The tibia, the homotype of the radius, presents a broad and expanded head at its proximal extremity, where it is marked by two shallow depressions which receive the condyles of the femur. The shaft of the bone is triangular, and presents three smooth surfaces separated by rough or sharp lines. The anterior ridge is called the spine of the tibia, and gives attachment above to the ligamentum patellae; the inner ridge gives attachment to the interosseous membrane, the fibres of which pass obliquely downwards and outwards to the fibula, as the fibres of the interosseous membrane of the forearm pass downwards and outwards from the radius to the ulna. The distal extremity of the radius is smooth and rounded, and terminates externally in the malleolus internus, the homologue of the styloid process of the radius. A concave pulley-like surface articulates below with the convex portion of the astragalus.
The tibia, unlike the radius, is firmly attached to the fibula, by a superior and an inferior articulating surface, but although the bone does not rotate upon the fibula, as the radius does upon the ulna, both bones are, when flexed, susceptible of a considerable amount of rotation upon the femur.
The mistake, made by many anatomists, of comparing the ulna with the tibia, and of regarding the right arm as corresponding with the left leg, and vice versa, arose from the absence in the human fibula of any process of bone which might be considered as a repetition of the olecranon; consequently, this latter process was compared to the patella, and the tibia, into which the ligamentum patellae is inserted, became the homotype of the shaft of the ulna. Even Soemmering and Meckel, usually so accurate, fall into this error. Patella, maximum os sesamoideum, vel appendix tibiae mo-bilis, olecrano nonnihil analoga videtur.* "The patella corresponds perfectly both in its situation and connection with this tendon, to the olecranon process of the ulna; and hence the tibia has no process which may be compared with the olecranon. The patella increases the analogy between the bones of the leg and those of the forearm."!
* Soemmering de Ossibus, t. i. p. 385.
+ Meckel's Anatomie Descriptive : Art. Patella.
But reference to the skeleton of the echidna will shew, that upon the upper extremity of the fibula there may be developed a process equally long, and of similar shape with the olecranon process of the ulna; and this, too, in a limb supplied with a patella imbedded in the common extensor muscle of the leg. Professor Owen, who calls this fibular lever the fibella, is the first, I believe, who has pointed out the true homologies of this part of the skeleton. The fibella in the human skeleton, though stunted, is prolonged upwards upon the posterior part of the tibia, in the form of a slight elevation or tubercle. This stunted fibella is noticed by Hildebrandt in his description of the fibula:— "Das obere ende des wadenbeins, welches unge-fahr dreikantig ist und der kopf, caput oder capitu-lum des wadenbeins heisst, hat nach hinten und aussen eine stumpfe spitze."*
* Hildebrandt's anatomie, b. ii., § 264.
But he omits to draw from thence the proper homological inference as to the true signification of the fibula; or to point out that with the prone position of the foot must correspond the prone position of the hand, the great toe and the thumb being both situated nearest to the mesial line of the body. If the upper extremity be placed in this, its true typical position, it will be seen that the radial side of the arm should be properly called internal, and the ulnar side external, just as the tibial side of the leg is called internal, and the fibular side external. The determination of these points is absolutely indispensable before attempting to trace the homologies either of the small bones of the hand and foot, or of the muscles which act upon the extremities.