A. Bones of the Shoulder-girdle.

The shoulder-girdle of the frog (Fig. 25) consists of four bones, two in the upper or scapular portion, and two in the lower or coracoid portion.

1. The scapular portion.
a. The suprascapula, pars suprascapularis scapulae, scapula superior s. omolita (Figs. 25 s′, 26, 27). This forms the upper, thinner portion. It is trapezoid in shape, its narrowest, thickest, and lowest border articulating with the scapula. The remaining three borders are free; the posterior and superior borders are very considerably thinned out, the inferior surface is curved towards the ventral surface. Only the thin upper and hinder borders (hh) are of hyaline cartilage; the middle portion (co) consists of calcified cartilage; the lateral part (o) and the anterior border (o′) of true bone.

The bony part forms a thin plate striated both radially and concentrically with the margin. The anterior border (o′) is bent on itself so as to form a groove (Fig. 27). The dorsal surface of this bony plate is quite smooth, and covered only by periosteum; the ventral surface, on the contrary, is for the most part covered by an even layer of calcified cartilage (Fig. 27 co′). This passes directly into the second portion of calcified cartilage (co); so that if the whole be dried, the thin hard lamina of bone can easily be split away from the underlying parchment-like cartilage. The layer of calcified cartilage is, however, not so complete that the suprascapula can be said to consist of calcified cartilage.

b. The scapula (Figs. 25 s, 28, 29) is a long, flat, four-cornered plate of compact bone, contracted in the middle, and wider at either end. From above (Fig. 28 l) it descends to the shoulder-joint with a slight curvature, with the convexity directed outwards. The inferior or ventral border (Fig. 28 m), which assists in the formation of the shoulder-joint, is cleft into two processes, a ventral and a dorsal. This may be best seen by looking at the border of the bone from behind or before (as in Fig. 29). The lower ventral process (Fig. 29 v) may be regarded as the acromion, or at least as a rudiment of the acromion, i. e., spina scapulae. The upper dorsal process evidently represents a rudimentary processus coracoideus. The anterior border (Fig. 28 a) is free, somewhat sharp, and slightly concave; the posterior (Fig. 28 p), also concave, is free externally; while internally, where the division of the ventral border commences, it takes part in the formation of the articular cavity of the shoulder (Figs. 28 and 29 c.a.).

Articulation with the suprascapula. The scapula consists of a shell of compact bony tissue and an inner portion of cancellous tissue; this latter passes, at the boundary between this bone and the suprascapula, through an intermediate layer of calcified cartilage, which is directly continuous with the cartilage of the suprascapula. The compact superficial layer ceases where the calcified cartilage begins, and the periosteum of the scapula passes directly into the perichondrium of the suprascapula.

2. The ventral portion of the shoulder-girdle consists of three parts, which lie one behind the other.

a. The larger and posterior bone, the coracoid (Figs. 24 co, 30), is contracted in the middle, and expanded at either end, especially at the sternal end. The inner half is flattened from above downwards; externally it becomes more cylindrical. The anterior and posterior borders are markedly concave; the outer border (Fig. 30 l) articulates by means of a cartilage with the proc. coracoideus of the scapula; the inner border (m) is separated from its fellow of the opposite side by the epicoracoids. This inner border is so broad, that it almost touches the inner end of the clavicle (Fig. 24 cl.). A coracoid foramen is thus formed between the two bones, similar to the foramen ovale of the pelvis.

b. The small anterior bones of the ventral portion of the shoulder-girdle (Figs. 24 cl., 31), I, like Cuvier and others, regard as clavicles. Dugès (n. 33) names them ‘acromial.’ Each bone is narrower than the coracoid, smaller internally (m), broader externally (l), and articulates by this broader end with the cartilage uniting the dorsal and ventral portions; the inner extremity rests upon the epicoracoid. The clavicle is grooved on its superior and posterior surfaces in almost its whole length; the groove widens externally (l), and contains

c. The precoracoid (Fig. 34 cl′), a slender bar of cartilage, connecting the anterior edge of the scapula with the sternum.

