peculiarly formed clavicle is no more immediately connected with the manubrium sterni, than was the case in the above-mentioned animals. The terminal surface presents a saddle-shaped depression which, for the anterior third of its extent is invariably directly attached to a remarkable cartilaginous structure (ep), whilst in the posterior twothirds, an articular cavity frequently intervenes. The cartilaginous piece is at least equal in length to one-third of the clavicle, and is applied to the anterior and lateral borders of the manubrium sterni. The extent of the joint between the clavicle and the cartilaginous piece varies to some extent according to my observations, and in a few examples appeared only as a small fissure extending from behind forwards. The whole sternal end of the clavicle is invested by a thin cartilaginous layer, which at the part corresponding to the joint forms an articular cartilage, covering the clavicle, whilst elsewhere it is directly continuous with the cartilaginous episternal piece, though the limits of the two in a histological point of view are sharply defined. The structural characters of the cartilaginous piece itself cause it to be just as strongly differentiated from the manubrium sterni as from the clavicle: and the surface of the former seems also to be invested by cartilage. As regards the intimate structure of the episternal cartilage, I found that with low microscopical powers, it presented a peculiar longitudinally striated appearance, just as though a fibrous investment was continued over it, from the clavicular to the sternal extremity. Closer investigation, however, showed that this apparent fibrillation was in reality due to the peculiar arrangement of the morphological elements of the cartilage. For immediately beneath the surface of the sternal extremity, roundish scattered cartilage cells appeared, which occasionally presented indications of division. At a short distance from this point, and towards the middle of the cartilage, the cells formed rows, at first of a somewhat oval form, and composed of three or four cells, but subsequently of a greater number, as from ten to fifteen, the group then presenting a fusiform shape. By comparing different rows, it was ascertained that each row proceeded from a single cell, multiplication by division in a longitudinal direction being exceedingly common amongst them. The intermediate substance (matrix of the cartilage) showed also some indications of longitudinal striation, though no doubt the impression to the eye was chiefly the result of the above-mentioned arrangement of the cells. The external investment of the cartilage, composed of connective tissue, was continued from the periosteum of the sternum into that of the clavicle, and consequently surrounded the fissure which existed between the clavicular surface of the cartilage and the clavicle, thus completing the articular cavity. The two cartilaginous episternal pieces, possessed a certain amount of elasticity. They were closely approximated to one another at the anterior border of the manubrium sterni, and the outer space was occupied by a short lancet-shaped cartilaginous prolongation (m), the minute structure of which was the same as that of the larger pieces of cartilage. From the above description, it follows that these cartilaginous pieces are not to be considered as detached portions either of the clavicle or of the sternum. From the clavicles they are separated by a welldefined line, and a well-marked histological difference; and at one part the separation is so perfect that a kind of joint is formed: whilst they are also differentiated from the sternum by their microscopical structure with equal clearness, so that they can neither be regarded as simple processes of the sternum, nor as sternal prolongations of the clavicles. They consequently exactly agree with the abovementioned intermediary pieces between clavicle and sternum; and if they are in those instances to be termed episternals, the same nomenclature must also be applied to these cartilaginous structures of the mole. The smaller degree of independency resulting from their more intimate connection with the sternum, constitutes no serious objection to this view: for we have already seen that in the Hedgehog the episternal cartilage is very intimately connected with the clavicle; and that on the other hand, in Didelphys the T-formed cartilage is more closely connected with the sternum than with the clavicle. At a much lower grade of development, yet still by the light of the structures just described in the mole, readily recognizable as belonging to the same series, are the homologous parts in the Squirrel. If in this animal, the sterno-clavicular articulation be examined from the anterior surface of the body (Fig. 9), the expanded sternal ex FIG. 9. Episternal apparatus of Sciurus vulgaris (Squirrel), seen from the outer side :e, clavicle; c', first rib. tremities of the clavicles appear to be directly applied to the obliquely bevelled anterior edge of the manubrium sterni. But an examination of the inner surface of the manubrium shows, that even in this instance, intervening structures are present. (Fig. 10.) Episternal apparatus of Sciurus vulgaris (Squirrel), seen from within:St, sternum; Ep, episternum; c, clavicle; c', first rib. Only a few preparations of the sterno-clavicular articulation are required to show that a remarkable piece is introduced between the sternum and the bony clavicle. This piece is formed of a fibrous mass, which is but slightly developed anteriorly, is softer than cartilage, and when longitudinal sections are carefully examined, appears to be chiefly composed of connective tissue. The sternal end of the clavicle is here also invested by cartilage, and from the cartilage the fibrous band which can be followed to the sternum directly springs. This band, whilst possessing much firmness in the central part, is of loose texture at the two extremities. The morphological elements of cartilage are not distinguishable in it, so that on histological grounds the whole intermediate piece is of a different character from that which I have found in other mammals. Tamias presents arrangements that