Connections.— The sternum articulates with fourteen ribs through the medium of their cartilages, and more directly with the two cla vicles.
Structure of the bone con sists of two very thin compact !minx, with an intervening cancellated structure, the cells of which are very large and have very delicate parietes. It is one of the most spongy bones of the body, and it is more than probable that to this circumstance the frequency of disease in it may be attributed. Absorption of this bone and great displace ment by bending inwards is very common, particularly in women who wear tight stays. Under such circumstances, or by disease, we have witnessed the sternum so depressed in wards that the depth, including all the thoracic integuments from the spinous processes of the dorsal vertebrae to the anterior surface of this bone, did not exceed three inches, in stead of from seven to nine inches.
Of the Development or Ossification of the Sternum. — As far as the middle of foetal life or a little later, the sternum is altogether cartilmino us, as represented fit:. 660. This bone is one of the slowest in its ossification ; it exhibits no bony points or centres of os sification up to the sixth month of foetal life. It is also of all bones the one in which the phenomena of ossification proceed with the least regularity. After the sixth month of foetal life, ossification begins with the for mation of osseous granules in the middle of the intervals between the points at which the cartilages of the ribs are connected.
" There are five of these granules for the sternum, exclusive of the ensiform appendage, and they form as many pieces (e, fig. 660.). The process of ossification makes its appear ance in the first between the fifth and sixth months ; and, soon following in the second and third, it reaches the fourth at the end of foetal life. The osseous centre of the last varies considerably in the time of its appearance. It may be found soon after birth, and may not be visible for a considerable time (one or two years) after that period." " In many cases, one or more of the divi sions of the sternum are formed from more nuclei than one, and there are peculiarities with respect to the number and position of these additional granules which require notice." Ossification of the 1st piece.—This some
times presents a single nucleus (fig. 1.), rounded and transversely strong ; sometimes it presents two nuclei, and in this case they may be either placed one above the other, or side by side. In the former case the up permost nucleus is the larger ; in the latter, both may be symmetrical and of equal size, or what is more common, they may be of un equal magnitude. It may occasionally present more than two osseous points. Albinus found three in one subject, and four in another.* Mr.Quain has a preparation in Uni versity College, where the very unusual num ber of six (fig. 660. c. V.) are to be seen. In this case, where there is a plurality of osseous points, the largest are generally situated alone; exceptions to this rule are very rare.
Ossification of the body, or the 2nd, 3rd, 4th, and 5th pieces. — The osseous nuclei which enter into the composition of the body of the sternum have generally a rounded form when they are single, and are situated in the middle line ; where they are in pairs or are placed laterally, they are more elongated, but smaller, and appear to represent only the half of one of the single nodules. The second piece has not often more than a single granule (b. 2, c. 2'.), but the rest are frequently formed from two nuclei, which are placed laterally to one another (c 3', 4'.), and not vertically as occurs in the first piece.
These different osseous points are always so arranged as to be situated between two costo-sternal articulations, so that a portion of the sternum is developed in each of the intervals comprised between the ribs. The last piece is the only exception, being com mon to the articulation of the 6th and 7th ribs. There are, therefore, four primitive pieces of the body of the sternum (b. 1, 2, 3, 4.), and each of these is sometimes formed by one point of ossification ; at other times by two lateral points. The first piece may be formed of one or many ossific points, which may be arranged vertically as well as laterally To the centres of ossification here described, M. Brechet I- has added two small epi-sternal granules, whose position is sufficiently shown by the indication of them, fig. 660. at el**. They occur only at rather advanced periods of life, but they do not appear to be constant.