Cartilages of the shoulder-girdle (Figs. 24, 32, 34). A cartilage (c), described by Dugès as the ‘paraglénal,’ connects the scapula with the coracoid, and converts the notch between the two processes (s and v) on the inner border of the scapula into an aperture opening posteriorly into the glenoid cavity, and which is filled up by the lining cartilage. This cartilage connects the two processes with the coracoid; being thicker between this and the proc. coracoideus, it takes an important share in the formation of the floor of the glenoid cavity. At the outer extremity of the clavicle the cartilage projects and is then continued in the groove on this bone towards the middle line (Figs. 32, 34), where it widens out, and is united by its external margin with the sternum, thus forming the precoracoid as already described, see p. 40. These connections are best seen in young animals, as is shown in Fig. 34.

The clavicle (cl) is of ossifying cartilage; the cartilage (c), the epicoracoid, is very large, and hyaline, and at cl runs directly into the epicoracoids, which are here united mesially by ligament. In the fully developed animal the whole of the epicoracoid changes to calcified cartilage, as also does the precoracoid lying in the groove of the clavicle.

B. Bones of the Fore Limb.

1. The arm has but one bone.

Figures twice natural size.

c Anterior extremity.  c′ Posterior extremity.  cd Crista deltoidea.
cm Crista medialis.  t trochlea.

The humerus, os humeri (Figs. 35, 36, 37). The shaft of the humerus is, on the whole, cylindrical, the middle part being the narrowest; its dorsal surface is slightly concave, the ventral surface convex. In the upper half the cylindrical form is indistinct, from the presence of a well-marked crest (crista deltoidea) upon the under or ventral surface. The hinder half of the bone has very different shapes in the two sexes: in the female it remains cylindrical as far as the elbow-joint; in the male, on the posterior surface, where the crista deltoidea ceases, a marked ridge (crista medialis) arises, which gradually winds inwards and terminates at the inner condyle. This sexual difference is found in all the three species described in the Introduction, and it would seem that this ridge increases in height during the pairing season. The muscle which arises from this process is the flexor carpi radialis (Fig. 74 Fc), and it attains, in the males, a very large size at the same period; indeed, it is at its origin twice as broad as in the female. Dugès‍35 has incorrectly stated that this crista is a peculiarity of Rana temporaria, and is wanting in Rana esculenta. Pouchet‍36 first observed that it was a sexual distinction.

The anterior articular extremity presents a rounded head, which is somewhat flattened externally. The inferior ventral border is sharp, and developed into the above-mentioned bony crest (crista deltoidea); the free border is arched and curved outwards. Above, on the outer surface of the head, is found an eminence which may be a rudimentary tuberc. maius. The inner half of the head is articular.

The posterior articular extremity consists of a rounded head (c′), attached to the posterior extremity of the shaft; internal to this is a small trochlea (t), placed upon the inner condyle. The external condyle is but small. The head is embraced by these condyles. Upon the under surface, above the head, is the fossa cubitalis anterior. The structure of the two articular extremities of the humerus differs; the upper, as in long bones generally (see page 16), consists of hyaline cartilage surrounded by calcified cartilage, which again is covered by hyaline articular cartilage; the lower extremity is composed of spongy bone, with roundish marrow spaces, and for the most part covered by a layer of calcified cartilage.

The shoulder-joint. The socket is formed by the posterior border of the scapula, by both the processes of that bone, the outer portion of the coracoid, and besides these by cartilage, which forms the articular surface already described (Fig. 33). In the macerated bone the floor of the socket opens dorsally into a fossa between the processes of the scapula (Fig. 32). This opening, in the natural condition, is closed by synovial membrane only; the cartilage covering the rest of the space being absent (the sac may easily be inflated, either from the joint itself or through the opening). The margin of the socket consists of a labrum, partly fibrous and partly cartilaginous, which is easily removed entire. The capsule, which is attached beneath the caput humeri, arises from this. From the articular cartilage of the ventral margin of the socket, and from the adjacent part of the scapula, proceeds a strong band, which is inserted into the flattened outer surface of the head.

