FIG. 118.

The occipito-axoid ligament (apparatus ligamentosus colli) is a broad and strong band, which covers in the odontoid process and its ligaments. It is attached below to the body of the axis, where it is continuous with the posterior common ligament; superiorly it is inserted by a broad expansion, into the basilar groove of the occipital bone. It is firmly connected opposite the body of the axis, with the dura mater; and is sometimes described as consisting of a central and two lateral portions.

A

The odontoid ligaments (alaria) are two short and thick fasciculi, which pass outwards from the apex of the odontoid process, to the sides of the occipital foramen and condyles. third and smaller fasciculus (ligamenmentum dentis suspensorium) proceeds from the apex of the odontoid process to the anterior margin of the foramen magnum.

These ligaments serve to limit the extent to which rotation of the head can be carried, hence they are termed check [or moderator] ligaments.

UPPER PART OF THE VERTEBRAL CANAL,

OPENED FROM BEHIND IN ORDER TO SHOW THE OCCIPITO-AXOID LIGAMENT. 1. Basilar portion of the sphenoid bone. 2. Section of the occipital bone. 3. Atlas, its posterior arch removed. 4. Axis, posterior arch removed. 5. Occipitoaxoid ligament, rendered prominent at its middle by the projection of the odontoid process. 6. Lateral and capsular ligament of the occipitoatloid articulation. 7. Capsular ligament of the articular process of the atlas and axis.

4. Articulation of the Atlas with the Axis. - The ligaments of this articulation are five in number,

Anterior atlo-axoid,

Posterior atlo-axoid,
Two capsular,
Transverse.

The anterior ligament consists of ligamentous fibres, which pass from the anterior tubercle and arch of the atlas to the base of the odontoid process and body of the axis, where they are continuous with the commencement of the anterior common ligament.

The posterior ligament is a thin, membranous layer, passing between the posterior arch of the atlas and the laminæ of the axis.

The capsular ligaments surround the articular processes of the atlas and axis; they are loose, to permit of the free movement which subsists between these vertebræ. The ligamentous fibres are most numerous on the outer and anterior part of the articulation; and the synovial membrane usually communicates with the synovial cavity between the transverse ligament and odontoid process.

POSTERIOR VIEW OF THE LIGAMENTS CONNECTING THE ATLAS, AXIS, AND OCCIPITAL BONE.

The posterior part of the occipital bone has been sawn away, and the arches of the atlas

and axis removed. 1. Superior part of the occipito-axoid ligament, which has been cut away in order to show the ligaments beneath. 2. Transverse ligament of the atlas. 3, 4. Ascending and descending slips of the transverse ligament, which have obtained for it the title of cruciform ligament. 5. One of the odontoid the opposite side. 6. One of the occipito-atloid ligaments; the fellow ligament is seen on the

capsular ligaments. 7. One of the atlo-axoid capsular ligaments.

The transverse ligament is a strong ligamentous band, which arches across the area of the ring of the atlas from a rough tubercle on the inner surface of one

articular process to a corresponding tubercle on the other; and serves to retain the odontoid process of the axis, in connexion with the anterior arch of the atlas. As it crosses the neck of the odontoid process, some fibres are sent downwards to be attached to the body of the axis, and others pass upwards to be inserted into the basilar process of the occipital bone; hence the ligament has a cross-like appearance, and is termed cruciform. A synovial membrane is situated between the transverse ligament and odontoid process; and another between that process and the anterior arch of the atlas.

Actions. It is the peculiar disposition of the transverse ligament in relation to the odontoid process, that enables the atlas, and with it the entire cranium, to rotate upon the axis; the perfect freedom of movement between these bones being insured by the two synovial membranes. The lower part of the ring formed by the transverse ligament with the atlas, is smaller than the upper, while the summit of the odontoid process is larger than its base; hence the process is retained in its place by the transverse ligament, when the other ligaments are cut through. The extent to which rotation of the head upon the axis can be carried, is determined by the odontoid or check ligaments. The odontoid process with its ligaments is covered in by the occipito-axoid ligament.

5. Articulation of the Lower Jaw. [This articulation is formed, at each side, by one of the condyles of the inferior maxillary bone below, and the anterior part of the glenoid cavity, and the inferior or articular root of the zygoma of the corresponding temporal bone above.] The lower jaw has properly but one ligament, the external lateral; the ligaments usually described are three in number; to which may be added, as appertaining to the mechanism of the joint, an interarticular fibrous cartilage and two synovial membranes, thus:

External lateral,
Internal lateral,

Capsular,

Interarticular fibrous cartilage.

