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pre-axial (radial) side of the forearm is now directed forwards and somewhat laterally, whilst in the hind limb the pre-axial (tibial) side of the leg is turned backwards and laterally, the pre-axial borders of the hand with thumb, and foot with great toe being in correspondence. In consequence of these changes in the position of the limbs, amounting in all in the upper segments to a rotation through an angle of 90°, the extensor surface of the fore limb is directed backwards, whilst that of the hind limb is directed forwards. In order to homologise the arrangement of the bones in the extended limb, it is necessary to place them so that their flexor or extensor surfaces are similarly disposed. It will then be observed (see diagram) that the medial or tibial side of the leg and foot (primitively pre-axial) corresponds to the lateral or radial side of the forearm and hand (primitively pre-axial), whilst the fibula and lateral border of the foot homologise with the ulnar or medial border of the forearm and hand (primitively post-axial), the result, as previously explained, of the torsion or twisting in opposite directions through an angle of 90° of the upper segment of the limb. In accordance with this view, it will be evident that in the fore limb there is nothing homologous with the patella, whilst in the hind limb there is no part to represent the olecranon.
In the axial mesoderm of each member, differentiation into cartilaginous segments begins about the second month; each of these cartilages becomes invested by a perichondrial layer which stretches from segment to segment, and ultimately forms the ligaments surrounding the joints, which are subsequently developed between the segments. Chondrification first begins in the basal part of the limb, and extends towards the digits.
The homodynamy of the carpal and tarsal elements may be tabularly expressed, and compared with the more generalised types from which they are evolved.
= Os triquetrum
= Absent, or fused with Navicular = Navicular, less its tuberosity.
Os multangulum majus =Os multangulum minus
Carpale (Tarsale), iv.)- Os hamatum
Carpale (Tarsale), v.
The pisiform is omitted from the above table, since it is now generally regarded as being a vestige of an additional digit placed post-axial to the little finger (digitus post-minimus). Its homologue in the foot is by some considered as fused with the calcaneus. The tuberosity of the navicular, formed, as has been stated, of three elements, of which the sesamoid bone in the tendon of the tibialis posterior may be one, is to be regarded as the homologue of the pre-axial sesamoid in the hand, which probably fuses with the navicular to form its tuberosity. The peroneal sesamoid probably corresponds to the hamulus (sometimes an independent ossicle) of the os hamatum. Similarly, on the pre-axial border of the hand and foot, vestiges of a suppressed digit (prepollex and prehallux) may occasionally be met with. The frequent occurrence of an increase in the number of digits seems to indicate that phylogenetically the number of digits was greater than at present, and included a prepollex or prehallux, and a digitus postminimus. The correspondence of the metacarpus with the metatarsus and the phalanges of the fingers with those of the toes is so obvious that it is sufficient merely to mention it.
The differences in size, form, and disposition of the skeletal elements of the hand and foot is easily accounted for by a reference to the functions they subserve.
In the hand, strength is sacrificed to mobility, thus leading to a reduction in the size of the carpal elements, and a marked increase in the length of the phalanges. The freedom of movement of the thumb, and its opposability to the other digits, greatly enhances the value of the hand as a grasping organ. In the foot, where stability is the main requirement, the tarsus is of much greater proportionate size, whilst the phalanges are correspondingly reduced. Since the foot no longer serves as a grasping organ, the great toe is not free and opposable like the thumb.
Limb Girdles. The free limbs are linked to the axial skeleton by a chain of bones which constitute their girdles. The fundamental form of these limb girdles consists each of a pair of curved cartilages placed at right angles to the axis of the trunk on either side, and embedded within its musculature. Each cartilage has an articular surface laterally, about the middle, for the reception of the cartilage of the first segment of the free limb. In this way each pectoral and pelvic cartilage is divided into an upper or dorsal half and a lower or ventral half. The dorsal halves constitute the scapula and ilium of the pectoral and pelvic girdles respectively. With regard to the ventral halves there is more difficulty in establishing their homologies. The original condition is best displayed in the pelvic girdle; here the ventral segment divides into two branches one anterior, which represents the pubis, the other posterior, which ultimately forms the ischium. Ventrally, the extremities of these cartilages unite to enclose the obturator foramen. In the pectoral girdle the disposition of the ventral cartilages is not so clear, consisting primitively of an anterior branch or precoracoid, and a posterior portion or coracoid; these, in higher forms, have undergone great modifications in adaptation to the requirements of the fore limbs. The posterior or coracoid element, the homologue of the ischial cartilage in the pelvic girdle, is but feebly represented in man by the coracoid process and the coraco-clavicular ligament.
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 :
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:
Pre-spinal ridge forming the floor of the prespinal fossa
Spine and acromion
Fore part of the blade and crest of the ilium, with its anterior spine or angle
Post-spinal part of scapula forming the floor of Hinder part of blade and crest of ilium
the post-spinal fossa
Medial or ventral surface
Posterior spine or angle
Posterior or sciatic border of ilium
Inner or true pelvic surface of ilium, including the surface for the articulation of the sacrum
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 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
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.
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.
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