ANATOMY

_Joints of the Axial Skeleton._

The bodies of the vertebrae except those of the sacrum and coccyx are

separated, and at the same time connected, by the _intervertebral

disks_. These are formed of alternating concentric rings of fibrous

tissue and fibro-cartilage, with an elastic mass in the centre known as

the _nucleus pulposus_. The bodies are also bound together by _anterior_

and _posterior common ligaments_. The odontoid process of the axis fits

into a pivot joint formed by the anterior arch of the atlas in front and

the _transverse ligament_ behind; it is attached to the basioccipital

bone by two strong _lateral check ligaments_, and, in the mid line, by a

feebler _middle check ligament_ which is regarded morphologically as

containing the remains of the notochord. This _atlanto-axial joint_ is

the one which allows the head to be shaken from side to side. Nodding

the head occurs at the _occipito-atlantal joint_, which consists of the

two occipital condyles received into the cup-shaped articular facets on

the atlas and surrounded by capsular ligaments. The neural arches of the

vertebrae articulate one with another by the _articular facets_, each of

which has a capsular ligament. In addition to these the laminae are

connected by the very elastic _ligamenta subflava_. The spinous

processes are joined by _interspinous ligaments_, and their tips by a

_supraspinous ligament_, which in the neck is continued from the spine

of the seventh cervical vertebra to the external occipital crest and

protuberance as the _ligamentum nuchae_, a thin, fibrous, median septum

between the muscles of the back of the neck.

The combined effect of all these joints and ligaments is to allow the

spinal column to be bent in any direction or to be rotated, though only

a small amount of movement occurs between any two vertebrae.

The heads of the ribs articulate with the bodies of two contiguous

thoracic vertebrae and the disk between. The ligaments which connect

them are called _costo-central_, and are two in number. The anterior of

these is the _stellate ligament_, which has three bands radiating from

the head of the rib to the two vertebrae and the intervening disk. The

other one is the _interarticular ligament_, which connects the ridge,

dividing the two articular cavities on the head of the rib, to the disk;

it is absent in the first and three lowest ribs.

The _costo-transverse ligaments_ bind the ribs to the transverse

processes of the thoracic vertebrae. The _superior costo-transverse

ligament_ binds the neck of the rib to the transverse process of the

vertebra above; the _middle_ or _interosseous_ connects the back of the

neck to the front of its own transverse process; while the _posterior_

runs from the tip of the transverse process to the outer part of the

tubercle of the rib. The inner and lower part of each tubercle forms a

diarthrodial joint with the upper and fore part of its own transverse

process, except in the eleventh and twelfth ribs. At the junction of the

ribs with their cartilages no diarthrodial joint is formed; the

periosteum simply becomes perichondrium and binds the two structures

together. Where the cartilages, however, join the sternum, or where they

join one another, diarthrodial joints with synovial cavities are

established. In the case of the second rib this is double, and in that

of the first usually wanting. The _mesosternal joint_, between the pre-

and mesosternum, has already been given as an example of a symphysis.

_Comparative Anatomy._--For the convexity or concavity of the

vertebral centra in different classes of vertebrates, see SKELETON:

_axial_. The intervertebral disks first appear in the Crocodilia, the

highest existing order of reptilia. In many Mammals the middle

fasciculus of the stellate ligament is continued right across the

ventral surface of the disk into the ligament of the opposite side,

and is probably serially homologous with the ventral arch of the

atlas. A similar ligament joins the heads of the ribs dorsal to the

disk. To these bands the names of anterior (ventral) and posterior

(dorsal) _conjugal ligaments_ have been given, and they may be

demonstrated in a seven months' human foetus (see B. Sutton,

_Ligaments_, London, 1902). The _ligamentum nuchae_ is a strong

elastic band in the Ungulata which supports the weight of the head. In

the Carnivora it only reaches as far forward as the spine of the axis.

The JAW JOINT, or _temporo-mandibular articulation_, occurs between the

sigmoid cavity of the temporal bone and the condyle of the jaw. Between

the two there is an interarticular fibro-cartilage or meniscus, and the

joint is surrounded by a capsule of which the outer part is the

thickest. On first opening the mouth, the joint acts as a hinge, but

very soon the condyle begins to glide forward on to the eminentia

articularis (see SKULL) and takes the meniscus with it. This gliding

movement between the meniscus and temporal bone may be separately

brought about by protruding the lower teeth in front of the upper, or,

on one side only, by moving the jaw across to the opposite side.