2. The forearm.

The radio-ulnar, os antibrachii (Fig. 38). The two bony elements of the forearm are fused into one, which, however, shows traces of its compound character. Of these two bones, that which forms the inferior (in man anterior) border is the radius, the other the ulna. The relative positions of the bony elements are such that they appear to be in a state of semi-pronation; by this the thumb-border of the arm is directed downwards (in man forwards). The two bones so united form a flattened, tube-like bone, the grooved surfaces of which are directed inwards and outwards respectively; the borders being dorsal and ventral. The shaft of the bone is narrower above, becoming gradually wider below. In the upper half, the groove showing the line of junction of the two bony elements is scarcely visible, and it appears as though this part consisted of ulna alone, the radius not reaching so far; below the groove is more marked. The upper articular extremity appears to consist of an olecranon (Fig. 38 u′) and a processus coronoideus (r′), between which is the greater sigmoid cavity (cavitas sigmoidea major) for the processus cubitalis humeri. A more careful examination shows this to be incorrect: the portion of the concave articular surface, which lies in the immediate neighbourhood of the ventral border, undoubtedly represents the articular surface of the capitulum radii, and the upper portion of the articular surface (at u′) belongs to the ulna and articulates with the trochlea (Figs. 35, 36 t) of the humerus. The inner border of the upper extremity of the bone articulates with the groove t of the humerus. The inferior extremity forms one of the usual cartilaginous epiphyses, enclosing the two bones. That part of the extremity corresponding with the radius is triangular and convex, it is prolonged upon the inner or under surface of the bone by means of a process; the ulnar portion of the bone forms a capitulum. The structure of the inferior extremity is that of epiphyses in general, except that there are two medullary cavities in a common sheath of calcified cartilage; the anterior head consists of cancellous tissue, with a covering of calcified cartilage.

3. Bones of the hand (Fig. 39).

a. Bones of the wrist, ossa carpi. These are arranged in two rows, proximal and distal. The proximal or anterior row has three bones, viz.:‍—

1. Os lunatum, the middle bone (Fig. 39 l), Dugès, n. 38, articulates with the radial part of the articular surface of the radio-ulnar by a concave surface; with the large bone of the second row (hc) it articulates by means of a rounded head; and with the two other bones of this row, os naviculare (n) and pyramidale (p), with the former by a saddle-shaped surface, with the latter chiefly by ligament. The bone is irregularly cuboid; the dorsal and palmar surfaces are non-articular. With Dugès I regard this bone as os lunatum; Mertens‍37 regards it as os naviculare.

2. Os pyramidale (Fig. 39 p), Dugès, n. 37, articulates with the ulnar part of the posterior articular extremity of the radio-ulnar. Anteriorly the bone has a facet which receives the capitulum ulnae, and alongside this a small flat surface which articulates with the radius. Posteriorly, opposite to the second row, it presents a long raised articular surface, extending from the dorsum to the palmar surface; and on the palmar aspect of the bone is a ridge running from before backwards. Mertens names this bone os lunatum.

3. Os naviculare (Fig. 39 n), Dugès, n. 39. The third bone of the first row does not articulate with the bones of the forearm. It is connected with the os lunatum by a saddle-shaped surface, posteriorly with the os capitatum by means of a concave surface. On the inner surface is a small concave articular facet for the os multangulum minus or trapezoides (Fig. 39 t′). The convex palmar surface of the bone articulates with the os multangulum majus (t); the radial side is non-articular, and with the os lunatum forms a ridge directed forwards. Mertens names this bone os triquetrum. It is evident, however, that if the arrangement in man obtains as a standard, the nomenclature of Dugès, which I have here used, is correct; but the os naviculare is no longer in contact with the articular surface of the os antibrachii.

