should be planned to end inferiorly in the intercondyloid fossa between the condyles and between the upper attachments of the two crucial ligaments. By this procedure the crucial ligaments can be studied singly, or together, and their relation to the lateral ligaments of the joint can be examined. It will be seen that the external lateral ligament and the anterior crucial ligament, constitute a pair of ligaments appropriated by the external condyle, to either side of which they are fixed; while the internal lateral and the posterior crucial ligaments belong to the internal condyle of the femur, and are attached on either side of it. When this relationship is observed, the internal lateral ligament may be divided. This will, in a measure, set free the internal condyle, and give greater space for the study of the crucial ligaments,

The Crucial Ligaments (ligamenta cruciata genu) are well named, because they cross each other like the limbs of the letter X in the interval between the two condyles of the femur. This crucial arrangement is seen whether they are viewed from the side, by the removal of the lower part of one condyle, or from the front or the back of the joint. The anterior crucial ligament is attached to the external condyle, whilst the posterior is fixed to the internal condyle of the femur. They are consequently sometimes termed external and internal.

The anterior crucial ligament springs from the intermediate rough area on the upper surface of the tibia, immediately in front of the inner tubercle which surmounts the tibial spine. From this it proceeds upwards, backwards, and outwards, to gain attachment to the posterior part of the inner surface of the external condyle of the femur.

The posterior crucial ligament springs from the posterior sloping part of the intermediate rough area on the upper surface of the tibia, behind the tibial spine, and behind also the attachments of the posterior horns of both semilunar cartilages. It proceeds upwards, forwards, and somewhat inwards, and crossing the anterior crucial ligament, is attached in the fore part of the intercondyloid fossa to the outer surface of the anterior oblique portion of the internal condyle. It receives one, or sometimes two, strong slips from the posterior horn of the external semilunar cartilage.

The anterior crucial ligament is tight in extension, and

the posterior crucial ligament is tight in flexion of the knee-joint.

Semilunar Cartilages.-These are two crescentic plates of fibro-cartilage which are placed on the condylar surfaces of the tibia. They deepen the surfaces upon which the condyles of the femur roll, and, being movable, they fill up the gaps which would otherwise arise during the movements of the joint. Each cartilage presents two fibrous extremities, or horns, which are attached to the rough intermediate surface on the upper end of the tibia. They are thick towards the circumference of the joint, but thin away to a fine free concave edge in the opposite direction. Both surfaces are smooth and covered with synovial membrane. They do not cover the entire extent of the condylar surfaces of the tibia. The central parts of the latter, as well as the sloping surfaces of the tubercles of the tibial spine, are free. On raising the cartilages from the surface upon which they rest, distinct impressions similar in shape and extent are seen on the subjacent encrusting cartilage of the tibia.

Dissection. Carefully define the attachments of the fibrous horns of the semilunar cartilages.

The external semilunar cartilage (meniscus lateralis) is usually somewhat thicker around its circumference than the internal cartilage. It forms the segment of a smaller circle, and its horns being fixed to the tibia close together, a very nearly complete circle is formed. The anterior fibrous horn is attached, immediately in front of the tibial spine, to the outer side of and partly under cover of the attachment of the anterior crucial ligament. The posterior horn is fixed to the summit of the tibial spine in the interval between the two tubercles. It likewise gives a strong slip to the posterior crucial ligament. The external lateral ligament is not in contact with the external semilunar cartilage. It is separated from it by the tendon of the popliteus, and the impress of the tendon is left on the cartilage in the form of a faint smooth groove on its

outer and posterior border. Behind, its circumference is attached to the posterior ligament.

The internal semilunar cartilage (meniscus medialis) is semicircular in form, and forms the segment of a much larger circle than the external cartilage.

Its anterior fibrous horn is fixed to the fore part of the intermediate rough area of the tibia in front of the attachment of the anterior crucial ligament; its posterior horn is attached to the back part of the intermediate rough area of the tibia, behind the tibial spine, and in front of the attachment of the posterior crucial ligament. The circumference of this cartilage is closely connected with the deep surface of the internal lateral ligament.

