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longus muscle is attached. The canal for the nutrient artery is directed towards the head of the bone.
The second metacarpal bone is recognised by its length and its broad and deeply notched base for articulation with the lesser multangular. It has a small half-oval facet for the greater multangular on the lateral side of its base, whilst on its medial aspect it presents a narrow vertical strip for the capitate, in front of which there are two half-oval surfaces for the third metacarpal. To the dorsal aspect of the base is attached the tendon of the extensor carpi radialis longus muscle, whilst the flexor carpi radialis is inserted into the volar surface.
The third metacarpal bone can usually be recognised by the pointed styloid process which springs from the dorsum of its base, towards the radial
FIG. 224.-THIRD RIGHT METACARPAL BONE.
NOTE. The bone is represented in the centre of the figure in the position which it occupies in the right hand viewed from the volar aspect. The views on either side, and below, represent respectively the corresponding surfaces of the bone turned towards the reader.
FIG. 225.-FOURTH RIGHT METACARPAL BONE. FIG. 226.-FIFTH RIGHT METACARPAL BONE. NOTE.-The bone in each figure is represented in the centre of the figure in the position which it occupies in the right hand viewed from the volar aspect. The views on either side, and below, represent respectively the corresponding surfaces of the bone turned towards the reader.
side. On the proximal surface of the base there is a facet for the capitate. On the lateral side there are two half-oval facets for the second metacarpal. On the medial side there are usually two small oval or nearly circular facets for the
fourth metacarpal. The extensor carpi radialis brevis muscle is inserted into the
dorsum of the base.
The fourth metacarpal bone may be recognised by a method of exclusion. It is unlike either the first, second, or third, and differs from the fifth, which it resembles in size, by having articular surfaces on both sides of its base. Proximally there is a quadrilateral surface on its base for articulation with the os hamatum. On its lateral side there are usually two small oval facets for the third metacarpal. Of these facets the dorsal one not infrequently has a narrow surface for articulation with the capitate. On the medial side there is a narrow articular strip for the base of the fifth metacarpal.
The fifth metacarpal bone can be recognised by its size and the fact that it has an articular facet only on one side of its base, namely, that on its lateral side for the fourth metacarpal. The carpal articular surface is saddle-shaped, and there is a tubercle on the medial side of the base for the insertion of the extensor carpi ́ ulnaris muscle.
As has been already pointed out, the openings of the arterial canals are usually seen on the volar surfaces of the metacarpals, those of the medial four bones being directed proximally towards the base or carpal end, differing in this respect from that of the first metacarpal, which is directed distally towards the head or phalangeal extremity. The opening of the latter canal usually lies to the medial side of the volar aspect of the body.
Ossification. The metacarpal bones are developed from primary and secondary centres; but there is a remarkable difference between the mode of growth of the first and the remaining four metacarpals, for whilst the body and head of the first metacarpal are developed from the primary ossific centre, and its base from a secondary centre, in the case of the second, third, fourth, and fifth metacarpals the bodies and bases are developed from the primary centres, the heads in these instances being derived from the secondary centres. In this respect, therefore, as will be seen hereafter, the metacarpal bone of the thumb resembles the phalanges in the manner of its growth, a circumstance which has given rise to considerable discussion as to whether the thumb is to be regarded as possessing three phalanges and no metacarpal, or one metacarpal and two phalanges. Broom (Anat. Anz. vol. 28), by a reference to reptilian forms, offers an explanation in regard to the difference in the mode of ossification of the first metacarpal on the ground that the most movable joint is that between the first metacarpal and the carpus, whereas on the other digits the most movable joints are those between the metacarpals and phalanges. In consequence those ends of the bones which enter into the formation of the joints where movement is most free are the ends where the epiphyses will appear. This is in accordance with the law to be suggested in connexion with the fibula. The primary centres for the bodies and bases of the second, third, fourth, and fifth metacarpals appear in that order during the ninth or tenth week of intra-uterine life, some little time after the terminal phalanges have begun to ossify; that for the body and head of the metacarpal bone of the thumb a little later. At birth the bodies of the bones are well formed. The secondary centres from which the heads of the second, third, fourth, and fifth metacarpals and the base of the first are developed, appear about the third year, and usually completely fuse with the shafts about the age of twenty. There may be an independent centre for the styloid process of the third metacarpal, and there is usually a scale-like epiphysis on the head of the first metacarpal which makes its appearance about eight or ten, and rapidly unites with the head. The occurrence of a basal epiphysis in the second metacarpal bone has been noticed.
Phalanges Digitorum Manus.
The phalanges or finger bones are fourteen in number-three for each finger, and two for the thumb; and they are named numerically in order from the proximal toward the distal ends of the fingers.
Phalanx Prima.-The first phalanx, the longest and stoutest of the three, has a semi-cylindrical body which is curved slightly forwards. The volar surface is flat, and bounded on either side by two sharp borders to which the fibrous sheath of the flexor tendons is attached. The dorsal surface, convex from side to side, is overlain by the extensor tendons. The proximal end, considerably enlarged, has a simple oval concave surface, which rests on the head of its corresponding metacarpal bone. On either side of this the bone displays a tubercular form, and affords attachment to
the collateral ligaments of the metacarpo-phalangeal joint, and also to the interossei
Phalanx Secunda.-The second phalanx resembles the first in general form, but is of smaller size. differs, however, in the form of its proximal articular surface, which is not a simple oval concavity, but is an oval area divided into two small, nearly circular concavities by a central ridge passing from volar to dorsal edge; these articulate with the condylic surfaces of the proximal phalanx. Into the margins of its volar surface near the proximal end are inserted the split portions of the tendon of the flexor digitorum sublimis, whilst on the dorsal aspect of the proximal end the central slip of the extensor digitorum communis muscle is attached.
