THE THORAX. 109 



centre of the sternum, on a line with the lower margin 

 of the articulation of the cartilage of the fourth rib with 

 it. The left auriculo-ventricular opening is about three- 

 quarters of an inch lower than the pulmonary orifice. 

 The orifice of the pulmonary artery is on a line with the 

 space between the junction of the second and the third 

 costal cartilages with the sternum, being to the left and 

 close to that bone. 



The orifice of the aorta is at the commencement of the 

 ascending portion of the arch, and is on a line with the 

 junction of the third costal cartilage of the left side with 

 the sternum. The arch attains the level of the upper 

 border of the second costal cartilage of the right side at 

 its junction with the sternum. (Note. These relations 

 vary slightly in the works of some authors, but those 

 mentioned above will be found correct for practical pur- 

 poses.) 



The region of the walls of the thorax may be conve- 

 niently described surgically, as sternal, costal, diaphrag- 

 matic, and spinal. 



The sternal region consists of the sternum itself and 

 of the structures which immediately cover it viz., in- 

 tegument, subcutaneous cellular tissue, aponeurosis of 

 pectoralis major, sterno-mastoid, rectus abdominis, and 

 occasionally fibres of the rectus stern alis muscles ; it is 

 surgically of importance, as being the seat of fractures, 

 of dislocations of the clavicle from it, of necrosis from 

 various causes and of growth. The mechanism of the 

 sterno-clavicular articulation, which possesses an inter- 

 articular fibro-cartilage and two synovia! membranes, 

 allows of motion in almost all directions, and largely 

 assists in the free play of the shoulder. The peculiarity 

 in the construction and the curved form of the clavicle, 



10 



