Skeleton

The sternum is developed according to G. Ruge by a fusion of the ventral ends of the ribs on each side thus forming two parallel longitudinal bars which chondrify and eventually fuse together in the mid line. The anterior seven or sometimes eight ribs reach the sternum, but the ventral ends of the ninth and sometimes the eighth probably remain as the xiphisternum. The morphological meaning of the sternum and surrounding parts cannot be settled entirely by a study of their development even when combined with what we know of their comparative anatomy or phylogeny. Pro fessor A. M. Paterson (The Human Sternum, London, 1904) takes deeply concave toward both the head and tail; such a vertebra is spoken of as amphicoelous and with one exception is always found in fishes which have centra. In the bony fish (Teleostei) and mudfish (Dipnoi) the vertebrae are ossified.

A vertebra from the tail of a bony fish like the herring has a ventral (haemal) arch surrounding the caudal blood-vessels and torresponding to the dorsal or neural arch which is also present. In the anterior or visceral part of the body the haemal arch is a different view from the foregoing and regards the sternum as derived from the shoulder girdle. To this point of view we shall return in the section on comparative anatomy. The position of the vertebral and sternal centres of ossification is shown in figs. 5, 6 and 7. The ribs ossify by one primary centre appearing about the sixth week and by secondary ones for the tubercle and head. The sternum is ossified by centres which do not appear opposite the attachment of the ribs but alternately with them, so that although the original cartilaginous structure is probably inter segmental the bony segments are segmental like those of the vertebral centra.

For further details see C. S. McMurrich, The Development of the Human Body (London, 1923). This includes bibliography, but G. Ruge's paper on the development of the sternum (Morph. Jahrb. vi. 188o) is of special importance.

Comparative Anatomy.

Just as in development the notochord forms the earliest structure for stiffening the embryo, so in the animal kingdom it appears before the true vertebral column is evolved. This is so important that the older phylum of Vertebrata has now been expanded into that of Chordata to include all animals which either permanently or temporarily possess a notochord. In the subphylum Adelochorda, which includes the worm-like Balanoglossus, as well as the colonial forms Rhabdopleura and Cephalodiscus, an ento dermal structure, apparently correspond ing to the notochord of higher forms, is found in the dorsal wall of the pharynx. In the subphylum Urochorda or Tunicata, to which the ascidians or sea-squirts be long, the notochord is present in the tail region only and as a rule disappears after the metamorphosis from the larval to the adult form. In the Acrania, which are

represented by Amphioxus (the lancelet) and are sometimes classed as the lowest division of the subphylum Vertebrata, the notochord is permanent and extends the whole length of the animal. Both this and the nerve cord dorsal to it are enclosed in tubes of mesodermal connective tissue which are continuous with the fibrous myo commata between the myotomes. Here then is a notochord and a membranous vertebral column resem bling a stage in man's development.

In the Cyclostomata (hags and lampreys) the notochord and its sheath persist through life, but in the adult lamprey (Petro myzon) cartilaginous neural arches are developed. In cartilaginous ganoid fishes like the sturgeon, the notochord is persistent and has a strong fibrous sheath into which the cartilage from the neural arches encroaches while in the elasmobranch fishes (sharks and rays) the cartilaginous centra are formed and grow into the notochord, thus causing its partial absorption. Each centrum is split and its two sides spread out deep to the muscles and form the ribs. In the elasmobranchs on the other hand the ribs lie among the muscles as they do in higher vertebrates, and the fact that both kinds of ribs are coexistent in the same segments in the interesting and archaic Nilotic fish Polypterus bichir shows that they are developed independently of one another. The sternum is never found in fishes with the possible exception of the comb-toothed shark (Notidanus). Among the Amphibia the tailed forms (Urodela) have amphicoelous vertebrae in embryonic life and so have some of the adult salamanders, but usually the intercentral remnants of the notochord are pressed out of existence by the forward growth of the centrum behind it, so that in the adult each vertebra is only concave behind (opisthocoelous). In the Anura (frogs and toads), on the other hand, the centra are usually concave forward (procoelous) and some of the posterior ones become fused into a long delicate bone, the urostyle. The ribs of urodeles have forked vertebral ends and are thus attached to the centrum as well as to the neural arch of a vertebra; this forking is supposed to be homol ogous with the double ribs of Polypterus already referred to. The sternum as a constant struc ture first appears in amphibians and is more closely connected with the shoulder girdle than with the ribs, the ventral ends of which, except in the salaman der Necturus, are rudimentary. It is not certain whether it is the homologue of the sternum of the fish Notidanus ; the subject is discussed by T. J. Parker and A. M. Paterson (The Human Sternum, London, 1904, p. 5o).