THE STERNUM, OR BREAST BONE 157 



The Cavity of the Thorax (cavumthoracis). The capacity of the cavity of the 

 thorax does not correspond with its apparent size externally, because (1) the space 

 enclosed by the lower ribs is occupied by some of the abdominal viscera ; and (2) 

 the cavity extends above the first rib into the neck. The size of the cavity of the 

 thorax is constantly varying during life, with the movements of the ribs and 

 Diaphragm, and with the degree of distention of the abdominal viscera. From 

 the collapsed state of the lungs, as seen when the thorax is opened, in the dead 

 body, it would appear as if the viscera only partly filled the cavity of the thorax, 

 but during life there is no vacant space, that which is seen after death being 

 filled up during life by the expanded lungs. 



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 thoracic vertebra, whereas in the male it is on a level with the lower 

 part of the body of the second thoracic 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, or Breast Bone. 



The sternum, or breast bone (Figs. 122 and 123), is a flat, narrow bone, situated 

 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, representing 

 the handle, is termed the manubrium sterni (presternum); the middle and largest 

 piece, which represents the chief part of the blade, is termed the gladiolus (meso- 

 sternum or corpus sterni); and the inferior piece, which is likened to the point 

 of the sword, is termed the ensiform or xiphoid process or appendix (processus 

 xiphoideus or metasternum). 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 (17.5 cm.), being rather longer 

 in the male than in the female. At the junction of the manubrium and gladiolus 

 is a distinct angle (angulus Ludomci), the gladiolus looking forward, the manu- 

 brium also looking forward, but to a less degree. This angle is on a level with 

 the second rib, and is produced by retraction of the upper portion of the thorax. 



First Piece. The first piece, or the manubrium sterni, is of a somewhat 

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

 middle piece. 



Surfaces. 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 Sternomastoid 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 of the Sternohyoid 

 and Sternothyroid muscles. 



Borders. The superior border, the thickest, presents at its centre the presternal 

 notch (incisura jugularis), and on each side an oval articular surface, the 

 clavicular facet (incisura clavicular is), directed upward, backward, and outward, 

 for articulation with the sternal end of the clavicle. The inferior border presents 

 an oval, rough surface, covered in the recent state with a thin layer of cartilage, 

 for articulation with the second portion of the bone (synchondrosis sternalis). 

 The junction of the manubrium with the gladiolus is marked by a transverse 

 ridge, which corresponds to the attachment on each side of the cartilage of 

 the second rib. The lateral borders are marked above by a depression (incisura 

 costalis /) for the first costal cartilage, and below by a small face^ ;( whieh, with a 



