THE HUMAN STERNUM 13 



costal cartilages on each side, and presenting absolutely no differentiation of 

 its component parts. 



Three human sterna of the third month, measuring respectively, 

 8, 9, and 10 mm., show clearly this condition (PI. II, Figs. 17, 18, 19). 

 The demarcation of sternal elements and of costal cartilages is shown in the 

 special arrangements of the cartilage cells in lines, along which, later on, 

 the joint cavities will be formed. In all three sterna the sterno-clavicular 

 joint is completely formed, with its inter-articular fibro-cartilage. But in 

 only one case is there a definite costo-sternal joint cavity (between the 

 mesosternum and the second right costal cartilage) (Fig. i8f). 



{d") The further changes in the cartilaginous sternum preceding 

 ossification occurs in rodents (rat, mouse) without cleavage of the parts of 

 the sternum or separation of costal cartilages. Opposite to the costal 

 attachments the cartilage is distinctly hyaline, poorer in cells, and stained 

 less deeply than in the intervals between the costal cartilages. In these 

 intervals the cells are massed together, stain deeply, and the process of 

 ossification is foreshadowed by the formation of 'primary areolae,' which 

 occur in the centre of the presternum, in the metasternum, and in the 

 first four so-called segments of the mesosternum (PI. Ill, Figs. 20, 21, 22). 



These areolae increase in size, while at the same time the perichon- 

 drium becomes enormously thickened, followed by the formation of 

 periosteal temporary bone in the usual way (Fig. 22). 



Thus at birth (in the mouse, PI. Ill, Fig. 23), while there is com- 

 plete fusion of the sternal elements together, and of the costal cartilages 

 with them, there is a distinct differentiation of elements entering into the 

 constitution of the sternum. Bone formation has begun, and cartilage only 

 exists at the extremities of the presternum and metasternum, in the 

 mesosternum opposite the costal attachments, and in the interval between 

 the attachments of the sixth and seventh costal cartilages (Fig. 24). 



These observations do not harmonize with accepted views, which have 

 been derived from the observations of distinguished and accurate observers, 

 notably Ruge,*° whose monograph on the subject has long been accepted 

 as having settled the question of the early development of the sternum. 

 There are several points in that memoir, however, which, on careful perusal, 