Two synovial membranes.

The external lateral ligament is a short and thick band of fibres, passing obliquely backwards from the tubercle of the zygoma, to the external surface

FIG. 120.

3

ARTICULA

EXTERNAL VIEW OF THE TION OF THE LOWER JAW. 1. Zygomatic arch. 2. Tubercle of the zygoma. 3. Ramus of the lower jaw. 4. Mastoid portion of the temporal bone. 5. External lateral ligament. 6. Stylomaxillary ligament.

of the neck of the lower jaw. It is in relation externally, with the integument of the face; internally, with the two synovial membranes of the articulation and the interarticular cartilage. The external lateral ligament acts conjointly with its fellow of the opposite condyle in the movements of the jaw.

The internal lateral ligament (sphenomaxillary) has no connexion with the articulation of the lower jaw, and is, therefore, incorrectly named; it is a thin aponeurotic expansion, extending from the extremity of the spinous process of the sphenoid bone to the margin of the dental foramen; and is pierced at its insertion by the mylo-hyoidean nerve.

Between the internal lateral ligament and neck of the jaw is a triangular space, in which are situated the internal maxillary artery, auriculo-temporal nerve, inferior dental artery and nerve, and part of the external pterygoid muscle; internally the ligament is in relation with the internal pterygoid muscle.

The capsular ligament (membrana maxillæ articularis) consists of numerous

irregular ligamentous fibres, which pass from the edge of the glenoid cavity to the neck of the lower jaw, and surround the articulation; on the inner side of the joint they form a pretty strong fasci

culus.

The interarticular fibrous cartilage (cartilago intermedia) is a thin oval plate, thicker at the edges than in the centre, placed horizontally between the head of the condyle of the lower jaw and the glenoid cavity. It is connected by its outer border with the external lateral ligament, and receives in front some fibres of insertion of the external pterygoid muscle. It divides the joint into two cavities, one being above, the other below the cartilage; but it is occasionally incomplete in the centre.

The synovial membranes are situated, one above, the other below the fibrous cartilage, the former being the larger of the two. When the fibrous cartilage is perforate, the synovial membranes communicate.

Besides the lower jaw, there are

several other joints provided with a complete interarticular cartilage, and, consequently, with two synovial membranes; they are, the sterno-clavicular, acromio-clavicular, and the articulation of the

ulna with the cuneiform bone. The interarticular fibrous cartilages of the kneejoint are partial, and there is but one synovial membrane.

The articulations of the heads of the ribs with the vertebræ have two synovial membranes separated by an interarticular ligament without fibrous cartilage.

Connected with the lower jaw, though. not with the joint, is the stylo-maxillary ligament, a process of the deep cervical fascia extended between the point of the styloid process and the angle of the jaw. It is attached to the jaw between the insertions of the masseter and internal pterygoid muscle, and separates the parotid from the submaxillary gland.

Actions.-The movements of the lower jaw are depression, by which the mouth is opened, elevation, by which it is closed; a forward and backward movement, and a movement from side to side.

FIG. 122.

IN THIS SKETCH A SECTION HAS BEEN MADE THROUGH THE JOINT TO SHOW THE POSITION OF THE INTERARTICULAR FIBRO

CARTILAGE, and the manner of its adaptation to the articulating surfaces. 1. Glenoid fossa. 2. Eminentia articularis. 3. Interarticular fibro-cartilage. 4. Superior syno

vial cavity. 5. Inferior synovial cavity. 6. An interarticular fibro-cartilage, removed from the joint, in order to show its oval and concave form; it is seen from below.

In the movement of depression the interarticular cartilage glides forwards on the eminentia articularis, carrying with it the condyle. If this movement be carried too far, the superior synovial membrane is ruptured, and dislocation of the fibrocartilage with its condyle into the zygomatic fossa occurs. In elevation the fibrous cartilage and condyle are returned to their original position. The forward and backward movement is a gliding of the fibro-cartilage on the glenoid articular

surface, in the antero-posterior direction; and the movement from side to side, in the lateral direction.

