With regard to the homologue of the pubic element in the pectoral girdle, there is much difference of opinion; in reptiles and amphibia it corresponds most closely to the precoracoid, but it is doubtful what represents it in mammals. According to Goette and Hoffman, the clavicle is a primordial bone, and not, as suggested by Gegenbaur, of secondary or dermic origin. If this be so, it corresponds to the ventral anterior segment of the pectoral girdle, and is therefore homologous with the ventral anterior (pubic) segment of the pelvic girdle. On the other hand, if Gegenbaur's view be accepted, the clavicle has no representative in the pelvic girdle. It must, however, be borne in mind that during its ossification it is intimately associated with cartilage, and that that cartilage may represent the precoracoid bar; nor must too great stress be laid upon the fact that the clavicle begins to ossify before it is preformed in cartilage, since that may be merely a modification in its histogenetic development.

According to another view (Sabatier), the subcoracoid centre (see Ossification of Scapula) is derived from the posterior ventral segment, and corresponds to the ischium, whilst the coracoid process is the remains of the anterior ventral segment (precoracoid), and is homodynamous with the pubis.

In no part of the skeleton does function react so much on structure as in the arrangement of the constituent parts of the pectoral or pelvic girdles. In man, owing to the assumption of the erect position and the bipedal mode of progression, the pelvic girdle acquires those characteristics which are essentially human, viz., its great relative breadth and the expansion of its iliac portions, which serve as a support to the abdominal viscera, and also furnish an extensive origin for the powerful muscles which control the movements of the hip-joint. The stability of the

FIG. 285.-DIAGRAM TO ILLUSTRATE THE HOMOLOGOUS PARTS OF THE SCAPULA AND ILIUM

ACCORDING TO FLOWER..

A, ideal type; three-sided rod. B, scapula rotated forward through quarter of a circle (90°), so that the primitive medial or vertebral surface is now directed anteriorly. C, ilium rotated backwards through quarter of a circle so that the primitive medial surface is now turned posteriorly. In the diagram the primitive medial or vertebral surface of each figure is coloured black, the pre-axial surfaces red, and the post-axial surfaces blue.

pelvic girdle is insured by the nature of its union with the axial skeleton, as well as by the osseous fusion of its several parts, and their union in front at the symphysis pubis.

Various attempts have been made to homologise the several parts of the ilium and scapula. All are open to objection; that by Flower is perhaps the most generally accepted. Assuming that the primitive type is represented by a prismatic rod, of which the dorsal end represents either the epiphysial border of the vertebral edge of the scapula or the iliac crest, whilst the ventral end corresponds to the glenoid or acetabular articular areas respectively, the surfaces of the three-sided rod are disposed so that one is vertebral or medial, another pre-axial, and the third post-axial. These surfaces are separated by borders, of which one is lateral, separating the pre-axial and post-axial surfaces, whilst the antero-medial and postero-medial margins separate the pre-axial and post-axial surfaces respectively from the vertebral or medial aspect. It is a necessity of Flower's theory that this part of the girdle undergoes a rotation along with the rest of the limb. Thus in the fore limb the surfaces of the primitive type are turned so that the vertebral surface looks forward, whilst in the case of the hind limb the vertebral surface is turned backward. A study of the accompanying diagram will enable the reader to realise how the ventral surface of the scapula is thus rendered homologous with the gluteal surface of the ilium, for by reference to the type, both these surfaces will be seen to correspond to the postaxial areas of the primitive condition. In accordance with this view the surfaces and borders of the scapula are homologised by Flower, as shewn in the subjoined table :—

BORDERS

Axillary border, posterior on most animals (attachment of triceps muscle)

1. Lateral

Spine continued into acromion 2. Antero-medial

Superior border, anterior in 3. Postero-medial
most animals, with scapulo-
coracoid notch

Anterior border (attachment of rectus muscle)

Linea arcuata interna continued into pubis

Posterior border with greater sciatic notch

Flower's views of this matter were strenuously opposed by Humphry, who maintained that there is strong presumptive evidence against any rotation of the superior parts of the girdles, since it is difficult to suppose that the scapula and ilium can undergo a rotation which is not participated in by the coracoid and ischium. According to this anatomist the homologous parts of the two bones are as stated below :—

FIG. 286.-DIAGRAM TO ILLUSTRATE THE HOMOLOGOUS PARTS OF THE SCAPULA AND ILIUM,
ACCORDING TO HUMPHRY.

