal would not be morphologically the head, and in each of the three first of these arches a rod of cartilage same, as they are formed in front of the anterior end of the chorda. is formed. The arches on opposite sides unite with each other in 2d, Proto-vertebræ are formed in the spine, but not in the basis the ventral mesial line, but those on the same side are separated cranii. 3d, The spine is transversely segmented in its cartilaginous from each other by intermediate branchial clefts; these clefts all stage of development, but the skull is not. 4th, The transverse close up in course of time, except the upper part of the first, which segmentation of the skull only appears when the bones are forined, remains as the external meatus of the ear, the tympanum, and the but the individuality of the segments becomes again concealed by Eustachian tube ; whilst the interval between the first visceral arch the fusion of the pre- and basi-sphenoids and the basi-occipital into and the maxillary lobes forms the cavity of the mouth. The con a continuous bar of bone, & condition which is not found in the version of the primordial cartilaginous and membranous cranium spine except in the sacro-coccygeal region. . 5th, The neural arches into the bones of the head takes place by the formation in it of in the spine are, like the bodies, ossified in cartilage, but in the numerous centres of ossification. The basic, ex., and so much of the cranium they are for the most part ossified in membrane. These supra-occipital as lies below the superior curved line, are formed differences in the mode of development of the spine and basis cranii from distinct centres in the cartilaginous floor of the skull; whilst may be summarised as below:the part of the supra-occipital above the curved line arises from
Spine. independent centres in the membranous cranium, the whole ulti
4th Stage, mately fusing together to form the occipital bone. The basi- or Unseginented Proto-vertebræ. Segmented Segmented post-sphenoid, the pre- with the orbito-sphenoids, the ali-sphenoid chorda.
cartilage. bones. with the external pterygoid and the internal pterygoid, also arise
Basis Cranii. in the cartilaginous floor, and they, together with the sphenoidal
2d Stage, 3d Stage, 4th Stage, spongy bones which are formed in the membranous cranium, fuse into the sphenoid bone. The palate is apparently formed by ossifica
Unsegmented Unsegmented Segmented Unsegmented chorda in part.
bones. tion of cartilage continuous with the bar in which the internal
tones. pterygoid arises. The central plate and each lateral mass of the It is evident, therefore, that, although both skull and spine are ethmoid also arise in the cartilage by distinct centres. The inferior developed in the walls of the cerebro-spinal groove, yet, to quote turbinal has also a distinct origin in cartilage. The petro-mastoid the words of Huxley, “though they are identical in general part of the temporal arises in cartilage from at least three centres, plan of construction, the two begin to diverge as soon as the one peri-, pro-, and opisth-otic, and soon blends with the squamous puts on the special character of a skull and the other that of a and tympanic elements which arise in the membranous cranium; vertebral column; the skull is no more a modified vertebral column subsequently the styloid process, which is ossified in the rod of than the vertebral column is a modified skull." cartilage in the second visceral arch, joins the temporal. The lower The limbs, at their first appearance, sprout like little buds or end of this same rod forms the lesser cornu of the hyoid ; the upper lappets from the sides of the trunk ; cartilage forms within them, end forms two small bones, the stapes and incus, situated within which assumes the shape of the future bones, and as the limbs the cavity of the tympanum. The cartilage of the third visceral grow outwards, manifestations of joints appear, and the subdivision arch forms the great cornu and body of the hyoid bone. The name of each limb into its several segments takes place. The clavicle, of Meckel's cartilage is applied to the rod found in the first visceral which ossifies before any of the other bones, begins to form, howarch ; its upper end is ossified into the malleus, a small bone ever, in fibrous membrane; and at a much later period the ends of situated in the tympanic cavity; whilst in the membrane sur the bone, which are formed in cartilage, unite with the intermediate rounding the rest of the cartilage the lower jaw-bone is formed. shaft. The scapula ossifies from one centre for its expanded plate The parietal and frontal bones arise altogether in the membranous and spine, two small centres each for the acromion and vertebral vault; and the nasal, lachrymal, malar, and superior maxillæ arise border, and one for the coracoid. In many vertebrates, more espein connection with the bones which form the face; the vomer is cially birds and reptiles, the coracoid is a distinct bone from the developed in the membrane investing the mes-ethmoid cartilage. scapula, but they articulate with each other to form the glenoid The human superior maxilla represents not only the superior maxilla fossa. Each of the three rod-like bones of which the innomirate of other vertebrates, but the pre-maxillary bone also; but the two bone is composed, ossifies from one centre for the shaft of the bone; bones become fused together at so very early a period that it is and one for each extremity; in the ilium these terminal centres are difficult to recognise their original independence. In the deformity situated at the crest and acetabulum ; in the ischium, at the tuber of hare-lip and cleft palate, they are not unfrequently separated by and acetabulum ; and in the pubis, at the symphysis and acetabua distinct fissure.