_Comparative Anatomy._--The joint between the temporal and mandibular

bones is only found in Mammals; in the lower vertebrates the jaw opens

between the quadrate and articular bones. In the Carnivora it is a

perfect hinge; in many Rodents only the antero-posterior gliding

movement is present; while in the Ruminants the lateralizing movement

is the chief one. Sometimes, as in the Ornithorhynchus, the meniscus

is absent.

_Joints of the Upper Extremity._

The _sterno-clavicular articulation_, between the presternum and

clavicle, is a gliding joint, and allows slight upward and downward and

forward and backward movements. The two bony surfaces are separated by a

meniscus, the vertical movements taking place outside and the

antero-posterior inside this. There is a well-marked capsule, of which

the anterior part is strongest. The two clavicles are joined across the

top of the presternum by an _interclavicular ligament_.

The _acromio-clavicular articulation_ is also a gliding joint, but

allows a swinging or pendulum movement of the scapula on the clavicle.

The upper part of the capsule is strongest, and from it hangs down a

partial meniscus into the cavity.

_Comparative Anatomy._--Bland Sutton regards the interclavicular

ligament as a vestige of the interclavicle of Reptiles and Monotremes.

The menisci are only found in the Primates, but it must be borne in

mind that many Mammals have no clavicle, or a very rudimentary one. By

some the meniscus of the sterno-clavicular joint is regarded as the

homologue of the lateral part of the interclavicle, but the fact that

it only occurs in the Primates where movements in different planes are

fairly free is suggestive of a physiological rather than a

morphological origin for it.

The SHOULDER JOINT is a good example of the ball and socket or

enarthrodial variety. Its most striking characteristic is mobility at

the expense of strength. The small size of the glenoid cavity in

comparison with the head of the humerus, and the great laxity of the

capsule, favour this, although the glenoid cavity is slightly deepened

by a fibrous lip, called the _glenoid ligament_, round its margin. The

presence of the coracoid and acromial processes of the scapula, with the

_coraco-acromial ligament_ between them, serves as an overhanging

protection to the joint, while the biceps tendon runs over the head of

the humerus, inside the capsule, though surrounded by a sheath of

synovial membrane. Were it not for these two extra safeguards the

shoulder would be even more liable to dislocation than it is. The upper

part of the capsule, which is attached to the base of the coracoid

process, is thickened, and known as the _coracohumeral ligament_, while

inside the front of the capsule are three folds of synovial membrane,

called _gleno-humeral folds_.

_Comparative Anatomy._--In the lower Vertebrates the shoulder is

adapted to support rather than prehension and is not so freely movable

as in the Primates. The tendon of the biceps has evidently sunk

through the capsule into the joint, and even when it is intra-capsular

there is usually a double fold connecting its sheath of synovial

membrane with that lining the capsule. In Man this has been broken

through, but remains of it persist in the _superior gleno-humeral

fold_. The _middle gleno-humeral fold_ is the vestige of a strong

ligament which steadies and limits the range of movement of the joint

in many lower Mammals.

The ELBOW JOINT is an excellent example of the ginglymus or hinge,

though its transverse axis of movement is not quite at right angles to

the central axis of the limb, but is lower internally than externally.

This tends to bring the forearm towards the body when the elbow is bent.

The elbow is a great contrast to the shoulder, as the trochlea and

capitellum of the humerus are closely adapted to the sigmoid cavity of

the ulna and head of the radius (see SKELETON: _appendicular_);

consequently movement in one plane only is allowed, and the joint is a

strong one. The capsule is divided into anterior, posterior, and two

lateral ligaments, though these are all really continuous. The joint

cavity communicates freely with that of the superior radio-ulnar

articulation.

The _radio-ulnar joints_ are three: the upper one is an example of a

pivot joint, and in it the disk-shaped head of the radius rotates in a

circle formed by the lesser sigmoid cavity of the ulna internally and

the _orbicular ligament_ in the other three quarters.

The _middle radio-ulnar articulation_ is simply an interosseous

membrane, the fibres of which run downward and inward from the radius to

the ulna.

The _inferior radio-ulnar joint_ is formed by the disk-shaped lower end

of the ulna fitting into the slightly concave sigmoid cavity of the

radius. Below, the cavity of this joint is shut off from that of the

wrist by a _triangular fibro-cartilage_. The movements allowed at these

three articulations are called pronation and supination of the radius.