are essentially similar to those of Sciurus. If the extreme shortness of the parts in the Mole be disregarded, very similar characters are found in the corresponding parts in various Carnivora (Meles, Lutra, Mustela, Felis, Hyæna). Amongst these animals the episternal on each side is represented by a band of considerable length, proceeding from the clavicle, which has been regarded either as properly belonging to the clavicle, partly from defective information, and partly from the similar mode of connection existing between the clavicle and scapula, or, as a structure developed to make up for the smaller size of the clavicle. Now, although the elongated form and purely ligamentous strcuture of the episternals here present may be considered as a sign of degradation, no objection need be raised to its homology with true epi sternal structures, any more than the small size of the piece of bone attached to the acromion only by a ligament, need be held to cast a doubt upon their representing the clavicle. It must be admitted also, that in other classes the clavicles and episternals alike present degradations of a similar character, as may be seen amongst the Edentata in Myrmecophaga didactyla and Chlamydophorus truncatus, where the episternal is still tense and of moderate length; but does not appear to contain any more solid parts in its interior. In the flying mammals I have long sought in vain for the various parts of the bony girdle of the chest, so well known to me in other mammals; and I drew the conclusion from their absence that a more intimate connection of the episternals with the sternum occurred, and consequently that the broad manubrium sterni of these animals resulted from the fusion of these parts.* The recent discovery of a structure that indubitably belongs to the series of episternal structures in these creatures has, however, changed my opinion on this point. If the sterno-clavicular articulation of a Cheiropterous animal is examined from before, the well-marked clavicle is seen to run to the side of the manubrium sterni, and covered with cartilage, to be inserted into a depression of the manubrium, which also possesses a cartilaginous investment. No intermediate piece of any description intervenes, and consequently the usual episternal fragment seems in this instance to vanish with the appearance of a more highly developed clavicle. On making an examination of the internal surface of the articulation, however, it may be seen that the whole of the terminal surface of the clavicle is not implicated in the formation of the joint, but that its posterior and inferior part gives attachment to a conical ligament which is connected with the sternum. The ligament lies imbedded to some extent in the capsular ligament, increasing its strength, but it is important to observe that it originates directly and immediately from the clavicle itself. Its position, therefore, quite independently of its magnitude, which is only a secondary consideration, clearly indicates that this fibrous band is to be placed in the category of the true episternals. In Pteropus a similar conformation of parts is present. The episternals of Quadrumana and Man are much less highly developed. *The examination of the embryos of the Cat exhibit a division of the manubrium sterni from the body of the sternum exactly at the point of junction of the first rib. In adult Rhinolophi I have been unable to remark anything particular on this point. Vespertilio Daubentoni is, however, provided with an articulation at this part of the formation, of which the ribs on both sides participate. From a consideration of the foregoing statements no reasonable doubt can be entertained that in the interarticular fibro-cartilage which separates the clavicle from the sternum, we have the equivalent of the symmetrical episternals. In Cercopithecus (C. ruber) I find a general agreement in the composition of the sternoclavicular articulation with that of man; the interarticular fibro-cartilage, however, being a good deal thicker behind, so that when viewed on that aspect it at once strikes the eye as being an independent structure. Here also, as in man, the presence of the cartilage causes the joint to be composed of two chambers or cavities. In order more easily to explain the structure of the sternoclavicular articulation, and at the same time to serve as a means of comparison, I have made a drawing of a somewhat oblique vertical section of it, and the interarticular fibro-cartilage may be seen to form a distinct portion of the skeleton, both in man and apes; whilst not only from its general relations, but also through its intimate connection with the sternal end of the clavicle and with the manubrium sterni, it is obviously to be regarded as a peculiar modification of the episternal piece, though certainly less developed than even in the Cheiroptera. The episternals are consequently portions of the shoulder girdle, which only vanish entirely when the clavicles are wholly absent, but which are always visible when the clavicles are present, and frequently form structures of very considerable magnitude. Now if we regard the interarticular fibro-cartilage of the sterno-clavicular articulation as the representative of the episternals, it follows that the occasionally present ossa supra-sternalia" described by various authors, and recently in particular by Luschka,* as sometimes occurring in man, must be considered to hold an intimate relation to the typical episternals of armadillos, &c. But a comparison of these ossifications lying in front of the manubrium sterni with the episternals of the armadillos, &c., is by no means satisfactory when we know that certain constantly present portions of the skeleton; namely the interarticular fibro-cartilages have already been determined to be the equivalents of the symmetrical episternals. Nevertheless I am unable at once to explain away these ossa supra-sternalia by considering them as merely accidentally and rarely occurring structures; for it is possible they may stand in genetic connection, not indeed with the more commonly developed symmetrical wings, but with the pro 66 * Zeitschrift für wissenschaft. Zoologie. Bd. iv. p. 36. |