Each of the long bones of the shafts of the limbs ossifies Since the time when Oken and Goethe propounded the theory from a single centre for the shaft, and one or more centres for each that the skull was built up of several vertebræ, the vertebral struc articular extremity. Each carpal and tarsal bone ossifies from a ture of the skull has led to much discussion amongst anatomists. single centre, except the os calcis, which possesses an independent Every one admits that the skuil is in series with the spine, that centre for its posterior surface. The metacarpal and metatarsal the cranial cavity is continuous with the spinal canal, and that the bones and the phalanges ossify each from two centres, one for the cranial vault is formed in the wall of the embryonic cerebro-spinal shaft and one for one of the extremities. In the metacarpal bones of canal. The skull also, like the spine, is transversely segmented, the fingers and the four outer metatarsals, the distal end is that which but whether we regard these segments as vertebræ or not will ossifies independently; in the metacarpal of the thumb, in the metadepend upon the conception we entertain of the meaning of the tarsal of the great toe, and in all the phalanges, the proximal end is term vertebra. If with Owen we define a vertebra to be
that which ossifies independently. As the method of ossification of those segments of the endo-skeleton which constitute the axis of the first metacarpal and first metatarsal corresponds with that of the the body and the protective canals of the nervous and vascular phalanges, some anatomists hold that these bones are really the first trunks," then we may support the vertebral nature of the cranial phalanges of their respective digits, and that the bone which is absent segments on the following grounds :-ist, The presence of a series in these digits, when compared with tho other digits, is not a of bones extending forwards from the foramen magnum along the phalanx, but a meta-carpal or tarsal bone. When the extremity
chondrosis. o, b, the
of a bone ossifies from a centre distinct from the centre from which and the sutures. A synchondrosis is the junction of two Synchon-
a cranial suture. b, b, the two
bones; &, opposite the suture; 1, All anatomists hold that the bones of the shaft and distal part of margins. In a young skull the the tibrous membrane, or peria limb belong to the appendicular part of the skeleton, but there is basi-occipital and basi-sphenoid two bones, which plays the part a difference of opinion as to the place in the skeleton to waich the
are united by synchondrosis, but
of a ligament, and which is
continuous with the interposed
a, Median longitudinal, consisting of Fig. 13.-Vertical sec
tion through a syn-
two halves of the frontal bone, and the
and the membranous vault, the direction of growth
the basi-cranial syn-
of sutures; its
growth, in breadth,
to the median longinot, as a rule, represented in the human carpus, but its homotype tudinal group, and is the os intermedium, found in many mammals.
The carpal | in height
tween the margins
crease in size. It
membrane is preGENERAL OLSERVATIONS ON THE ARTICULATORY AND
maturely ossified in
a particular locality,
Fig. 14.-Vertex view of a boat-shaped or scaph
occasioned. One of the most usual of these deformities is
stoppage of the growth of the skull in breadth, and, by pivot joint, in which the movement takes place about the way of compensation, great increase in its length, so as to axis of one of the bones, which is the axis of rotation of produce a very elongated and somewhat boat-shaped the joint ; examples of this joint are found in the joint cranium.
between the atlas and odontoid process of the axis and The movable joints are divided into the amphiarthrodial the radio-ulnar joint. Another form is the ginglymus or and the diarthrodial joints. An amphiarthrosis or half-hinge joint, in which the axis of rotation of the joint is joint has only a feeble range of
perpendicular to the axis of the two bones; the movemovement. It consists of two
ments of the hinge are called flexion when the angle bones, each of which has its
between the two bones is diminished, and extension when articular surface covered by a
the angle is increased. An important modification of the plate of cartilage, and which
ginglymus is the screwed-surfaced joint, examples of which plates are firmly connected to
are found in the elbow and ankle ; here the plane of gether by an intermediate disc of
flexion is not perpendicular, but oblique to the axis of the fibro-cartilage. The centre of this Fig. 15.–Vertical section through
joint. The saddle-shaped and oblong joints are also modidisc is soft, or may even be
an amphiarthrodial joint. b, b, fied hinges, but allow motion about two axes ; in the
the two bones; C, C, the plate of hollowed out into a ity, lined cartilage on the articular sur
oblong both axes are on the same side of the joint; but in by a smooth synovial membrane, mediate fibro- cartilage; hy h
the saddle-shaped there is an axis of rotation on each side of and containing a little fluid. the external ligaments. the joint. The best example of the saddle-shaped is found Ligamentous bands, continuous with the periosteum invest between the metacarpal bone of the thumb and the trapeing the bones, invest the fibro-cartilage, and assist in bind uam; of the oblong between the fore-arm and the carpus. ing the bones together. The best examples of amphi- In the ball-and-socket joint a spheroidal head fits into a arthrodial joints are furnished by the articulations between cup, and rotation takes place about any diameter of the the bodies of the true vertebræ.