6. Articulation of the Ribs with the Vertebra. -The ligaments of these articulations are remarkable for their strength, being in fact so strong as to render dislocation impossible; the neck of the rib must break before displacement could occur; they are divisible into two groups: -1. Those connecting the head of the rib with the bodies of the vertebræ; and, 2. Those connecting the neck and tubercle of the rib with the transverse processes.

1st Group.

Anterior costo vertebral or stellate,
Capsular,

Interarticular ligament,

Two synovial membranes.

2d Group.

Anterior costo-transverse,
Middle costo-transverse,
Posterior costo-transverse.

The anterior costo-vertebral or stellate ligament (lig. capituli costarum, fig. 111) consists of three short bands of ligamentous fibres which radiate from the anterior part of the head of the rib. The superior band passes upwards to be attached to the vertebra above; the middle fasciculus is attached to the intervertebral substance; the inferior, to the vertebra below.

In the first, eleventh, and twelfth rib, the three fasciculi are attached to the body of the corresponding vertebra.

The capsular ligament is a thin layer of ligamentous fibres surrounding the joint in the interval left by the anterior ligament; it is thickest above and below the articulation, and protects the synovial membranes.

The interarticular ligament is a thin band which passes between the sharp crest on the head of the rib and the intervertebral substance. It divides the joint into two cavities, each being furnished with a separate synovial membrane

The first, eleventh, and twelfth rib have no interarticular ligament, and consequently but one synovial membrane.

The anterior costo-transverse ligament (lig. transversarium internum) is a broad band composed of two fasciculi, which ascend from the crest-like ridge on the neck of the rib, to the transverse process immediately above. This ligament separates the anterior from the dorsal branch of the intercostal nerve.

The middle costo-transverse ligament (lig. cervicis externum) is a strong interosseous ligament passing directly between the posterior surface of the neck of the rib, and the transverse process against which it rests. It is absent in the first and two inferior ribs.

The posterior costo-transverse ligament (lig. transversarium externum) is 3 small but strong fasciculus, passing obliquely from the tubercle of the rib to the apex of the transverse process. The articulation between the tubercle of the rib and the transverse process is provided with a small synovial membrane.

There is no anterior costotransverse ligament to the first or last rib; and only rudimentary posterior costo-transverse ligaments to the eleventh and twelfth rib. Several accessory

[FIG. 125.

SECTION OF ONE-HALF OF THE BODY, AND ONE TRANSVERSE PROCESS OF A DORSAL VERTEBRA, AND OF PART OF A RIB, IN SITU, to show the interosseous or middle costotransverse ligament. 1. Transverse process. 2. Neck of the rib. The middle costo-transverse ligament is seen between them. 3. The synovial membrane between the tubercle of the rib and the articular facet of the transverse process.]

bands (ligamenta accessoria) are found passing between the transverse processes and the ribs; a broad band of this kind connects the last rib with the transverse process of the first or second lumbar vertebra.

Actions. The movements permitted by the articulations of the ribs are upwards, downwards, and slightly forwards and backwards; the movement increasing in extent from the head to the extremity of the rib. The forward and backward movement is trifling in the seven superior, but greater in the inferior ribs; the eleventh and twelfth are very movable.

7. Articulation of the Ribs with the Sternum, and with each other. — The ligaments of the costo-sternal articulations are

Anterior costo-sternal,
Posterior costo-sternal,

Superior costo-sternal,
Inferior costo-sternal,

Synovial membranes.

The anterior costo-sternal ligament (lig. costo-sternale radiatum) is a thin band of ligamentous fibres, passing in a radiated direction from the extremity of the costal cartilage to the anterior surface of the sternum, and intermingling its fibres with those of the ligament of the opposite side, and with the tendinous fibres of origin of the pectoralis major muscle.

The posterior costo-sternal ligament, much smaller than the anterior, consists of a thin fasciculus of fibres, situated on the posterior surface of the articulation. The superior and inferior costo-sternal ligaments are narrow fasciculi corre sponding with the breadth of the cartilage, and connecting its superior and inferior border with the side of the sternum.

The synovial membrane is absent in the articulation of the first rib, its cartilage being usually adherent to the sternum; that of the second rib has an interarticular ligament, with two synovial membranes; the synovial membranes of the other costal cartilages are single.

The sixth and seventh ribs have several fasciculi of strong ligamentous fibres, passing from the extremity of their cartilages to the anterior surface of the ensi