Union of the points of osscation of the body of the considering the union of the different parts which compose the body of the sternum, it is necessary to make a distinc tion between the lateral conjunction—that is, the union of the osseous points which are situated on each side of the median line— and the vertical conjunction, or the union of the pieces of the sternum properly so called. The lateral conjunction, or the union of these osseous germs, which form a pair in the same interval, always precedes the vertical conjunc tion.
The vertical conjunction, or the union of the different pieces of the body of the sternum together, commences with the two inferior portions. After this union, the body of the bone consists only of three parts. The 2nd piece then unites with the lower : the sternal foramen is formed sometimes at the junction of these last mentioned parts, sometimes at the place where the two lateral points of the 4th and of the 3rd portions of the body are united. If the interruption to the progress of ossification should occur at the point where the lateral parts of two sternal pieces would meet, the foramen is likely to have con siderable size ; for it may be the result of an "arrest of development" proceeding from four centres each constituting a part. (Fig. 660. d.) The union of the divisions of the body of the sternum takes place precisely in the inverse order of their appearance In fact the appearance of the osseous points proceeds from above downwards, while their union proceeds from below upwards : a fact which verifies the assertion, that the order of development of osseous points is not always correlative to the order of junction.
The lowest or 5th piece is joined to the 4th soon after puberty ; the 4th and the 3rd are united, between 20 and 30 years of age ; and the body of the sternum is usually not completed by the junction of the 3rd piece to the 2nd before 35 or 40 years. Lastly, the 1st division does not in general join the rest of the sternum at any period ; but should its union happen to take place, it is only to be met with in advanced age.
Of the osscation of the is generally accomplished by one nodule. Some times there are two ; and then they are rarely symmetrical. The process commences in the upper part of the cartilage, and very rarely extends through the whole. The time of ap pearance of the osseous point is extremely variable. Sometimes it is visible towards the 3rd or 4th year ; sometimes not until the 12th or even the 18th year ; according to the observation of Beclard, between the 2nd and 18th years.
From the 40th to the 50th year, and some times later, the appendix becomes united to the body of the sternum, From the varieties of ossification or development of the sternum, it will be evident that it is impossible to assign to it a limited number of osseous points.