THE HUMAN STERNUM 17 



{a) Ossification of the Human Sternum before Birth 



The following facts have been derived from an examination of two 

 hundred and thirty-six sterna of human embryos between the foetal ages of 

 three and nine months. 



Dates of Ossification. 



The approximate periods of ossification in the component parts of 

 the sternum are shown in the diagram (PL V, Fig. 35), which essentially 

 corroborates the accepted account of the dates at which the several centres 

 appear. The first nuclei appear, generally, in the sixth month for the pre- 

 sternum and the first piece of the mesosternum. In the seventh month 

 centres appear for the second and third pieces of the mesosternum. No 

 further addition to the number of centres occurs during the eighth and 

 ninth months ; and in full-time foetuses (of which one hundred and twenty- 

 two examples were examined) the fourth piece of the mesosternum and 

 the metasternum are usually cartilaginous (Table I). 



Position and Number of Centres of Ossification. 



In Table II, a summary is given of the situation and number of 

 centres of ossification in the component parts of the sternum. A more 

 intimate analysis did not appear necessary ; and for the sake of brevity and 

 clearness, reference has been omitted to the separation or fusion of double 

 centres, and to obliquity in their position (such oblique double nodules 

 being grouped as lateral). The results tabulated, it should be noted, are 

 reached from an examination of specimens, of which the large majority are 

 full-time foetuses. The cases of younger sterna are not only fewer in 

 number, but are also less advanced in ossification. 



While there is considerable variability, both in number and position, 

 the centres of ossification are, as a general rule, single and median, and are 

 five in number ; one for the presternum, one each for the first three 

 elements of the mesosternum, and one for the metasternum. Exceptions 

 to this rule are found in all situations, except in the metasternum, which, 

 in the few caSes in which bone formation has begun, is invariably provided 

 with a single median centre. 