A, primitive rod-like ilium of kangaroo, prismatic on section. B, scapula. C, ilium. The corresponding surfaces are similarly coloured.

The difficulty arising in this scheme of attempting to homologise the attachments of the triceps and rectus femoris, Humphry explains by pointing out that the former muscle also arises from the lateral surface of the scapula, whilst the rectus overruns the lateral surface of the ilium above the acetabulum, so that there is a correspondence in the origins of both these muscles from the lateral surface of their respective bones; but in consequence of the rotation of the extensor surfaces of the limbs in opposite directions the triceps has been turned backwards on to the posterior border of the scapula, whilst the rectus has been turned forwards on to the anterior border of the ilium. Sufficient has been said to enable the reader to recognise that all attempts to determine in detail the homologies of these parts are beset with difficulty. It is wiser, therefore, in our present state of knowledge to be content with establishing a general correspondence, and so avoid the error of endeavouring to establish a closer homological relationship than actually exists.

In man, since the erection of the figure no longer necessitates the use of the fore limb as a means of support, the shoulder girdle has become modified along lines which enhance its mobility and determine its utility in association with a prehensile limb. Some of its parts remain independent (clavicle and scapula), and are united by diarthrodial joints, whilst others have

become much reduced in size or suppressed (coracoid, precoracoid, see ante). The dorsal part of the girdle (scapula) is not directly united with the axial skeleton as is the ilium, but is only indirectly joined to it through the medium of the clavicle, which is linked in front with the presternum. The same underlying principles determine the differences in mobility and strength between the shoulder, elbow, and wrist, and the hip, knee, and ankle joints of the fore and hind limbs respectively, whilst the utility of the hand is further enhanced by the movements of pronation and supination which occur between the bones of the forearm. In the leg such movements are absent, as they would interfere with the stability of the limb.

THE ARTICULATIONS OR JOINTS.

SYNDESMOLOGY.

By DAVID HEPBURN.

Syndesmology is that branch of human anatomy which treats of the articulations or joints.

A junctura ossium (articulation or joint) constitutes a mode of union or connexion subsisting between any two separate segments or parts of the skeleton, whether osseous or cartilaginous. It has for its primary object either the preservation of a more or less rigid continuity of the parts joined together, or else the permission of a variable degree of mobility, subject to the restraints of the uniting media.

Classification of Joints. In attempting to frame a classification of the numerous joints in the body, several considerations must be taken into account, viz., the manner and sequence of their appearance in the embryo; the nature of the uniting media in the adult, and also the degree and kind of movement permitted in those joints where movement is possible.

In this way we obtain two main subdivisions of joints :

(1) Those in which the uniting medium is co-extensive with the opposed surfaces of the bones entering into the articulation, and in which a direct union of these surfaces is thereby effected.

(2) Those in which the uniting medium has undergone more or less of interruption in its structural continuity, and in which a cavity of greater or less extent is thus formed in the interior of the joint.

To the first group belong all the immovable joints, many of which are only of temporary duration; to the second group belong all joints which possess, as their outstanding features, mobility and permanence.

SYNARTHROSES.