The head of that bone twists, in the orbicular ligament, round its

central vertical axis for about half a circle. Below, however, the whole

lower end of the radius circles round the lower end of the ulna, the

centre of rotation being close to the styloid process of the ulna. The

radius, therefore, in its pronation, describes half a cone, the base of

which is below, and the hand follows the radius.

_Comparative Anatomy._--In pronograde Mammals the forearm is usually

permanently pronated, and the head of the radius, instead of being

circular and at the side of the upper end of the ulna, is transversely

oval and in front of that bone, occupying the same place that the

coronoid process of the ulna does in Man. This type of elbow, which is

adapted simply to support and progression, is best seen in the

Ungulata; in them both lateral ligaments are attached to the head of

the radius, and there is no orbicular ligament, since the shape of the

head of the radius does not allow of any supination. The olecranon

process of the ulna forms merely a posterior guide or guard to the

joint, but transmits no weight. No better example of the maximum

changes which the uses of support and prehension bring about can be

found than in contrasting the elbow of the Sheep or other Ungulate

with that of Man. Towards one or other of these types the elbows of

all Mammals tend. It may be roughly stated that, when pronation and

supination to the extent of a quarter of a circle are possible, an

orbicular ligament appears.

The WRIST JOINT, or _radio-carpal articulation_, lies between the radius

and triangular fibro-cartilage above, and the scaphoid, semilunar, and

cuneiform bones below. It is a condyloid joint allowing flexion and

extension round one axis, and slight lateral movement (abduction and

adduction) round the other. There is a well-marked capsule, divided into

anterior, posterior, and lateral ligaments. The joint cavity is shut off

from the inferior radio-ulnar joint above, and the intercarpal joints

below.

The _intercarpal joints_ are gliding articulations, the various bones

being connected by palmar, dorsal, and a few interosseous ligaments, but

only those connecting the first row of bones are complete, and so

isolate one joint cavity from another. That part of the intercarpal

joints which lies between the first and second rows of carpal bones is

called the _transverse carpal joint_, and at this a good deal of the

movement which seems to take place at the wrist really occurs.

The _carpo-metacarpal articulations_ are, with the exception of that of

the thumb, gliding joints, and continuous with the great intercarpal

joint cavity. The carpo-metacarpal joint of the thumb is the best

example of a saddle-shaped joint in Man. It allows forward and backward

and lateral movement, and is very strong.

The _metacarpo-phalangeal joints_ are condyloid joints like the wrist,

and are remarkable for the great thickness of the palmar ligaments of

their capsules. In the four inner fingers these _glenoid ligaments_, as

they are called, are joined together by the _transverse metacarpal

ligament_.

The _interphalangeal articulations_ are simple hinges surrounded by a

capsule, of which the dorsal part is very thin.

_Comparative Anatomy._--The wrist joint of the lower Mammals allows

less lateral movement than does that of Man, while the lower end of

the ulna is better developed and is received into a cup-shaped socket

formed by the cuneiform and pisiform bones. At the same time, unless

there is pretty free pronation and supination, the triangular

fibro-cartilage is only represented by an interosseous ligament, which

may be continuous above with the interosseous membrane between the

radius and ulna, and suggests the possibility that the fibro-cartilage

is largely a derivative of this membrane. In most Mammals the wrist is

divided into two lateral parts, as it is in the human foetus, but free

pronation and supination seem to cause the disappearance of the

septum.

_Joints of the Lower Extremity._

The _sacro-innominate articulation_ consists of the _sacro-iliac joint_

and the _sacro-sciatic ligaments_. The former is one of the

amphiarthroses or half-joints by which the sacrum is bound to the ilium.

The mechanism of the human sacrum is that of a suspension bridge slung

between the two pillars or ilia by the very strong _posterior

sacro-iliac_ ligaments which represent the chains. The axis of the joint

passes through the second sacral vertebra, but the sacrum is so nearly

horizontal that the weight of the body, which is transmitted to the

first sacral vertebra, tends to tilt that part down. This tendency is

corrected by the great and small _sacro-sciatic ligaments_, which

fasten the lower part of the sacrum to the tuberosity and spine of the

ischium respectively, so that, although the sacrum is a suspension

bridge when looked at from behind, it is a lever of the first kind when

seen from the side or in sagittal section.

The _pubic symphysis_ is the union between the two pubic bones. It has

all the characteristics of a symphysis, already described, and may have

a small median cavity.

[Illustration: (From David Hepburn, Cunningham's _Text-book of

Anatomy_.)