124 THE SKELETON. 



THE THORAX. 



The Thorax, or Chest, is an osseo-cartilaginous cage containing and protecting 

 the principal organs of respiration and circulation. It is conical in shape, being 

 narrow above and broad below, flattened from before backward, and longer behind 

 than in front. It is somewhat reniform on transverse section. 



Boundaries. The posterior surface is formed by the twelve dorsal vertebrae and 

 the posterior part of the ribs. It is concave from above downward, and presents 

 on each side of the middle line a deep groove in consequence of the direction 

 backward and outward which the ribs take from their 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, the intercostal spaces. These are eleven in number, and are 

 occupied by the intercostal muscles. 



The upper opening of the thorax is reniform in shape, being broader from side 

 to side than from before backward. It is formed by the first dorsal 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 part of the ring is on a 

 lower level than the posterior. The antero-posterior diameter is about two inches, 

 and the transverse about four. The loiver opening is formed by the twelfth dorsal 

 vertebra behind, by the twelfth rib at the sides, and in front by the cartilages of 

 the eleventh, tenth, ninth, eighth, and seventh ribs, which ascend on either side 

 and form an angle, the subcostal angle, from the apex of which the ensiform 

 cartilage projects. It is wider transversely than from before backward. It slopes 

 obliquely downward and backward, so that the cavity of the thorax is much deeper 

 behind than in front. The Diaphragm closes in the opening forming the floor of 

 the thorax. 



In the female the thorax differs as follows from the male : 1. Its general 

 capacity is less. 2. The sternum is shorter. 3. The upper margin of the sternum 

 is on a level with the lower part of the body of the third dorsal vertebra, ^hereas 

 in the male it is on a level with the lower part of the body of the second dorsal 

 vertebra. 4. The upper ribs are more movable, and so allow a greater enlargement 

 of the upper part of the thorax than in the male. 



The Sternum. 



The Sternum (arepvov, the chest) (Figs. 79, 80) is a flat, narrow bone, sit- 

 uated in the median line of the front of the chest, and consisting, in the adult, of 

 three portions. It has been likened to an ancient sword ; the upper piece, repre- 

 senting the handle, is termed the manubrium ; the middle and largest piece, which 

 represents the chief part of the blade, is termed the gladiolus ; and the inferior 

 piece, Avhich is likened to the point of the sword, is termed the ensiform or xiphoid 

 appendix. In its natural position its inclination is oblique from above downward 

 and forward. It is slightly convex in front, concave behind, broad above, becoming 

 narrowed at the point where the first and second pieces are connected, after which 

 it again widens a little, and is pointed at its extremity. Its average length in the 

 adult is about seven inches, being rather longer in the male than in the female. 



The First Piece of the sternum, or Manubrium ( pre-sternum), is of a somewhat 

 triangular form, broad and thick above, narrow below at its junction with the 

 middle piece. Its anterior surface, convex from side to side, concave from above 

 downward, is smooth, and affords attachment on each side to the Pectoralis major 

 and sternal origin of the Sterno-cleido-mastoid muscle. In well-marked bones the 

 ridges limiting the attachment of these muscles are very distinct. Its posterior 

 surface, concave and smooth, affords attachment on each side to the Sterno-hyoid 

 and Sterno-thyroid muscles. The superior border, the thickest, presents at its 

 centre the pre-sternal notch ; and on each side an oval articular surface, directed 



