136 



THE SKELETON 



costal cartilage becomes subdivided, one part remaining attached to the sternum and forming the 

 xiphoid process, whilst the end still continuous with the rib acquires a new attachment to the 

 eighth cartilage. The ends adherent to the sternum may remain separate and give rise to a 

 bifid xiphoid process, though much more frequently they unite, leaving a small foramen. 



At first, therefore, the sternum and costal cartilages are continuous. A joint soon forms 

 between the presternum and mesosternum, and others between the costal cartilages and the 

 sternum (except in the case of the first) quickly follow. The division of the mesosternum into 

 segments is a stiU later formation and arises during the process of ossification. 



Fig. 163. — Posterior Surface of the Manxjbrium (Pre-sternum), with Sternal Ends 

 OF Clavicles and the First Costal Cartilages. 



Sterno-hyoid Sterno-thyreoid 



On the other hand, a view has been advanced by Professor A. M. Paterson that the sternum 

 is not a bilateral structure, but is laid down, as shown in human sterna of the thu-d month, as a 

 simple median band of hyahne cartilage, in complete fusion with the costal cartilages on each 

 side and presenting no differentiation of its component parts. From a study of the earliest 

 stages of the development of the sternum, its comparative anatomy and structure. Professor 

 Paterson has, moreover, brought forward evidence which indicates its independence in the first 

 instance of costal elements and its genetic association with the shoulder girdle. 



Ossification. — The ossification of the sternum is slow and ii-regular. The process begins 

 in the presternum (manubrium) by a single centre about the sixth month of intra-uterine life, 

 though occasionally other accessory centres are superadded. 



Fig. 164. — Two Stages in the Formation of the Cartilaginous Sternum. (After Ruge.) 



The mesosternum (body) usually ossifies from seven centres. The upper segment ossifies 

 from a single median nucleus al)out the eiglith month, and below this, three pairs of ossific 

 nuclei appear, which may reinaiti for a long time soi)arate. Of these, two pairs for the second 

 and third sogrneiits are vi.siblc at l)irth, and those for the lower segment make their appearance 

 toward the end of the first year. The various lateral centres unite in pairs, so that at the sixth 

 year the sternum con.sists of six stcrnebric, the lowest (metastornum) being cartilaginous. 

 Very often, however, there are only four centres of ossification in the gladiolus, as shown in 

 fig. 165. Gradually the four pieces representing the mesosternum fuse with one another, and 



