148 



MAMMALIAN ANATOMY 



the ventral ends of the cartilages of the ribs. It is separated from 

 the vertebral column by the entire dorso-ventral diameter of the 

 thorax. Owing to the conical shape of the thorax, this diameter is 

 small at the cephalic end and large at the caudal end, and the cephalic 

 end of the sternum is therefore much closer to the vertebral column 

 than is the caudal end. For example : if it is distant from the ver- 

 tebral column at the cephalic end by twenty millimetres, it is distant 

 at the caudal end by at least one hundred and ten millimetres. The 

 sternum has an oblique position in the body, and is directed from the 

 cephalic end ventrally and toward the tail. Since the sternum moves 

 with the ribs back and forth in respiration and in muscular efforts, 

 it does not always lie ventral to the same vertebrae. It appears to 

 reach from a point ventral to the fifth cervical to a point ventral 

 to the twelfth dorsal. The sternum can be felt lying in a groove 

 between the heavy pectoral muscles which rise in part from its ventral 

 surface. 



The sternum is composed of eight oblong bones, called sternebrae. 

 They are arranged end to end, and are united by plates of cartilage. 

 These cartilages are large in young animals, and their sides furnish 

 most of the attachment for the costal cartilages. In fully developed 

 sterna the cartilages are thin disks and the ribs are fastened also to the 

 adjoining sides of the sternebrse. The sternebrse, with the exception 

 of the first and the eighth, are similar in shape and vary little in size. 

 In a typical one the length is about three times the width, which is 



equal to the height. There 

 are six surfaces : the anterior 

 or cephalic, the posterior or 



/ i:;iSjif& \ Tubercles 



s """" --' caudal the dorsal 



and the two lateral. 



TYPICAL STERNEBRA, VENTRAL VIEW. 



Right Lateral Surface. 



surfaces are quadrilateral. 

 I lie lateral margins are emar- 

 ginate, and the dorsal and ven- 



tral margins gently arcuate. The cephalic surface (Fig. 100) has the 

 dorso-ventral diameter greater than the transverse diameter, but in the 

 caudal surface (Fig. 101) the two diameters are about equal. 



The lateral surfaces are much longer than wide. The cephalic 

 and caudal margins are straight. The dorsal and ventral margins are 