The Transverse Ligament (ligamentum transversum genu) is a fibrous band which stretches across from the fore part of one semilunar cartilage to the corresponding part of the other, constituting thereby a bond of union between them.

Dissection. The condyles of the femur should now be detached by dividing the external lateral ligament and the crucial ligaments close to their femoral attachments.

Attachment of Parts to Upper Surface of the Tibia.

antr. horn of ext. semilunar cart.

articular surface of tibia.

external semilunar cartilage.

postr. horn of extl. semilunar cart.

postr. crucial ligt,

postr. horn of intl. semilunar cart.

FIG. 67.-Upper articular surface of right tibia showing attachments of semilunar cartilages and crucial ligaments.

The ligamentous structures are attached to the intermediate area on the upper surface of the tibia in the following order from before backwards :-(1) The anterior

horn of the internal semilunar cartilage on the inner side of the extreme anterior part of the area. (2) The anterior crucial ligament and the anterior horn of the external semilunar cartilage: these are placed side by side, but the attachment of the former, which lies to the inner side, overlaps that of the external semilunar cartilage. (3) The posterior horn of the external semilunar cartilage on the summit of the tibial spine between its two tubercles. (4) The posterior horn of the internal semilunar cartilage immediately behind the tibial spine. (5) The posterior crucial ligament at the hinder part of the area.

The Ankle-joint (articulatio talocruralis).-The anklejoint is a diarthrodial articulation of the ginglymus or hinge variety. The articulation takes place between the bones of the leg and the astragalus, and the weight of the body is transferred through it to the foot. It is a joint of great strength; its stability being ensured not only by the powerful ligaments which surround it, but also by the close interlocking of the articulating surfaces.

The bones which enter into the formation of the anklejoint are the lower ends of the tibia and fibula and the superior surface of the astragalus. The lower ends of the leg bones are very firmly united together by an interosseous and other ligaments which give the joint a certain amount of elasticity or spring. They form a deep hollow resembling a mortice. The upper surface of the astragalus is received into this cavity.

The ligaments of the ankle-joint are:—

1. The anterior.

3. The external lateral.

2. The posterior.

4. The internal lateral.

Dissection. The remains of the annular ligaments, together with the tendons which are in relation to the joint, should be removed and the ligaments defined. The anterior and posterior ligaments should be first dissected. They may then be removed in order to bring the powerful external and internal lateral ligaments more fully into relief, and at the same time display the articulating surfaces, and thus permit the play of these surfaces to be seen when the joint is flexed and extended.

The Anterior and Posterior Ligaments are feeble bands which are placed in front of and behind the joint. They are attached to the margins of the articulating surfaces, except in front and below, where the anterior ligament is fixed to the neck of the astragalus. The fibres of these ligaments have for the most part a transverse direction. ant. inf. tib.-fib. ligt.

ant. fasc. ext. lat. ligt.

fibular surface..

mid. fasc. extl. lat. ligt.

tibial surface.

intl. malleolar surface.

intl. lateral ligt.

post. fasc. extl. lat.

ligt.

transverse ligament.

synovial pad.

post. inf. tibio-fib. ligt.

FIG. 68. -Socket for the astragalus; right ankle-joint. Note how the transverse ligament forms a part of this socket. On the articular surface of the astragalus a corresponding facet may be distinguished.

The External Lateral Ligament is divided into three distinct bands-an anterior, a middle, and a posterior. The anterior fasciculus (ligamentum talofibulare anterius) is a flattened band which passes from the anterior border of the lower end of the fibula to the outer and back part of the neck of the astragalus. The middle fasciculus (ligamentum calcaneo-fibulare) round and cord-like, passes from a point a little in front of the tip of the external malleolus to the external surface of the calcaneum. The posterior fasciculus (ligamentum talofibulare posterius) the strongest of the three, is a powerful band of fibres which proceeds almost horizontally inwards from the deep pit behind the lower articular surface of the fibula to a prominent tubercle on the back of the astragalus.

This tubercle is sometimes detached, and forms a supernumerary ⚫ tarsal bone which may represent the os trigonum found in some mammals. In such cases it has been mistaken for a fracture.

The Internal Lateral Ligament (ligamentum deltoideum)