Phalanx Tertia. The third or terminal or ungual phalanx is the smallest of the three; it is easily recognised by the spatula-shaped surface on its distal extremity which supports the nail. The articular surface on its proximal end resembles that on the proximal end of the second phalanx, but is smaller. On the volar aspect of this end of the bone there is a rough surface for the insertion of the tendon of the flexor digitorum profundus muscle. The dorsal surface of the same extremity has attached to it the terminal portions of the tendon of the extensor digitorum communis muscle. The phalanges of the thumb resemble in the arrangement of their parts the first and third phalanges of the fingers.
The arterial canals, usually two in number, placed on either side of the volar aspect and nearer the distal than the proximal end of the bone, are directed towards the finger-tips.
Ossification. The phalanges are ossified from primary and secondary centres.
III. Phalanx, ungual or terminal
From the former, which appear as early as the ninth week of
1. About ten weeks. Here the ossific nuclei of the terminal phalanges and the medial four metacarpal bones
2. A little later. The centre for the metacarpal for the thumb is now present, as also the centres for the proximal row of phalanges. The centres of the medial row of phalanges have appeared in the case of the middle and ring fingers.
During the third month. All the primary centres for the metacarpal bones and phalanges are now developed.
4. About the fourth to fifth month.
5. About the sixth to seventh month.
fœtal life, the body and distal extremities are developed; whilst the latter, which begin to appear about the third year, form the proximal epiphyses which unite with the bodies
from eighteen to twenty. Dixey (Proc. Roy. Soc. xxx. and xxxi.) has pointed out that the primary centre in the distal phalanges commences to ossify in the distal part of the bone rather than towards the centre of the body. This observation has been confirmed by Lambertz, who further demonstrates the fact that ossification commences earlier in the distal phalanges than in any of the other bones of the hand. Of the other phalanges, those of the first row, beginning with that of the third finger, next ossify, subsequent to the appearance of ossific centres in the shafts of the metacarpal bones, whilst the second or middle row of the phalanges is the last to ossify about the end of the third month. Sewell has recorded a case in which the proximal phalanx had a distal as well as a proximal epiphysis.
Two little oval nodules (sesamoid bones), which play in grooves on the volar aspect of the articular surface of the head of the first metacarpal bone, are constantly met with in the tendons and ligaments of that metacarpo-phalangeal articulation. Similar nodules, though of smaller size, are sometimes formed in the corresponding joints of the other digits, more particularly the index and little finger; as Thilenius has pointed out (Morph. Arbeiten, vol. v.), these are but the persistence of cartilaginous elements which have a phylogenetic interest.
THE BONES OF THE INFERIOR EXTREMITY.
THE PELVIC GIRDLE AND THE PELVIS.
The pelvic girdle is formed by the articulation of the two hip bones with the sacrum dorsally, and their union with each other ventrally, at the joint called the symphysis pubis.
The hip bone (os coxæ) (O.T. innominate) is the largest of the "flat" bones of the skeleton. It consists of three parts-the os ilium, the os ischium, and the os pubis -primarily distinct, but fused together in the process of growth to form one large irregular bone. The coalescence of these elements takes place in and around the acetabulum, a large circular articular hollow which is placed on the lateral side of the bone. The expanded wing-like part above this is the os ilium; the stout V-shaped portion below and behind it constitutes the os ischium; while the <-shaped part to the medial side, and in front and below, forms the os pubis. The two latter portions of the bone enclose between them a large aperture of irregular outline called the foramen obturatum (obturator foramen), which is placed in front and below, and to the medial side of the acetabulum.
The ilium, almost a quadrant in form, consists of an expanded plate of bone, having a curved superior border, the crista iliaca (iliac crest). Viewed from the side, this forms a curve corresponding to the circumference of the circle of which the bone is the quadrant; viewed from above, however, it will be seen to display a double bend-convex anteriorly and laterally, and concave posteriorly and laterally. The iliac crest is stout and thick, and for descriptive purposes is divided into a labium externum (external lip), a labium internum (internal lip), and an intermediate surface (linea intermedia), which is broad behind, narrowest about its middle, and wider again in front. About 2 inches from the anterior extremity of the crest the external lip is usually markedly prominent and forms a projecting tubercle, which can readily be felt in the living. Attached to these surfaces and lips anteriorly are the muscles of the flank, whilst from them posteriorly the latissimus dorsi, quadratus lumborum, and sacro-spinalis muscles derive origins. The crest ends in front in a pointed process, the spina iliaca anterior superior (anterior superior iliac spine). To this the lateral extremity of Poupart's inguinal ligament is attached, as well as the sartorius muscle, which also arises from the edge of bone immediately below it, whilst from the same process and from the
anterior end of the external lip of the iliac crest the tensor fascia latæ muscle takes origin.
The anterior border of the ilium stretches from the anterior superior iliac spine to the margin of the acetabulum below. Above, it is thin; but below, it forms a thick blunt process, the spina iliaca anterior inferior (anterior inferior iliac spine). From this the rectus femoris muscle arises, whilst the stout fibres of the
FIG. 229.-THE RIGHT HIP BONE SEEN FROM THE LATERAL SIDE.
ilio-femoral ligament of the hip-joint are attached to it immediately above the acetabular margin. Posteriorly, the crest terminates in the spina iliaca posterior superior (posterior superior iliac spine). Below this, the posterior border of the bone is sharp and irregularly notched, and descends to a prominent angle, the spina iliaca posterior inferior (posterior inferior iliac spine). In front of the posterior inferior iliac spine the edge of the bone becomes thick and rounded, and sweeps forwards and downwards, round a wide notch called the incisura ischiadica major (greater sciatic notch), to join the posterior border of the ischium behind the acetabulum.