centre of the head appears about one year after birth. The great trochanter begins to ossify about the third or fourth year; the lesser about the fourteenth. All the pieces have united about the age of twenty-one.

THE PATELLA.

(Plate XXXVI.)

The patella is a little bone developed in the extensor tendon of the knee, in order to protect the knee joint, and to increase the leverage of the extensor muscle by making it act at a greater angle. It is a principle in mechanics that the efficiency of a force which acts upon a lever is greatest when its direction is at right angles to the lever, and decreases as the obliquity of that direction is increased. The patella is the best example of a "sesamoid" bone. In shape, it is somewhat triangular, with rounded angles, the apex being downwards.

Its anterior surface is convex, and marked by longitudinal streaks, indicative of the insertion of the fibres of the extensor tendon.

Its posterior surface is smooth, and crusted in the recent state with cartilage, in order to play upon the trochlea of the femur. It is divided by a vertical ridge adapted to the groove in the femur, and on each side of the ridge are the articular facettes corresponding to the condyles of the femur. The external articular facette is the larger of the two in adaptation to the external condyle, and by this we may distinguish the right patella from the left. Besides this, the outer edge of the patella is much thinner than the inner edge, which is also another good distinction. Below the articular surface that is, at the apex-there is a rough surface for the attachment of the "ligamentum patella," or continuation of the extensor tendon. The base of the bone is thick and irregular, for the insertion of the extensor tendon.

The patella is developed from a single centre, which appears

about the second year. It is not fully ossified until about the age of fourteen or fifteen.

The patella being developed actually in the substance of the extensor tendon of the knee is very liable to be broken by a sudden and violent action of the extensor muscles, as when we attempt to preserve the balance of the body when it is in danger of falling backwards. In this position—that is, when the knee is half-bentthe upper part of the patella is not supported by its trochlea: there is a hollow under it, and here consequently the patella snaps transversely, like a stick broken across the knee. The broken ends are separated widely, and therefore in the vast majority of cases their reunion takes place by ligamentous substance, not by bone.

But even when the knee is extended, violent muscular contraction is able to snap the patella. Desault speaks of both patella being broken by convulsions in a patient after he had been cut for the stone. Opera dancers sometimes break the patella in practising the step called the "Entrechat."

THE TIBIA.

(Plate XXXVI.)

The tibia is the larger of the two bones of the leg, and is placed on the inner side. It entirely supports the condyles of the femur, and transmits the weight of the body to the foot. Its direction is not oblique like the femur, but vertical; so that in well-formed legs the two tibiæ should be parallel. Let us examine in succession the upper end, the shaft, and the lower end.

HEAD.

The upper end is generally called the "head" of the tibia. It is very broad in the transverse direction for the support of the condyles of the femur: and we point to this great breadth as one of the peculiarities of the human skeleton. The articular surfaces for the condyles are very shallow in the dry

bone, but deepened in the recent state by discs of fibro-cartilage (termed the "semilunar cartilages"). These cartilages convert the shallow articular surfaces of the tibia into variable sockets; that is, sockets which adapt themselves to the varying forms of the condyles in flexion and extension of the knee. The outer articular surface (facette) is round, but the inner is oval, with the long diameter from before backwards, in adaptation to the internal condyle. Between the articular surfaces is a projection termed the spine, which is generally topped by two little tubercles. In front of the spine is the depression in which the anterior crucial ligament is attached, and behind the spine is another much larger one, in which the posterior crucial ligament is attached. These depressions serve also for the attachments of the semilunar cartilages.

ternal and internal.

TUBEROSITIES, ex- The lateral masses which support the articular surfaces are called the "tuberosities" of the tibia. The external tuberosity presents at its back part a small articular surface for the head of the fibula: this articular surface is on a kind of bony ledge, and its direction is oblique. The internal tuberosity is much larger, and projects more than the other. It has a groove behind for the insertion of the "semi-membranosus." About one inch and a half below the head of the tibia is the "tubercle" for the insertion of the common extensor tendon of the leg (ligamentum patella). You will observe that the insertion takes place into the lower part of the tubercle, which is rough; the upper part is smooth, to allow the easy play of the tendon (a bursa being interposed between the tendon and the bone).

