155 



vertebral extremities to their angles. The anterior surface is flattened or slightly 

 convex, and inclined forward from above downward. It is formed by the sternum 

 and costal cartilages. The lateral surfaces are convex; they are formed by the 

 ribs, separated from each other by spaces. Each space is called an intercostal 

 space (spatium inter costale). These are eleven in number, and are occupied by 

 the intercostal muscles. 



r First thoracic 



FIG. 120. The thorax. Ventral view. 



The superior or upper aperture of the thorax, the inlet (aperiura thoracis supe- 

 rior), is reniform in shape, being broader from side to side than from before back- 

 ward. It is formed by the first thoracic vertebra behind, the upper margin 

 of the sternum in front, and the first rib on each side. It slopes downward 

 and forward, so that the anterior boundary is on a lower level than the posterior. 

 The antero-posterior diameter is about two inches (5 cm.), and the transverse 

 about four (10 cm.). The parts which pass through the upper opening of the 

 thorax are, from before backward in or near the middle line, the Sternohyoid 

 and Sternothyroid muscles, the remains of the thymus gland, the trachea, 

 oesophagus, thoracic duct, the inferior thyroid veins, and the Longus colli muscle 



