Structure and patella is entirely composed of cancellated texture, the anterior surface being covered by a thin lamella of very fibrous compact tissue already referred to. This bone is developed by a single point of ossification, which commences about the second year.
The patella exists pretty generally among Mammalia, also among Birds. It is most de veloped in the I'achydermata and the Solipeds, and also in the Monotremata; and least so in the Carnivora and Quadrumana. It is absent in Cbeiroptera and Alarsupiata.* Leg.—The bones that form the second segment of the inferior extremity are the Tibia and Fibula.
Tibia, (shin-bone ; Germ. das Schienbein.) This bone is situated between the inferior ex tremity of the femur and the astragalus. Its length is to that of the femur as five to six. It forms the principal support of the leg, on the inside of which it is placed, and its volume is five times that of the fibula. After the femur, it is the longest bone in the body, being longer than the humerus.
The upper or femoral extremity of the tibia is thicker and broader than the remaining parts of the bone, and is properly the head of the bone. Its transverse extent is much greater than its antero-posterior. Its superior surface presents two bony processes lying on the same plane, denominated condyles of the tibia. Each of these has upon its superior surface a superficial concave articular facet, oval with long axis from before backwards; to these surfaces the term condyle has been improperly applied ; but they are more correctly called the glenoid cavities of the tibia, (cavitatcs glcnoidete, ex terns et interna). These cavities correspond to the condyles of the femur, having the semi lunar cartilages interposed ; the outer cavity approaches more to the circular form than the internal one; it is likewise much less deep, and at its posterior part it is even convex. The internal one, on the other hand, is uniformly concave, and its antero-posterior axis greatly exceeds its transverse. These sur faces are separated in the centre by a pyra midal eminence whose apex appears bifurcated, the subdivisions of which are separated by a narrow rough space. This is the spine of the tibia, (acclivitas intercondyloidea); it corres ponds to the intercondyloid fossa of the femur where the crucial ligaments are attached. Anterior and posterior to this spine are two rough depressions, the posterior more hollowed than the anterior : into the former the posterior crucial ligament is inserted, and the latter re ceives the anterior crucial ligament.
The circumference of the head is rough and perforated by a vast number of minute vas cular foramina. Each condyle projects late rally beyond the plane of the corresponding surface of the shaft, the internal to a greater extent than the external. These lateral pro jections are distinguished by the name of Tu berosities. The internal tuberosity gives in sertion at its lower part to the internal lateral ligament of the knee-joint; posteriorly this tuberosity is grooved, and one of the tendons of the semi-membranosus is inserted into the groove, and separates the internal lateral ligament from the bone in this situation. At the pos
terior part of the external tuberosity there is a small articular facet, nearly circular and plane, with which the fibula is articulated.
In front of the head of the tibia there is a rough triangular surface, the apex of which is directed downwards and forms a promi nence, which is smooth at its superior part, but rough inferiorly. The 14,eamentum patella: is inserted in the latter situation; the smooth portion indicates the position of a bursa which intervenes between the ligament and the bone. This prominence is called the anterior tube rosity, and by some anatomists the spine. From the inferior rough portion of this tube rosity there passes upwards and outwards a prominent line, most prominent at its ter mination, where the tibialis anticus muscle has one of its attachments.
The shaft of the tibia has the form of a triangular prism in almost its whole extent: at its inferior third this form is less distinct, in consequence of the angles being rounded off. Of the three surfaces the anterior is that which presents the greatest dimensions : it is smooth and slightly convex in its entire extent, in clined backwards and inwards, subcutaneous, except at its upper part, where an aponeurotic expansion connected with the tendons of the semi-tend inosus, sartori us, and graeilis muscles. The inferior fourth of this surface is much more convex than the upper portion, and looks directly inwards. The external surfixe is in dined backwards and outwards, and is con cave in its three superior fifths, convex in the rest of its exteot. The depth of the superior concave portion is proportionate to the de velopement of the tibialis anticus muscle, to which it gives insertion. The inferior con vex portion is of less extent than the superior, and as it descends it experiences a change of aspect so as to look directly forwards. This change is in accordance with the altered di rection of the tendons of the tibialis antieus and extensor muscles of the toes, which lie in contact with the bone in this situation. The posterior surface is expanded at its extremities and contracted in the centre. At its superior part a triangular surface is marked off from the rest, towards the upper extremity by au oblique line, which proceeds from below up wards, and from within outwards; into this line are inserted the poplitiens, solceus, tihialis posticus, and the long flexor muscle of the toes. The space which intervenes between this line and the posterior margin of the head of the bone is covered by the poplitaus muscle and forms part of the floor of the poplitcal space. Immediately below this oblique line, the orifice of the nutritious canal is situated, penetrating the bone obliquely downwards ; this canal is the largest of the medullary canals of the long bones; and Cruveilhier states that he has traced a nervous filament passing into it in company with its artery. All that portion of the posterior surface which is below the oblique line is smooth and divided by a ver tical line, which is variously developed in dif ferent subjects; the tibialis posticus muscle and the long flexor of the toes arc attached to this surface.