sphere; the joint therefore is multi-axial ; the hip and A diarthrosis admits of more or less perfect movement.
shoulder joints are the best examples. Some joints, in In it the two articular surfaces are each covered by a plate
which the forms of the articular surfaces are more complex, of encrusting cartilage, the free
are called composite; in them the movements of a hinge surface of which is smooth and
and of a ball-and-socket joint may be combined; the knee polished ; between these surfaces
may be cited as an example of this form of articulation. is a cavity containing a glairy
In a large number of movable joints only portions of the fluid, the synovia, for lubricating
opposite articular surfaces are in contact with each other the smooth surfaces of the cartil
at a given time; but, as the joint describes its path of age and facilitating the movements
movement, different parts of the surfaces come into contact of the joint. This cavity is en
with each other successively, and it is not unusual to find closed by ligaments, which are
the articular surface both of the cartilage and the subattached to the bones, and the
jacent bone mapped out into distinct areas or facets, which inner surface of these ligaments
are adapted to corresponding facets on the opposite artiis lined by a synovial membrane
cular surface in particular positions of the joint. When which secretes the synovia. Some- Fig.16.–Vertical section through
the corresponding facets on opposite articular surfaces times a plate or meniscus of fibro a diarthrodial joint. b, b, the break contact with each other, the space between becomes
two bones; c, C, the plate of cartilage is interposed between, cartilage on the articular sur occupied by synovia, or in some joints, more especially the without, however, being attached
knee, by folds of synovial membrane enclosing clumps of
vesting ligament, the dotted to the encrusting cartilages of a
fat, which have been called synovial pads. In the simple diarthrodial joint, so as more or letters is placed in the cavity hinge, in that with screwed surfaces, in the oblong and less perfectly to subdivide the of the joint.
composite joints, the principal ligaments are situated at cavity enclosed by the ligaments into two spaces. The the sides of the joint, and are called lateral ; they not only articular surfaces of diarthrodial joints are retained in prevent lateral displacement of the bones, but, by a apposition with each other, some
tightening of their fibres, check excessive movement fortimes by investing ligaments, at
wards or backwards during flexion and extension. In the others by surrounding muscles and
saddle-shaped and ball-and-socket joints, the joint is intendons; at others by atmospheric
cluded within a bag-like ligament called capsular. In the pressure, aided by the adhesive
pivot joints the cavity in which the pivot fits is completed character of the interposed synovia.
by a transverse or a ring-shaped ligament. The form of the articular or mov
The Muscles are the organs which, by their contraction Muscles. able surfaces varies very materi
or shortening, move the bones on each other at the joints. ally in different examples of these
The muscles constitute the flesh of the body. They are so joints, and the modifications in
arranged as to be capable not only of moving the various form determine the direction of
bones on each other, but the entire body from place to the movements of the joints. In
place. Hence the muscles are organs both of motion and some, as the carpal and tarsal
locomotion. As they can be brought into action at the
Fig.17.-Vertical section through joints, the surfaces are almost flat, a diarthrodial joint, in which will of the individual, they are called voluntary muscles. so that they glide on each other; two by an interposed fibro-car
Some of the muscles are engaged in the movement of other the movement is comparatively tilage or meniscus, Fc. The structures than the bones, such as the eye-ball, tongue, slight, and about an axis perpen
other letters as in Fig. 16.
cartilages of the larynx, &c. About 400 muscles are dicular to the moving surfaces : these are called plane usually enumerated, and the names applied to them express surfaced joints. In other joints the articular surfaces may either their position, or relative size, or shape, or direction, be regarded as produced by the rotation of a straight or or attachments, or mode of action. The word muscle is curved line about an axis lying in the same plane; these itself derived from the Latin musculus, a little mouse, from are called rotation joints, and they present various modifica- a fancied resemblance between that animal and some of tho tions according to the direction and relation of the rotat most simply formed muscles. It is customary to dising line to the axis. One form of a rotation joint is the tinguish in a muscle a central part, or belly, and two ex
face of each bone; I, I, the in
line within which represents