154 MAMMALIAN ANATOMY 



from the cephalic end toward the tail and dorsally. It is not perfectly 

 straight, but follows the curves of the vertebral column, and is at first 

 convex and then slightly concave ventrally. 



The ventral wall of the thorax is shorter than the dorsal wall. 

 It is formed by the sternum and the costal cartilages. It is triangular, 

 almost pointed, at the cephalic end, and curves dorsally into the walls 

 where the thorax is widest at the caudal end. It is almost flat at first, 

 but gradually becomes more convex transversely as it passes toward 

 the tail. It is directed from the cephalic end ventrally and toward the 

 tail, and extends from a point ventral to the sixth cervical vertebra to 

 a point ventral to the junction of the bodies of the eleventh and 

 twelfth thoracic vertebrae ; but, of course, as this position is varied in 

 life, the one o-iven may not be the one found after death. 



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The lateral walls of the thorax arch dorso-ventrally and laterally, 

 more strongly at the caudal than at the cephalic end, and are directed 

 from the inlet toward the tail and laterally. Their dorso-ventral 

 height increases from the cephalic end to a point ventral to the tenth 

 thoracic vertebra, whence it decreases rapidly, owing to the obliquity 

 of the plane of the outlet. The walls are formed of the ribs and the 

 dorsal part of the costal cartilages. The first two or three pairs of 

 ribs are almost vertical, but the rest are directed from the vertebral 

 end in a curve ventrally and toward the tail, the obliquity increasing 

 in the caudal ribs. 



The cavity of the bony thorax is conical, compressed from side to 

 side in front, but behind the fifth vertebra the transverse diameter 

 exceeds the vertical. It is a third longer than high at its highest 

 caudal part. It does not always, however, have these dimensions. It 

 is enlarged by the filling of the lungs in inspiration and diminished 

 when the air is driven out in expiration. The enlargement is due 

 principally to the movement of the ribs toward the head and laterally, 

 but also to the movement toward the tail, or the flattening, of the 

 diaphragm, the muscular sheet which forms a partition between the 

 thoracic and abdominal cavities. 



The enlargement of the thorax by the movements of the ribs may 

 be explained as follows. An examination of the articulation of a 

 rib to the vertebra shows that the rib cannot be pulled dorsally or 

 ventrally, nor in a cephalic or a caudal direction, because of the close 

 union between its tubercle and the vertebral transverse process ; but 



