o-.ap. x.j THE THORAX. 171 



thorax is very pliable and elastic, and if a constant 

 impediment exists to the entrance of air, as afforded, 

 for example, by greatly enlarged tonsils, the thoracic 

 walls may yield in time to the unbalanced pressure 

 brought to bear upon them at each inspiration. The 

 weakest part of the thorax is along the costo-chondral 

 line on either side, and it is here that the parietes 

 yield most conspicuously in such cases, and by this 

 yielding the deformity is produced." 



The sternum* The upper edge of the sternum, 

 corresponds to the disc between the second and third 

 dorsal vertebrae, and the sterno- xiphoid joint to the 

 middle of the ninth dorsal. In the foetus at full term 

 the upper edge of the sternum is opposite the middle 

 of the first dorsal vertebra (Symington). A trans- 

 verse ridge may be felt upon its anterior surface that 

 corresponds to the junction of the manubrium and 

 gladiolus, and is in a line with the second costal 

 cartilages. The skin over the sternal region is the 

 part of the surface most frequently the seat of keloid. 

 The bone is rarely fractured, being soft and spongy, 

 and supported by the elastic ribs and their cartilages, 

 as by a series of springs. In the old, when the car- 

 tilages are ossified and the chest more rigid, the 

 tendency to fractui-e is increased. The sternum is 

 most often found fractured in connection with injuries 

 to the spine, although it may be broken by simple 

 direct violence. The bone may be fractured by 

 violent bending of the spine backwards, and by abrupt 

 bending of it forwards. In the former instance the 

 lesion is probably due to muscular violence, to the 

 abdominal muscles and the sterno-mastoid pulling one 

 against the other. In the latter instance the lesion is 

 commonly brought about by the violent contact of the 

 chin with the bone. In all instances the fracture is 

 usually transverse, and most often occupies the line 

 between the manubrium and the gladiolus. As these 



