24 



PROFESSOR PATERSON. 



ossification is foreshadowed by the formation of " primary 

 areolte," which occur in the centre of the pre-sternum, in the 

 meta-sternum, and in the first four so-called segments of the 

 meso-sternum (figs. 20, 21, 22). 



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

 perichondrium becomes enormously thickened, followed by the 

 formation of periosteal temporary bone in the usual way (fig. 22). 



Thus at birth (in the mouse, fig. 23), while there is complete 

 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 cartihige only exists at the extremities 

 of the pre-sternum and meta-sternum, in the meso-sternum 

 opposite the costal attachments, and in the interval between the 

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



II. Ossification. 



The following facts have been derived from an examination 

 of 236 sterna of human embryos between the foetal ages of three 

 to nine months. 



(A) Dates of ossification. — The approximate periods of ossi- 

 fication in the component parts of the sternum are shown in Table 

 I., which essentially corroborates the accepted account of the 



Table I. — Ossification of the Sternum {percentages). 