SHAFT.

The shaft of the tibia is triangular. It is a little twisted outwards, to determine the obliquity of the foot; consequently the inner malleolus advances a little before the ankle joint, and the outer one recedes a little behind it. This, observe, corresponds with the obliquity of the neck of the thigh bone, the position of its trochanters, and the oblique direction of the muscles; the object of all being to give a natural inclination outwards to the lower extremity. The narrowest part of the shaft is about the lower third; hence the frequency of fracture here. Let us examine each of its surfaces.

The internal surface is subcutaneous.

We notice on it, below

the internal tuberosity, the insertions of the "sartorius," "gracilis," and "semitendinosus." Behind these is a rough surface for the attachment of the internal lateral ligament of the knee.

The external surface is slightly hollowed along its upper half for the origin and lodgment of the "tibialis anticus:" its lower part is turned forwards, so as to present a smooth surface adapted for the play of the tendons which run over the front of the anklejoint.

The posterior surface presents along its upper third a rough line, slanting from the outer towards the inner side. It indicates part of the tibial origin of the "soleus;" the remainder of this origin runs down the inner edge of the shaft to the extent of about three inches. This origin is an important piece of anatomy since it concerns the operation of tying the posterior tibial artery. Above the "oblique line" is a triangular surface, indicating the insertion of the "popliteus." The surface of the bone below the ridge is occupied by the origin of the "flexor longus digitorum," and part of the "tibialis posticus." Just below the line is the canal for the medullary artery. It is the largest of all the canals in the long bones, runs very obliquely from above downwards, and when divided in amputations sometimes occasions troublesome hemorrhage. I have many times traced a nerve through this canal with the artery into the medullary cavity.

The crest or "shin" of the tibia is the densest and strongest

part of the bone (see cut, fig. 34); for this reason, that the chief pressure on the tibia is at the anterior part; which is at once obvious if we consider the direction of the force in walking, running, or leaping. This form of the tibia, therefore, is not a mere matter of accident, or the result of the pressure of the muscles which surround it.

FIG. 34.

T

Section through the tibia, T, and fibula, F, to show the thickness of their walls.

With regard to the edges of the tibia, the anterior, called the "crest," is very sharp, and readily felt beneath the skin, but only along the upper two-thirds of the shaft: along the lower third the front of the bone is round, for the passage of the extensor tendons and the anterior tibial vessels and nerve. The external edge is turned towards the fibula, and gives attachment to the interosseous membrane (represented by the dotted line in the cut) which connects the two bones. The internal edge runs from the hinder part of the head of the tibia down to the inner malleolus. It gives attachment to the deep fascia covering the muscles of the back of the leg, beneath those of the calf.

Lower end.

FIG. 35.

The lower end of the tibia is expanded transversely in order to form a hinge-joint with the astragalus. For this purpose its articular surface is uniformly concave from before backwards; but the plane of the joint is horizontal (as seen in cut, fig. 35), like that of the knee, for the advantageous support of the weight of the body. The joint is secured on the inner side by the Astragalus. massive projection termed the "malleolus internus." One side of this is smooth and crusted with cartilage, to articulate with the lateral surface of the astragalus; the other is subcutaneous. At its apex there is a deep notch for the attachment of the very powerful internal lateral ligament of the ankle; and behind is a longitudinal groove, which transmits the tendons of the "tibialis posticus" and the "flexor longus digitorum."

Tunnel.

Os Calcis.

Section to show that the plane of the ankle-joint is horizontal.

Lastly, on the outer surface of the lower end is the rough excavation for the reception of the fibula. There is no sensible movement between the bones, but only just enough to give a slight amount of elasticity. The security of the angle requires that they be firmly rivetted together by a strong interosseous ligament; and their contiguous surfaces are rough accordingly.