The second or distal row also consists of three bones:‍—

4. Os capitato-hamatum (Fig. 39 hc), Dugès, n. 40, 41, 42, corresponds with the os capitatum and hamatum‍38, which are here fused together. It is the largest bone, and articulates with all three bones of the first row; it is crescentic, with the concavity directed posteriorly, and on the convex surface has three facets for the three outer ossa metacarpi.

5. Os multangulum minus, s. trapezoides (Fig. 39 t′), Dugès, n. 43, is a small bone lying next to the foregoing; it articulates with the os metacarpi II, and anteriorly with the os naviculare. Dugès considers it to be the os trapezium.

6. Os multangulum majus, s. trapezium (Fig. 39 t), is a small bone, which is placed on the convex palmar auricular side of the os naviculare. Dugès regards this bone as os metacarpi pollicis (Dugès, n. 44), Mertens as multangulum minus. It appears to me that the interpretation here given is justified, because it articulates with the rudiment of the thumb, and as it possesses no resemblance to a long bone it cannot be regarded as an os metacarpi.

Structure of the carpal bones. All the carpal bones consist of calcified cartilage, though variations due to age occur in the amount of the deposit.

Wrist joint. As stated above, the os lunatum and os pyramidale are articulated with the forearm. In addition to the capsules uniting the parts, there are strengthening ligaments, two of which require special mention. One of these arises from the palmar surface of the posterior extremity of the ulna, and passes to the sharp palmar ridge of the os pyramidale, where it is attached, and is inserted into the os capitato-hamatum; the second ligament passes in a similar manner on the palmar surface from the radius to the os naviculare. Between the two ligaments there is on the volar surface a deep excavation.

Movements of the hand. The two bones of the forearm being firmly united, pronation and supination, as in man, are impossible. The normal position, as already remarked (see radio-ulnar), is that of semi-pronation (compare Figs. 2 and 74). If the animal be in its natural posture, with the arm bent at the elbow (Fig. 1) it can touch the ground with the ulnar border only; but when the whole surface of the hand rests upon the ground, the action is brought about as follows: the plane of the carpus is in extension parallel with that of the forearm; in flexion, almost at right-angles with that plane (Fig. 74) The os lunatum glides on the prolongation of the articular surface on the inner palmar surface of the radius, while the os pyramidale is held firmly by the ligament just described. The hand follows the movement of the os lunatum, and at the same time the three outer ossa metacarpi gliding with it inwards, the fingers of the hand turn inwards.

b. The Bones of the metacarpus, ossa metacarpi (Fig. 40 m). Of these there are five: the second to the fifth are of about equal length; the first, forming the rudiment of the thumb, hidden beneath the skin, is much shorter. This metacarpal bone of the thumb is regarded by Dugès and others as the first phalanx. My reason for describing it as a metacarpal has already been given. This bone also presents differences in the two sexes. Meckel’s‍39 observation that it exists only in the male is incorrect; it is a large, broad and sickle-shaped mass of calcified cartilage or even true bone, while in the female it is simply a small cartilaginous rod; as this almost entirely disappears in dried specimens, Meckel’s statement is explained. The second finger has in the frog assumed the functions of the thumb, and, in the males, in the breeding season swells and undergoes an essential alteration of structure. The metacarpal of this finger also displays differences in the sexes, being in the male stronger, broader, and on the inner side provided with a spine-like prolongation, for the tendon of the abductor digiti II (pollicis) longus, which attains an unusual development during the breeding season. The remaining metacarpal bones, the third, fourth, and fifth, are of similar form, the last being provided with a tubercle. The anterior articular extremities are rather broad and concave, and the posterior have rounded heads.

c. The phalanges, phalanges digitorum. The hand has four fingers and the rudiment of a fifth, which latter is completely hidden under the skin, and as already explained, consists of a metacarpal only. Of the four fingers, the fourth is the longest, the third the shortest, the second and fifth of intermediate and about equal length; the fourth and fifth fingers have each three phalanges, the second and third have each two. Their shape is not unlike that of the human phalanges.