THE THORAX AS A WHOLE 79 



The Sacrum and Coccyx. The sacrum and coccyx 

 are the result of the fusing of the lower nine vertebrae 

 into two bones, five to form the sacrum and four the 

 coccyx. 



The sacrum is much larger than the coccyx, is 

 located between the two iliac bones, articulating 

 above with the fifth lumbar vertebra, below with the 

 coccyx, and is perforated with foramina which transmit 

 the spinal nerves. 



The coccyx is pyramidal. Its vertebrae are very 

 rudimentary, and it possesses a trace of the neural 

 arch and transverse processes of the typical vertebra. 



THE THORAX AS A WHOLE 



The thorax is an osseocartilaginous cage, conical and 

 flattened from before backward. The short antero- 

 posterior diameter is characteristic of man, but in the 

 lower mammals and human fetus it is longer than the 

 transverse diameter. The posterior surface is convex 

 forward, formed by the twelve thoracic vertebrae and 

 back part of ribs; on either side the sulcus pulmonalis 

 is formed by the ribs as they project backward, so 

 that the weight of the body is more equally distrib- 

 uted around the vertebral column. The anterior 

 surface is flattened and slightly convex, is formed by 

 the sternum and costal cartilages, and lies at an angle 

 of 20 or 25 degrees with the posterior. A horizontal 

 anteroposterior diameter taken from the base of the 

 ensiform is 8 inches (20 cm.); the transverse at the 

 eighth or ninth ribs is 11 inches (28 cm.); the vertical 

 anteriorly is 6 inches (15.5 cm.); and posteriorly is 

 12 inches (31.5 cm.). The lateral surfaces are convex; 

 formed by the ribs, separated from each other by the 

 spaces (intercostal). 



The superior or upper opening of the thorax, the inlet, 

 is reniform in shape, being broader from side to side 



