30 



THE THORAX. 



The intercostal spaces are eleven in number, and somewhat wider above than 

 below, but varying with the elevation or depression of the ribs. 



In man the transverse diameter of the thorax exceeds the antero-posterior, whereas in 

 quadrupeds the dorso- ventral diameter is usually the greater. In the human foetus also the 

 sagittal diameter preponderates, and at the time of birth it is but little less than the 

 coronal. In the female the thorax is relatively shorter, and more rounded than in the male. 



OSSIFICATION OP THE STEBNUM AND BIBS. 



The ossification of the sternum begins about the sixth month, and usually by a single 

 centre in the manubrium. The next centre appears at the seventh month in the upper 



fc- 



Fig. 30. OSSIFICATION OF THE STERNUM. (R. Quain.) 



A, the cartilaginous sternum before the middle of fetal life. 



B, the sternum at birth. 1, 2, 3, & 4, the nuclei for the manubrium and upper three pieces of the 

 body. 



C, the sternum soon after puberty, showing cartilage between the manubrium and body, and 

 imperfect union of the first, second and third pieces of the body, while the third and fourth are 

 united. 



D, a sternum at birth with an unusual number of ossific centres, six in the manubrium, 1', which 

 is very uncommon ; two pairs in the lower pieces of the body 3' & 4', which is not unusual ; 2, the 

 single centre of the first piece of the body. 



E, Example of perforated sternum ; this figure also shows two suprasternal bones, * *. C and E 

 are reduced below the size of nature. 



segment of the body, and ossification follows in the next two segments shortly before birth. 

 In the lower segment ossification begins in the first year or later, in the xiphisternum usually 

 not before the sixth year, and often much later. In the manubrium there are sometimes two 



Fig. 31. ONE OF THE MIDDLE RIBS 



AT ABOUT TWENTY YEARS ' OF AGE. 



(R. Quain.) 



1, body ; 2, epiphysis of the head ; 

 3, that of the tuberosily. 



centres of ossification, one above the 

 other, and occasionally several are 

 met with. In the upper segment of 

 the body the centre is most commonly 

 single, but in each of the following 

 segments there are frequently two. 

 placed one on each side of the middle 



line. The lower segments of the body unite together after puberty, but the upper one often 

 remains separate till after the twenty-fifth year. The xiphisternum is united to the body in 

 middle life ; the manubrium and body are only exceptionally joined by bone. The bony 

 parts formed from the lateral centres of the lower segments of the body not unfrequently 



