PART II 

 THE THORAX 



THE thorax (6o>pa^ breast-plate) is enclosed by the dorsal vertebra.-, 

 the ribs, costal cartilages, and sternum ; its apex is bifid and extends 

 an inch and a-half above the first rib. The floor, formed by the dia- 

 phragm, is convex, and is higher on the right side than on the left by 

 the depth of a rib, being pushed up by the liver ; the centre is the 

 lowest part, being on the level of base of the xiphoid. On the right 

 side, after an ordinary expiration, the floor reaches the level of the 

 fifth rib in front, and, after a forced expiration, to the level of the fourth. 

 The lowest part of the floor extends from the base of the xiphoid to 

 the eleventh or twelfth rib (p. 196). 



The sternum consists of the manubrium, gladiolus, and xiphoid ; 

 to the last piece the linea alba is attached. At the top of the manubrium 

 is a notch between the clavicles which receives the trachea in flexion of 

 the neck. Below and to the outside of this notch the cartilages of the 

 first ribs blend with the manubrium. 



The downward slope of the first ribs brings the top of the manubrium 

 opposite the second dorsal vertebra, from which it is separated by the 

 space of two inches. 



Down each lateral half of the sternum the pectoralis major arises, 

 and from the upper part of the front of the manubrium arises the 

 pointed tendinous head of the sterno-mastoid. At the back of the manu- 

 brium arise the sterno-hyoid and sterno-thyroid, and from the lower end 

 of the gladiolus the triangularis sterni. The diaphragm is attached 

 to the back of the xiphoid cartilage, and the linea alba to the tip. 



The sternum is developed in lateral halves, together with the ribs, 

 in the mesoblastic layer, in curved plates extending from the vertebral 

 column. When these plates fail to meet along the middle line 

 the thorax is open in front, the condition being called cleft sfcr/nnn. 

 For an extreme case of this defect, with ectopia cordis, see p. 318. 

 Ectopia differs from cleft sternum in that not only is the sternum cleft, 

 but the want of fusion in the middle line implicates the soft tissues 

 as well as the bcne, the heart being out of place. 