Intervening membrane

The general characteristics of this group are partly positive and partly negative. Thus, there is uninterrupted union between the opposed surfaces of the bones joined together at the plane of the articulation, i.e. there is no trace of a joint cavity, and further, there is an entire absence of movement. Developmentally, these joints result from the approximation of ossific processes which have commenced from separate centres of ossification, and therefore the nature. of the uniting medium varies according as the bones thus joined together have originally ossified in membrane. or in cartilage. In the former case union is effected by an interposed fibrous membrane continuous with, and corresponding to, the periosteum. To such articulations. the term sutura (Fig. 287) is applied. In the latter case the uniting medium is a plate of hyaline cartilage. Such articulations are called synchondroses (Fig. 288). In all the synchondroses, and in many of the sutures, the uniting medium tends to disappear in the progress of

FIG. 287.-VERTICAL SECTION
THROUGH A SUTURE.

ossification, and thus the plane of articulation becomes obliterated, so that direct structural continuity between the osseous segments takes place. The primary features common to all synarthroses are (a) continuous and direct union of the opposing surfaces; (b) no joint cavity; (c) no movement.

Intervening -hyaline cartilage

FIG. 288.-SECTION THROUGH
THE OCCIPITO-SPHENOID SYN-

CHONDROSIS.

Sutura.--This form of synarthrosis is found only in connexion with the bones of the skull. In a large number of cases the bones which articulate by suture present irregular interlocking margins, between which there is the interposed fibrous membrane to which reference has already been made. When these interlocking margins present well-defined projections they are said to form a sutura vera (true suture); on the other hand, when the opposed surfaces present ill-defined projections, or even flat areas, they are described as sutura notha (false suture). In each of these subdivisions the particular characters of the articulating margins are utilised in framing additional descriptive terms. Thus true sutures may possess interlocking margins whose projections are tooth-like (sutura dentata), e.g. in the interparietal suture; saw-like (sutura serrata) (Fig. 289), e.g. in the interfrontal suture; ridge-like, or comparable to the parallel ridges on the welt of a boot (sutura limbosa). Similarly false sutures may articulate by margins which are scalelike (sutura squamosa), e.g. in the squamoso-parietal suture; or by rough opposed surfaces, sutura harmonia, e.g. in the suture between the palatine processes of the maxillary bones. There is one variety of synarthrosis which, in the adult, can scarcely be called a suture, although the differences are of minor importance, viz., schindylesis, which is an articulation between the edge of a platelike bone, such as the rostrum of the sphenoid, and the cleft in another, such as the vomer.

SERRATA.

Synchondrosis.-Illustrations of this group can be found only FIG. 289.-SUTURA in the young growing individual, because as age advances and growth ceases, the process of ossification affects the hyaline cartilage which constitutes the uniting medium, and the plane of articulation disappears. Under this heading we may include the planes of junction between all epiphyses and the diaphyses to which they severally belong. The occipito-sphenoid (Fig. 288) and the petro-jugular articulations in the base of the skull provide other well-marked examples.

AMPHIARTHROSES-DIARTHROSES (MOVABLE JOINTS).

The leading features of this group are capability of movement and permanence. In very few instances do such joints ever become obliterated under normal conditions. Determining their permanence, and regulating the amount of possible movement, there is always more or less of interruption in the continuity of the structures which bind the osseous elements together. That is, there is always some evidence of a cavum articulare (joint cavity), although as a matter of course such interruption can never be so extensive as to entirely disassociate the articulating elements. Therefore in all movable joints a new class of structures is found, viz., ligamenta (the ligaments), by means of which continuity is maintained even when all the other uniting media have given place to an articular cavity. The further subdivision of this group is founded upon the amount of movement permissible, and the extent to which the articular cavity takes the place of the original continuous uniting medium. Thus we obtain the amphiarthroses, or partly movable, and the diarthroses, or freely movable.

An amphiarthrosis (Fig. 292) presents the following characteristics: (a) partial movement; (b) union by ligaments and by an interposed plate or disc of fibro-cartilage, in the interior of which there is (c) an incomplete or partial joint cavity, which may be lined by a rudimentary stratum synoviale (synovial membrane)