A. Bones of the Pelvic Girdle (Fig. 41).

The pelvis has a characteristic V-shape, the pubic and ischiatic bones of the two sides together forming a vertical plate (a p), which divides anteriorly into the two iliac bones (il). The pubic and ischiatic bones being united by their median surfaces, one can only speak of the pelvic cavity as the space between the two ilia.

1. The ilia, ossa ilii (Fig. 42 il). In these a broader part or body can be distinguished, which takes part in the formation of the acetabulum (a). The hinder and broadest part of the bone is joined to the corresponding portion of its fellow by ligament; anteriorly the two diverge, bounding a cone-shaped cavity, the pelvis. The suture, by which the body of the iliac bone is united with the ischiatic and pubic bones, runs from above downwards through the middle of the acetabulum (Fig. 42, from p through the acetabulum to is). Inferiorly and in front the united ilia form a prominence (s), which represents the symphysis oss. pub. of man. The expanded portions of the ilium rest directly over the articular cavity by means of a somewhat constricted part or root. The ala is a sabre-like bone, with the sharper, more convex border directed upwards; the more concave, thicker border downwards. The two surfaces, of which the inner is grooved, are directed respectively inwards and outwards. The broader part of the blade springs directly from the neck or root (at il′), and at this end of the sharp convex border is a process, process. sup., from which the tendon of the m. vastus externus arises. The anterior end of the blade is a hollow cylinder, containing cartilage, movably articulated to the transverse process of the last vertebra or sacrum.

2. The ischia, ossa ischii (is), are irregular, four-cornered plates of bone, united together by the whole extent of their inner surfaces. The vertical plate formed by the union of the two bones takes only a very small share in the formation of the articular cavity. Above, it has a somewhat sharp border, continued with the ilium into a process (is′) corresponding with the united tubera ischii; the hinder border, which is united below with the pubic cartilage, is curved.

3. The pubes, os pubis (Figs. 41, 42, 43 p). Ossified pubic bones do not exist; even in old animals they are composed of calcified cartilage. Each is a triangular cartilage, wedged between the ilium and ischium, but also taking part in the formation of the acetabulum. The pubes of opposite sides are, as shown in section in Fig. 43, united by ligaments in the median plane.

B. Bones of the Hind-Limb.

1. The thigh.

The femur, os femoris (Fig. 44). The shaft is cylindrical, with a slight double or S-shaped curve. In the anterior portion the convexity is directed upwards, in the two hinder thirds downwards. Towards the hinder extremity it is flattened from above downwards. The anterior extremity is a rounded articular head placed directly on the shaft like that of the humerus. This head is somewhat flattened from side to side; and in structure resembles that of the humerus. The inferior extremity is rounded below, but flattened above (towards the hollow of the knee) and behind.

Hip-joint. The articular cavity is deepened by a fibrous ring, labrum cartilagineum, which fits closely upon the caput femoris; from this labrum the joint-capsule arises, to be inserted behind the head; close to the lower border of the cavity a ligamentum teres arises, which is inserted into the caput femoris.

2. The leg.

Tibio-fibula, os cruris (Fig. 45). This consists of a single bone, the os cruris; it is often called the tibia, but presents, however, distinct indications of being formed of two bones, the tibia and fibula. The shaft is not quite straight; it is thinner in the middle and flattened from above downwards. On the upper as also on the lower surface of the bone is a groove, passing from the middle towards each extremity; this deepens as it proceeds, and is an indication of the compound nature of the bone. In the middle of each surface of the bone is the opening of a canal, through which a bristle may be passed: the canal has bony walls. The medullary cavity is, for the most part, double, being single only in the middle. The anterior extremity is articular, and presents a median groove, which is prolonged on to the under surface. The posterior extremity forms a transverse condyle, which has at its inner part a notch for the tendon of the m. tibialis posticus.

The knee-joint consists of a capsule strengthened by the insertions of numerous tendons. Within this the articular surfaces of the femur and tibio-fibula are connected by means of fibro-elastic bands, which spring from the latter bone and cross each other, thus representing the lig. cruciata. The articular surfaces are moreover deepened by loose semilunar cartilages, corresponding with the inter-articular cartilages of the knee-joint of man. In addition, on either side of the joint, are found auxiliary ligaments.

3. The foot (Fig. 47).

a. The ankle, ossa tarsi. The tarsus consists of two rows of bones.

α. The proximal row consists of two long tubular bones (Fig. 47 ac), which are parallel to each other, and undoubtedly correspond with the astragalus (talus) and calcaneum. The bones are separated in the middle, but approach each other at either end. The outer of these bones (c) I have named calcaneum, following Cuvier and Dugès; the inner, astragalus (a). The two bones are surrounded and enclosed at their anterior and posterior extremities in a common epiphysis of calcified cartilage, which has the usual structure found elsewhere. The ligamentum calcanei, which will be described below (see Muscles of Foot, Fig. 88 lc), may be regarded as an unattached process of the heel‍40.

β. The distal row of the tarsals are all composed of calcified cartilage. They are:‍—

1. Os cuboideum (Fig. 47 cb), a flat plate of cartilage, concave in front, convex behind, placed between the common epiphysis of the astragalus and calcaneum (ac) and the ossa metatarsi II and III. This corresponds, as Dugès suggests, with the separated os cuboideum and cuneiforme III of other batrachians, e. g. Bufo.

2. On the inner side of the above, between the astragalus and metatarsus I lies a small body, concave in front, convex behind, which we may regard, with Dugès, as the analogue of the os naviculare (Fig. 47 n).

3. This latter supports a cartilage which forms a projection on the inner border, covered with skin. This consists of closely-united pieces, a small round, basal piece (h), and a flattened beak-like piece (h′). Dugès considers these pieces as the first and second ossa cuneiformia; I rather incline to Meckel’s opinion that they represent a rudimentary hallux, the first being the os metatarsi, the second a phalanx. Both are composed of calcified cartilage; in R. temporaria the lime-deposit seems to be less than in R. oxyrhinus and R. esculenta, and the projection consequently softer‍41.

b. The metatarsal bones, ossa metatarsi (Fig. 47, m I-V). The five ossa metatarsi are long, thin tubular bones, the anterior articular extremities of which are broader above and narrow below, hence are wedge-shaped, and anteriorly present flat surfaces for articulation with the tarsus. The shafts are long and thin; the posterior extremities have rounded heads. The comparative length of these is as follows: the fourth is the longest, then come the third, fifth, second, and first in order. In structure they resemble other tubular bones.

c. The phalanges, phalanges digitorum (Fig. 47). The total lengths of the separate toes correspond with that of the ossa metatarsi, but the distinctive differences are increased, partly by the difference in number and partly by the varying lengths of the phalanges. The first and second toes have each only two phalanges, the third and fifth have each three, the fourth, which is the longest, has four. The terminal phalanges are somewhat hooked towards the plantar surface.